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Sputter deposition

Author: Geym

Jul. 29, 2024

Sputter deposition

Sputter deposition Technology DetailsTechnology PVDMaterials Ag, Al, Al2O3, Au, Cr, Cu, Fe, Ge, Ni, Pt, SiO2, Ti, TiO2, Zn

Sputter deposition is a physical vapor deposition method of thin film deposition in which a high-purity source material (called a cathode or target) is subjected to a gas plasma (typically argon). The energetic atoms in this gas plasma collide with the target material and knock off source atoms which then travel to the substrate and condense into a thin film.

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Equipment

The LNF has 4 sputter deposition tools

Endeavor M1 Aluminum Nitride Sputter Tool

Materials deposited: AlN, Al
The Endeavor M1 tool is designed specifically for Piezoelectric AlN on either 4" or 6" wafers. It is a high temperature process that works only on certain substrates with specific seeding materials and the handler is designed to process both 4" (via 6" carrier) or 6" wafers.

Lab 18-1

Lab 18-2

PVD 75 Sputter Tool

Materials

The following materials can be deposited via Magnetron sputter deposition at the LNF. Please note some materials are rarely used in these cases, additional work will be needed to validate process conditions and the rate of deposition.

Method of operation

Sputtering is not typically done on liftoff samples or shadow masks. Therefore it is typically done as a blanket deposition that will be patterned and etched later if patterning is needed. Samples should be clean and vacuum-safe and should be able to handle a small amount of plasma heating. It is recommended that if there is any organic processing done to samples that they are cleaned in an oxygen plasma.

Samples are introduced into a vacuum chamber (either loaded in a vented chamber or sent in from a loadlock) with the proper source target materials. The sample is heated and etched before deposition if the process requires it. Ar is then introduced to the chamber until a desired sputtering pressure is reached and then power is applied the targets to strike a sputtering plasma. The target power is then ramped up slowly while being shielded from the samples by a shutter. Once the pre-sputter sequence is finished, the shutter opens and the deposition is timed to achieve the desired thickness of film. The shutter closes, power is ramped down and the sputtering Ar gas is pumped out. Multiple films may be run by repeating this process with a different source targets. The samples are then removed from the chamber and the tool is pumped back to a low base pressure.

Applications

Figures of merit

The figures of merit are described in the generic PVD page

Responses of figures of merit with parameter changes

Deposition Rate, Stress, Resistivity and Step Coverage

Although many films (especially reactive films and insulating compounds) may vary in their response, typical sputtering variables can be summarized in this trend chart:

Uniformity

Uniformity is set by the material being deposited and the geometry of the system: throw distance, target size, substrate rotation and deposition angle.

In sputtering, uniformity is determined by throw distance and the shape of the deposition cone.
- In the PVD 75 and Lab 18 tools, smaller sources eject material from the face of a 3" target that is angled to cover around 1/2 of the substrate area. The substrate is rotated to coat the entire area. Varying the angle will vary the throw distance and change the amount deposited on the center and edge. The supplies have a set throw angle that is optimized for best uniformity.
- In the ALN tool, the wafer is centered over two targets which are larger than the substrate. Uniformity can be only be adjusted using the DC supply which raises and lowers the power to the center target.

Review Article: Tracing the recorded history of thin-film ...

Thin films, ubiquitous in today's world, have a documented history of more than years. However, thin-film growth by sputter deposition, which required the development of vacuum pumps and electrical power in the s and the s, is a much more recent phenomenon. First reported in the early s, sputter deposition already dominated the optical-coating market by . Preferential sputtering of alloys, sputtering of liquids, multitarget sputtering, and optical spectroscopy for process characterization were all described in the s. Measurements of threshold energies and yields were carried out in the late s, and yields in reasonable agreement with modern data were reported in the s. Roll-to-roll sputter coating on flexible substrates was introduced in the mid-s, and the initial demonstration of sustained self-sputtering (i.e., sputtering without gas) was performed in . The term magnetron dates to , and the results of the first magnetron sputtering experiments were published in the late s. The earliest descriptions of a parallel-plate magnetron were provided in a patent filed in , rotatable magnetrons appeared in the early s, and tunable &#;unbalanced&#; magnetron sputtering was developed in . Two additional forms of magnetron sputtering evolved during the s, both with the goal of efficiently ionizing sputter-ejected metal atoms: ionized-magnetron sputtering and high-power impulse magnetron sputtering, with the latter now being available in several variants. Radio frequency (rf) glow discharges were reported in , with the initial results from rf deposition and etching experiments published in the s. Modern capacitively-coupled rf sputtering systems were developed and modeled in the early s, and a patent was filed in that led to pulsed-dc and mid-frequency-ac sputtering. The purposeful synthesis of metal-oxide films goes back to at least , leading to early metal-oxide and nitride sputtering experiments in , although the term &#;reactive sputtering&#; was not used in the literature until . The effect of target oxidation on secondary-electron yields and sputtering rates was reported in . The first kinetic models of reactive sputtering appeared in the s; high-rate reactive sputtering, based on partial-pressure control, was developed in the early s. While abundant experimental and theoretical evidence already existed in the late s to the early s demonstrating that sputtering is due to momentum transfer via ion-bombardment-induced near-surface collision cascades, the concept of sputtering resulting from local &#;impact evaporation&#; continued in the literature into the s. Modern sputtering theory is based upon a linear-transport model published in . No less than eight Nobel Laureates in Physics and Chemistry played major roles in the evolution of modern sputter deposition.

Metallographic studies of Moche artifacts, coated with gold films whose thicknesses ranged from &#; Å to 1 μm, exhibit evidence of post-deposition heat treatment (annealing) to obtain a film/substrate interdiffusion zone, presumably for better adhesion. An excellent example of craftsmanship is depicted in Fig. 4 .

An autocatalytic solution-growth technique involving oxidation/reduction reactions was developed by the Moche Indians utilizing minerals available in the local area, the northern highlands of Peru, beginning &#;100 BC to deposit gold (as well as silver) films on copper and bronze artifacts. 13 The technique is still in use today, although carried out in a more efficient manner, and referred to as electroless plating. 14 Moche artisans first dissolved gold in a hot aqueous solution of equal parts potassium aluminum sulfate [KAl(SO 4 ) 2 ], potassium nitrate (KNO 3 ), and salt (NaCl), a process that took several days. The resulting mixture was then buffered with sodium bicarbonate (NaHCO 3 ) to form a weakly alkaline solution (pH &#; 9), which was allowed to boil for several minutes before immersing the copper artifact to be plated. The overall reaction is

Highly skilled ancient Egyptian craftsmen mastered the art of gold sheathing&#;the direct application of thin gold layers onto wooden and plaster objects (mostly for noble families) to provide the impression that the object is solid gold&#;at least as early as BC. 11,12 Striking examples were found in the tomb of Queen Hetepheres (wife, and half-sister, of Pharaoh Sneferu, Fourth Dynasty, Old Kingdom, &#;&#; BC). Other spectacular specimens of early thin-film technology were found in the tomb of Pharaoh Tutankhamun (&#;King Tut,&#; 18th Dynasty, ruled &#;&#; BC). Gold sheets were beaten into position over carved wooden structures to provide embossed hieroglyphic texts and decorations as shown in Fig. 3 .

Today, gold leaf can be beaten to &#;500 Å thick (partially transparent to visible light) by highly skilled craftsmen. 5 In fact, the production of gold leaf, primarily for decorative purposes, remained a viable industry for craftsmen until the development, in the mid-s, of roll-to-roll sputter and evaporative coating technologies (see Sec. IV K ).

(Color online) Fresco from a tomb (&#; BC) in Saqqara, Egypt, which depicts the gold melting and purification process, as well as the initial thinning of purified bulk gold with a rounded stone. The reed blowpipes, tipped with baked clay, were used to both increase and control the temperature of the charcoal fire in the ceramic pot. Reproduced with permission from Darque-Ceretti et al., Rev. Mater. 16 , 540 (). Copyright by Creative Commons Attribution License 4.0.

Gold ore was purified by melting it in a mixture of &#;alum&#; [the mineral alunite: KAl 3 (SO 4 )(OH) 6 ], salt (NaCl), and chalcopyrites (e.g., CuFeS 2 ). The process produces sulfuric and hydrochloric acids (H 2 SO 4 and HCl) which dissolve base metal impurities. 9,10 The purified gold still had several to a few tens of atomic percent of silver, copper, or both, depending upon where it was mined, thus giving rise to variations in color. Flattening of purified bulk gold was initiated by beating with a rounded stone and mechanical rolling, followed by many stages of thinning and sectioning of composite structures consisting of Au leaf sandwiched between layers of animal skins, parchment, and vellum. 6 Figure 2 shows an image of a fresco from a tomb (&#; BC) in Saqqara, illustrating melting and purification of Au during which the temperature is adjusted by craftsmen using blowpipes (reeds) with clay tips.

The gold used to produce early thin films was mined by the Egyptians in the Eastern Desert, between the Nile River and the Red Sea. Ancient mining sites in Wadi Hammamat, along the trade route from Thebes (modern-day Luxor) to the Red Sea port of Al-Quseir, are accurately located on a papyrus map drawn by a scribe of Ramses IV during a quarrying expedition in approximately BC (Refs. 7 and 8 ) and now on display in the Museo Egizio, Turin, Italy.

The adjective &#;thin&#; in the term &#;thin films&#; is ambiguous and poorly defined. It is used to describe, depending on the application, &#;coating&#; layers ranging in thickness from less than a single atomic layer (a partial monolayer) to films that are a significant fraction of a millimeter thick. The earliest documented purposefully made inorganic thin films were gold layers produced chemo-mechanically, for decorative (and later, optical) applications, by the Egyptians during the middle bronze age, more than years ago. 1,2 Gold films, with thicknesses < Å (&#; atoms), have been found in ancient tombs, including the Pyramid of Djoser (actual name, Netjerykhet, second King of the Third Dynasty, Old Kingdom; ruled from &#; to BC) 3 in Saqqara, 1,3&#;5 southwest of Cairo, Egypt. The films were often gilded on copper and bronze statues, jewelry, and religious artifacts using mercury-based, compositionally graded, interfacial adhesion layers as discussed in Ref. 1 .

(Color online) (Left panel) Copper interconnect metallization in a transistor, courtesy of IBM, . The bus bar (metallic frame) in the lower part is &#;20 μm wide, about 1/5 the size of a human hair (average diameter, &#;100 μm). A colorized scanning electron micrograph view after removal of insulating layers by chemical etching is shown. Figure courtesy of International Business Machines Corporation, International Business Machines Corporation; http://www-03.ibm.com/ibm/history/ibm100/us/en/icons/copperchip/ . (Middle panel) Coated architectural glass in office buildings. Figure courtesy of Hy-Power Coatings Ltd. (Nano) Brampton, ON, Canada. (Right panel) TiN, TiAlN, and TiB 2 (left to right) coated tools. Figure courtesy of KYOCERA SGS Precision Tools, Inc., Munroe Falls, OH.

The use of thin films to enhance the physical and chemical properties of materials is ubiquitous in today's world. Examples are shown in Fig. 1 : copper metallization layers for electronic communication among billions of transistors in a silicon integrated-circuit; coated architectural glass in office buildings for which the thin films are designed to enhance energy efficiency and comfort by, depending on the time of year and latitude, reflecting ultraviolet and infrared sunlight, while transmitting visible light, to minimize air conditioning usage, or reflecting infrared radiation from within offices to minimize heating; and coated cutting tools developed to reduce friction and wear during use and, hence, increase tool lifetimes. Other common examples include magnetic thin films for electronic data storage; transparent conductive oxide and absorber layers in solar cells; thin film resistors and dielectrics; catalytic layers for toxic-gas sensing; superconducting thin films for high-frequency devices, data storage, and magnetic circuitry; corrosion-, friction-, and wear-protective layers on automotive and airplane engine parts (spark-plug electrodes, pistons, cylinders, turbine blades, etc.); and multiple layers on eyeglasses to correct vision, minimize ultraviolet light transmission, and provide scratch resistance.

Kenneth Kingdon and Irving Langmuir (American surface chemist, Nobel Laureate in Chemistry for his research in surface science), from the General Electric Research Laboratory, dropped the &#;l&#; in favor of the word sputtering in their Physical Review paper on &#;The removal of thorium from the surface of a thoriated-tungsten (light bulb) filament by positive ion bombardment.&#; 47 Nevertheless, in the same year, in an article on the sputtering of tungsten published in the Philosophical Magazine by the &#;Research Staff of the General Electric Company and communicated by the Laboratory Director,&#; the term &#;cathode disintegration&#; was used in place of sputtering. 48 With time, however, the term sputtering prevailed and is now universal.

Historical footnote: While sputtering was in use for the deposition of thin films by the mid-s, the etymology of the word sputtering remains unclear. The term &#;spluttering,&#; an intensified form of the English word sputtering, meaning &#;to spit with explosive sounds&#; (a cognate for the Dutch word &#;sputteren&#;), 41 may have been used as early as the late s. 42 In a book chapter, Gottfried (Fred) Wehner (German-born American physicist, &#;) and Gerald Anderson 43 noted that a search of the literature revealed that Joseph John (J. J.) Thomson (English physicist, &#;; Nobel Prize in Physics, ) still used the term spluttering in : &#;A well-known instance of this is the spluttering of the cathode in a vacuum tube;&#;&#; 44 The quote implies that the term was in use even earlier. Don Mattox pointed out 45 that the third edition () of the Shorter Oxford Dictionary 46 lists, in addition to sputter (verb, ) and sputter (noun, ), the English term splutter meaning &#;to spit out a spray of particles in noisy bursts.&#;

(Color online) Schematic illustration of the essential features of a basic dc-diode sputtering system. Argon gas flows through a controlled leak valve into an evacuated deposition chamber; some Ar atoms are ionized by a dc potential applied between the copper target and an electrically conducting substrate on a metal substrate table. Ar + ions are accelerated toward the target in order to sputter-eject copper atoms, which are deposited on the substrate, as well as the chamber walls, to form a Cu film.

Sputter deposition, in its simplest configuration (Fig. 5 ), is carried out in an evacuated chamber which is backfilled with a low pressure of a rare gas such as argon. Argon is often the rare gas of choice for sputtering, primarily due to two reasons: (1) argon comprises approximately 1% of the earth's atmosphere and, hence, is relatively inexpensive and (2) the mass of argon (39.95 amu) is a reasonable match, resulting in significant collisional momentum transfer, to a wide range of metals in the middle of the periodic table (see discussion in Sec. IV H ). A dc voltage is then applied between a metal target (the source of the film atoms) such as Cu and an electrically conducting substrate upon which the film is deposited. The voltage breaks down the gas to form a glow discharge consisting of Ar + (with a much smaller fraction of Ar 2+ ) ions and electrons. The positively charged ions are accelerated to bombard the target and, via momentum transfer, sputter-eject target atoms, some of which are deposited on the substrate. A reactive gas can also be added in order to form compound films (see Sec. IV J ). Important commercial examples are the reactive sputtering of titanium and titanium-aluminum alloys in Ar/N 2 mixtures to deposit hard ceramic TiN and TiAlN coatings 38&#;40 on cutting tools, drill bits [see Fig. 1 (right panel)], gear hobs, and dies.

Historical footnote: The term &#;PVD&#; appears to have been coined by John M. Blocher, from Battelle Columbus (OH) Laboratory, while chairing a session on Vapor Deposition at the Electrochemical Society meeting in Houston, TX. Blocher sought to distinguish deposition techniques by employing chemical reactions (he was also the first to use the term &#;chemical vapor deposition&#;) from processes such as sputtering and evaporation. 36 The expression physical vapor deposition first appeared in print in a book entitled Vapor Deposition, edited by Powell et al., published in . 37

The physical-vapor deposition (PVD) techniques of sputtering and evaporation were developed in the middle to late s following the evolution of vacuum and electrical-power technologies beginning in the mid-s. 1 The first publication focused on sputter deposition of thin films was in , 25 as discussed in Sec. IV A , while the first documented evaporation experiments were carried out in the early s by Josef Stefan (&#;), 26&#;28 an Austrian physicist best known for his research in thermal conductivity of gases and blackbody radiation of solids, 29 and the early s by Heinrich Hertz (&#;), 30,31 a German physicist well known for his work on electromagnetics 32 and contact mechanics. 33 Reliable early measurements of the vapor pressures of solids (sublimation) and liquids (evaporation) were made by Martin Knudsen (&#;, Danish physicist) using what is now termed a Knudsen cell, 34,35 an isothermal enclosure with a very small orifice.

This is an example of a film-growth methodology which today is termed chemical vapor deposition. 1,23 As discussed in a review article by Rolsten, 24 carbon reduction of oxides was an important method for obtaining relatively pure metals, in order to investigate their fundamental physical properties, during the s and the early s.

The earliest reported purposeful growth of metal films from the vapor phase was in when Johann Schroeder, a German pharmacist, described a method for reducing arsenic oxide [As 2 O 3 ] with charcoal 21,22 through the overall endothermic reaction

The first thin films grown from the vapor phase, as discussed in Ref. 1 , were likely metal layers deposited accidently on the ceramic pots and rocks surrounding hot charcoal fires used to reduce metal ores, a process which can be traced back more than years. 15 Based upon both archeological evidence and metallurgical analyses, copper smelting (extraction from ore) and metal working originated independently in the Balkans (Serbia and Bulgaria) by &#; BC and in Anatolia by at least BC. 16&#;19 The Roman philosopher Pliny the Elder (23&#;79 AD) discussed this process in his 37-book Naturalis Historia, the first encyclopedia, published in &#;79 AD. 20

The rapid development of sputter deposition in the middle to late s required the evolution of vacuum technology, beginning in the s, and the invention of dc power supplies (batteries) in the late s and the early s. For a more detailed discussion of early vacuum technology and power supplies, see Ref. 1 .

The earliest actuated mercury pump was developed by Swedenborg (&#;), Swedish scientist/theologian, as described in his book Miscellanea. 74 The pump consists of a metal funnel attached to a plate which holds a glass bell jar to be evacuated. The lower end of the funnel is attached to a leather tube with a metal lever. The funnel and leather tube are filled with mercury, and the lever is used to compress the mercury and force air out of the bell jar through a set of inward and outward opening valves. Basically, the solid piston of von Guericke's mechanical pump was replaced by a mercury column. Over the next 155 years, a large variety of mercury pumps were reported, as reviewed in detail in a wonderful paper, with more than 130 references, by Thompson in . 56 In addition to the important Sprengel momentum-transfer pump described above, a type of mercury-based vacuum-siphon pump, with a three-way stopcock, was developed by Heinrich Geissler (&#;), a German glassblower, in , which could achieve a vacuum of &#;100 mTorr. 75 The first public mention of the pump was in a pamphlet published in by Mayer 76 (also see Refs. 56 and 77 ).

The first mercury &#;pump&#; is attributed to Evangelista Torricelli (&#;), an Italian physicist and mathematician who, in , 72 invented the barometer to measure atmospheric pressure. 73 (The modern pressure unit Torr is in honor of Torricelli.) His initial experiments were carried out with an &#;100-cm-long glass tube, open at one end, filled with liquid mercury, and tightly closed with a fingertip. The tube was then inverted and partially immersed in a mercury reservoir, and the fingertip was removed from the tube opening. Some of the mercury flowed out of the tube leaving space at the top such that the height of the liquid column corresponded to the ambient atmospheric pressure. The empty volume at the top of the barometer was &#;Torricelli's void;&#; he had produced vacuum!

Historical footnote: Egyptian frescos clearly illustrate that siphon pumps were used to decant liquids, including wine, 63 from large earthen storage jars, by &#; BC (Ref. 64 ) (and probably much earlier). Hero of Alexandria (&#;10&#;70 AD), a Greek mathematician and engineer, wrote extensively about siphon pumps in his famous essay Pneumatica, 65 in which he borrowed heavily from earlier treatises by Philo of Byzantium (&#;280&#;220 BC), a Greek engineer who spent most of his life in Alexandria, 66 and, especially, Ctesibius of Alexandria (285&#;222 BC), 67,68 a Greek inventor and mathematician who is considered the father of pneumatics. Marcus Vitruvius Pollio (&#;75&#;15 BC), commonly known as Vitruvius, a Roman architect and engineer, reported in his 10-volume De Architectura 69 on Greek and Roman architecture, technology, and natural sciences that Ctesibius wrote a book in which he described the invention of, among many other things, an air pump, with valves, connected to a keyboard and rows of pipes (a water organ, in which water is the actuator) 70 and a force pump for water (the up-stroke of a piston draws water, through a valve, into the cylinder; on the down-stroke, the water is discharged through a valve into an outlet pipe). 71 Unfortunately, the original writings of Ctesibius were lost.

Improvements in vacuum technology required better gauging in order to measure the increasingly lower pressures produced. In , Herbert McLeod (&#;), a British chemist, developed what today is termed the McLeod mercury gauge, 58,59 which operates based upon Boyle's law. Boyle (&#;), another British chemist, showed in that for a closed system at constant temperature, the product of pressure P and volume V remains constant. 60 In operation, the gauge compresses a known volume V 1 of gas at the unknown system pressure P 1 to a much smaller known volume V 2 in a mercury manometer with which the pressure P 2 is measured. 61,62 Thus, by Boyle's law, the system pressure P 1 is given by the expression P 2 V 2 /V 1 . Liquid mercury wets glass and thus forms the required glass/metal seals in the gauge.

In operation, mercury was added to funnel A and the stopcock at C opened, allowing mercury droplets to fall, trap air, and reduce the pressure in chamber R. Air and mercury were exhausted through the spout of bulb B. The mercury collected in basin H was poured back into funnel A for continuous pumping. The second version of the Sprengel pump, described in the same paper, 53 is shown in Fig. 7(b) . It was approximately 1.8 m tall, and Sprengel reported using 4.5&#;6.8 kg of mercury during operation. The pump contained a mercury pressure gauge (similar to the one described below) attached to the evacuated chamber and a mechanical-piston backing pump S. Later versions incorporated continuous mercury recycling. With the combination of the mechanical and mercury pumps, a 0.5 l chamber could be evacuated in &#;20 min. The importance of Sprengel's work was recognized by the Royal Society of London which elected him as a Fellow in . A later version of the pump, presently housed in the Dr. Guislain Museum, Ghent, Belgium, is shown in Fig. 7(c) .

(Color online) Drawings of (a) a prototype and (b) an initial version of Sprengel's mercury transfer pump. Reproduced with permission from Sprengel, J. Chem. Soc. 18 , 9 (). Copyright by Royal Society of Chemistry. (c) A later version of the pump, presently housed in the Dr. Guislain Museum, Ghent, Belgium. Photograph courtesy of Luca Borghi for Himetop, The History of Medicine Topographical Database.

An initial prototype of the Sprengel mercury pump is shown in Fig. 7(a) . 53 Droplets of mercury (a heavy metal which is liquid at room temperature), falling through a small-diameter (2.50&#;2.75 mm) glass tube, trap and compress air by momentum transfer. The tube, labeled CD in Fig. 7(a) , was &#;76 cm long and extended from funnel A to enter glass bulb B through a vulcanized-rubber stopper. The bulb has a spout several mm above the lower end of tube CD.

Much better vacuum was required in order for scientists in the s to obtain longer gas-phase mean-free paths, higher deposition rates, and increased purity in films grown from the vapor-phase. This was solved by a German chemist, Herman Sprengel (&#;), who developed a practical mercury momentum-transfer pump in . 53 The pump is related to a trombe in which water falls from an upper reservoir, while trapping air, into a pipe and deposits the air in a lower reservoir at higher pressure (a type of air compressor), 54,55 which had been known for &#;some hundreds of years.&#; 56 The base pressure claimed by Sprengel in his initial publication was &#;6 × 10 &#;4 Torr and limited by leaks in vulcanized rubber joints connecting the glass tubes (the rubber connectors were cemented to the glass tubes, and the joints were bound with copper wire). While lower pressures were achieved with later versions of the pump, 57 pressures of 10 &#;3 &#;10 &#;4 Torr were sufficient to provide ballistic environments (i.e., gas-atom mean-free paths of the order of, or larger than, system dimensions) for investigating gas discharges and sputter deposition in the small evacuated chambers of that era.

Progress in vacuum technology (the word vacuum is derived from the Latin vacuus, meaning empty space) was essential for providing cleaner deposition environments necessary for the advancement of thin-film science. In , Otto von Guericke (&#;) of Magdeburg, Germany, a scientist, inventor, and politician, developed a mechanical piston pump that achieved a vacuum of 2 Torr (&#;0.003 atm). 49,50 (For comparison, a typical household vacuum cleaner produces enough suction to reduce standard atmospheric pressure, 760 Torr, to &#;610 Torr). 51 von Guericke's third-generation vacuum system, a model of which is shown in Fig. 6 , 52 consisted of a bell jar separated from the piston pump by a cylinder with a stopcock. The pump was equipped with two valves near the entrance to the nozzle at the bottom of the bell jar; the first valve was located between the nozzle and the cylinder and the second valve between the cylinder and the atmosphere. During the piston down-stroke, valve one is closed to stop air from entering the nozzle and the bell jar, while valve two is forced to open by the air displaced from the cylinder. During the piston return stroke, valve two is closed and valve one is forced to open by the pressure of the remaining air in the bell jar and the nozzle. The percentage of pressure decrease per complete piston stroke diminishes continuously as the bell-jar pressure is reduced toward the base pressure.

A practical problem with voltaic piles, especially with larger ones used to obtain higher voltages, is that the weight of the disks squeezes the electrolyte out of the cloths. In , William Cruickshank (&#;), a surgeon and Professor of Chemistry at the Royal Military Academy, Woolwich (southeast London), solved this problem and designed the first electric battery for mass production. 104 In the initial version, Cruickshank arranged 60 pairs of equal-sized zinc and silver sheets cemented together with rosin and beeswax in a long resin-insulated rectangular wooden box such that all zinc sheets faced one direction and all silver sheets the other. Grooves in the box held the metal plates in position, and the sealed box was filled with an electrolyte of brine or dilute ammonium chloride (NH 4 Cl), which has higher conductivity.

Historical footnote: Volta not only introduced the term electromotive series, but was the first to use the term &#;semiconductor&#; in describing &#;materials of semiconducting nature&#; in a paper published in the Philosophical Transactions of the Royal Society 103 and presented at a Royal Society of London meeting on March 14 of that year.

(Color online) (a) Schematic illustration of a voltaic pile. (b) Photograph, attributed to GuidoB and licensed under the Creative Commons Attribution-Share Alike 3.0 Unported, of a single voltaic pile. The battery is on display at the Tempio Voltiano Museum, Como, Italy. (c) Sketch of a double voltaic pile consisting of two sets of eight pairs of silver and zinc plates. Reproduced with permission from Volta, Philos. Mag. 7 , 289 (). Copyright by Taylor and Francis Publishing.

Volta's interest in electrochemistry led him to discover that voltage can be obtained from stacks consisting of several pairs of different metal disks, each pair separated by an electrolyte (initially pieces of cloth saturated in brine), connected in series to form a &#;voltaic pile.&#; 101 The first metals used were copper and zinc, but Volta found, based upon electrometer measurements, that silver and zinc produce a larger electromotive force, a term Volta introduced in . 102 Figures 8(a) and 8(b) show an illustration and a photograph, respectively, of an early voltaic pile. Such devices could only provide a few volts; obtaining larger potentials required a series (i.e., a battery) of many large voltaic piles. An example of a small double voltaic pile is shown in Fig. 8(c) . 100

The invention of the electrochemical battery to provide low-voltage dc power is generally attributed to Count Alessandro Volta (&#;), 1 Professor of Natural Philosophy at the University of Pavia, Italy, based upon his work in the s resulting in a classic paper published first in French 99 and then in English 100 in .

Gentlemen's Magazine reported the following in . 98 &#;Could one believe that a lady's finger, that her whalebone petticoat, should send forth flashes of true lightening, and that such charming lips could set on fire a house? The ladies were sensible of this new privilege of kindling fires without any poetical figure, or hyperbole, and resorted from all parts to the public lectures of natural philosophy, which by that means became brilliant assemblies.&#;

Historical footnote: Georg Bose, one of the first to work on charge-storage devices, was a scientific stuntman who became famous for public demonstrations. He was known, for example, to produce flames by lighting alcohol floating on the surface of water via a spark generated by his friction machine. However, his most famous stunt was the &#;Electric Kiss.&#; 97 An attractive young woman in the audience was invited to stand on a block of an insulating material (Bose was, by all accounts, a charming and persuasive fellow), and she was given a moderate static charge from an electrostatic generator. Gentlemen in the audience would then be invited to kiss her, but, as they tried to approach her lips, a strong spark would discourage the attempt, while greatly amusing both the young woman and the rest of the audience.

Daniel Gralath (&#;), physicist (founder of the Danzig Research Society) and Mayor of Danzig, Poland, repeated the Leyden jar experiments and was the first to combine several jars, connected in parallel (see Ref. 1 ), to increase the total stored charge. 94 The term &#;battery&#; was reputedly coined by Benjamin Franklin, 95,96 who likened the group of jars to a battery of cannon.

Benjamin Franklin (American scientist and inventor, &#;) was the first to understand that charge is stored in the glass dielectric, not the electrodes. He realized that the water merely served as an electrode and was not essential. To prove it, he produced flat capacitors consisting of a sheet of glass between metal-foil electrodes. 90 Franklin discussed his findings in a letter dated . 91 The Leyden jar was also used by Franklin in his famous kite experiment in a thunderstorm to &#;capture lightning in a bottle.&#; 92 (Note, however, that it is disputed whether the experiment was actually performed.) 93

The earliest devices were hand-held glass jars partly filled with water (the inner conductor) in contact with a nail inserted through a cork stopper in the top of the jar. Both Kleist and van Musschenbroek reported receiving significant shocks when they touched both the nail and the outside of the jar! Eventually, van Musschenbroek realized that adding a conductor (other than his own body) such as a metal foil to the outside of the jar was far more practical.

In order to store charge in Leyden jars, the glass cylinder of an electrostatic generator was rotated, via a hand crank, against a leather (or wool) strip pressing on the glass. The friction resulted in positive charge accumulating on the leather and negative charge (electrons) on the glass. The electrons were collected by an insulated (perhaps comb-shaped) metal electrode. When sufficient charge accumulated, a spark jumped from the generator collector to the central collector electrode of a nearby Leyden jar, where the charge was stored. Originally, the capacitance of the device was measured in units of the number of &#;jars&#; of a given size or by the total area covered with metal. A typical 0.5-l Leyden jar had a maximum capacitance of approximately 1 nF. 89

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In , the Dutch scientist Pieter van Musschenbroek (&#;) of Leiden University [mathematics, philosophy, medicine, and astrology (the latter is closer to theology than science!)] and Ewald Georg von Kleist (&#;), a German lawyer, cleric (Bishop of Pomerania, Prussia), and physicist, are credited with independently inventing what today is known as the Leiden jar (Leyden jar), 84,85 an early form of the modern capacitor, to provide pulsed power. Both Kleist and van Musschenbroek studied at Leiden University, and it is likely that Kleist developed his interest in electricity from lecture demonstrations in the Physics Department. However, it appears that van Musschenbroek obtained the idea for his research from Andreas Cunaeus (&#;), a lawyer who often visited van Musschenbroek's laboratory and had learned Kleist's experiments. 86 Cunaeus carried out the initial experiments that led to the Leyden jar while attempting to reproduce even earlier results by Andreas Gordon (&#;), a Professor at Erfurt, Germany, and Georg Mattias Bose (&#;) at the University of Wittenberg, Germany. 87 The device accumulates static electricity between electrodes on the inside and outside of a glass jar. The first mention in the literature of the Leyden jar experiments was by Trembley in February, (there is some controversy regarding the exact publication date). 88

Historical footnote: The concept of static electricity was known by the ancient Greeks (e.g., rubbing amber on wool) and first recorded by Thales of Miletus (624&#;546 BC), 80 a pre-Socratic Greek philosopher, mathematician, and one of the Seven Sages of Greece. 81&#;83

In addition to vacuum, electrical power was necessary for initiating early experiments in thin-film sputter deposition. von Guericke also played an important role in this field through his development in of a crude friction-based electrostatic generator which transformed mechanical work into electrical energy. 78,79 The generator was based on the triboelectric effect (although the term did not exist at the time), in which a material becomes electrically charged (&#;static electricity&#;) through friction.

From the mid- to late-s, several papers were published on the use of optical emission to investigate the transition between a continuous glow discharge and an arc, the electrical structure and configuration of dc glow discharges, and the nature of metal atoms sputter ejected from the cathode (target). Michael Faraday (&#;), an English scientist, reported on the electrical and optical characterization of glow discharges in .105 His glass discharge tube contained brass electrodes and was operated in air, nitrogen, oxygen, hydrogen, and other gases at a pressure of 4.4 inches of mercury (&#;112 Torr). While Faraday must certainly have deposited films during these experiments, this was not the objective. His later work on the optical properties of vacuum-arc-deposited thin metal films was discussed in his fifth Bakerian Lecture106 in and published in the same year.107

Historical footnote: The Bakerian Medal and Lecture of the Royal Society of London was established by a gift from Henry Baker (&#;) in . It is awarded annually to a person (one per year) working in the fields of &#;natural history or experimental philosophy&#; (i.e., the physical sciences). Baker was described by Turner, Senior Assistant Curator of the Museum of Science in Oxford:108 &#;Henry Baker was a typical polymath in the eighteenth-century manner. Although he did not contribute to scientific research in any significant way, he did make valuable contributions to the dissemination of scientific knowledge, particularly in the field of microscopy, an enthusiastic participator in the scientific and literary life of London.&#;

Historical footnote: Michael Faraday, known primarily for his research in electromagnetics and electro-chemistry, is considered by science historians to be one of the most influential scientists and the best experimentalist, in history,109,110 even though he had no formal education past grade school. Faraday, during the years between and , was invited to present five Bakerian Lectures to the Royal Society.

Heinrich Geissler, in , used his mercury pump (see the Historical Footnote in Sec. III&#;A) to evacuate small glass enclosures and develop the &#;Geissler tube&#; to study the optical and electrical properties of glow discharges in rare gasses, air, mercury, etc.75 He reported observing a wide variety of discharge colors due to optical emission resulting from the decay of excited gas atoms. This gave rise to the production, beginning in the s, of the first gas-discharge lamps which were sold primarily as novelty and artistic items.111 Julius Plücker,112 also at the University of Bonn, and his ex-graduate student Johan Wilhelm Hittorf113 used Geissler tubes to study gaseous electronic effects resulting from ion bombardment of metal targets (see Sec. IV&#;D).

In , William Robert Grove (&#;), a Welsh lawyer (later, judge) and physicist, published the earliest recorded description of sputter deposition and ion etching experiments.25 A sketch of his equipment is shown in Fig. 9. Vacuum was achieved with a mechanical piston pump, similar to that developed by von Guericke as described in Sec. III&#;A, with power supplied by Grove's version of a trough-style dc voltaic pile (following Cruickshank, Sec. III&#;B)104 combined with an &#;induction coil&#; step-up transformer supplied by Heinrich Ruhmkorff (&#;),114 a famous German instrument maker living at the time in Paris. The electrodes consisted of a copper plate, with a polished electroplated silver surface, and a rod, which passed through a leather stopper in the top of the glass vacuum chamber, with a steel needle attached to its end. The gas used to sustain the discharge was stored in a bladder.

. 9.

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System used by William Grove to investigate target &#;disintegration&#; (sputtering) in a gas discharge. Reproduced with permission from Grove, Philos. Trans. R. Soc. 142, 87 (). Copyright by Taylor and Francis Publishers. See text for details.

. 9.

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Historical footnote: Based upon a passing comment in Grove's later papers, the small vessel attached to the rod electrode shown in Fig. 9 contained &#;potassa fusa&#; [the ancient name for potassium hydroxide (KOH), a caustic deliquescent desiccant which can capture large quantities of water]. This was an early adsorption (i.e., getter) pump.

The experiments were carried out at rather high pressures, ranging from &#;100 to 500 mTorr, with the steel needle target quite close to the silver-plated substrate (generally a separation of 0.25&#;cm, &#;but this may be considerably varied&#;). When using a mixture of hydrogen and air with the silver plate positive and the steel needle serving as the (negative) cathode, Grove observed thin-film deposition on the silver substrate. The layer was primarily iron oxide, i.e., reactive sputtering (although the term did not come into existence until more than a century later, Sec. IV&#;J). The color of the oxide film &#;presented in succession yellow, orange, and blue tints&#; with increasing thickness (longer deposition time). Grove reported optical interference effects (as he noted later in the paper), which for a given substrate/film combination can be calibrated to provide film thickness versus color as is commonly done today for SiO2 and Si3N4 dielectric layers on Si(001) wafers used in microelectronic device fabrication.

When Grove switched polarity such that the silver plate became the cathode (negative), he reported that the iron oxide film was removed by ion etching. Continuing the experiment, a polished region &#;occasioned by molecular disintegration&#; remained. Thus, Grove had not only removed the original oxide film, but also sputter-etched into the silver substrate layer. The word &#;sputter&#; did not yet exist (as discussed in Sec. II), and Grove described the process throughout the paper as &#;molecular disintegration.&#;

Grove repeated the above experiment by sputtering the steel target in an &#;air vacuum&#; (a term Grove attributed to Faraday) to produce a more fully oxidized film on the silver-plated copper substrate and then switched gases and electrode polarity to &#;sputter clean&#; the silver plate in a nitrogen discharge. In actuality, there must have been a thin silver-oxide layer remaining due to the competition between the rates of silver oxidation from the discharge, arising from the relatively poor vacuum, and sputter etching. However, this layer, a few tens of Å in thickness,115 would have been too thin for Grove to observe. Several more experiments in which he substituted different metals for the target needle and changed discharge gases were also reported. The results were similar, but he described observing differences depending upon the atomic masses of the target material and the gas, the ionization potential of the gas, and the oxidation tendency of the metals.

Interestingly, Grove realized that oxygen can form negative ions which are accelerated by the applied target voltage toward the substrate (anode). The significance of the fact that oxygen has a high electron-attachment probability116 was not fully appreciated until almost 130 years later. Researchers investigating the growth of piezoelectric and transparent conducting oxides (TCOs) in the early s117&#;119 and high-temperature oxide superconductors in the late s,120 all of which are typically deposited today by reactive magnetron sputtering (see Secs. IV&#;F and IV&#;J), were confronted with the deleterious effects of O&#; and O 2 &#; irradiation.

Negative ions, accelerated by the same potential used to produce sputtering by positive ions incident at the target, bombard the growing film with energies which can produce residual defects, change the preferred orientation of polycrystalline layers, degrade film properties, and decrease deposition rates by resputtering.115,117&#;121 Irradiation of the growing film by fast neutral O and O2 species can also occur, as attached electrons are stripped from the corresponding ions in the plasma.118,120,122 Early solutions involved increasing the discharge pressure in order to decrease particle mean-free paths and hence lower the average energies of ions and fast atoms via collisions,121 while later solutions focused on off-axis deposition and facing-target sputtering (Sec. IV&#;F&#;1).119,123&#;127

A few years after Grove's seminal paper, John Gassiot (&#;), a highly successful English businessman and gentleman scientist, presented his Bakerian Lecture (March 4, ) &#;On the stratifications and dark bands in electrical discharges as observed in Torricellian vacuums,&#; which was published in the Proceedings of the Royal Society of London128 (also see Ref. 129). Most of the lecture was concerned with his observations of alternating bright and dark bands formed in rarefied-air discharges contained in glass tubes partially filled with clean boiled mercury and evacuated with a mechanical pump [as air is extracted, the mercury level sinks in the tube and a &#;Torricelli vacuum&#; is formed (see the Historical footnote, Sec. III&#;A)].1,72,73 During these experiments, Gassiot noted that the luminous discharge regions move under the influence of a magnetic field. However, he conceded in the conclusion of Ref. 128 that &#;I refrain for the present from any observations as to the action of the magnet on the discharge.&#;

Gassiot also reported that for discharges formed between two platinum wire electrodes hermetically sealed about 4 inches (&#;10&#;cm) apart in a discharge tube: &#;a black deposit takes place on the sides of the tube nearest the negative terminal. This deposit is platinum (analyzed by Michael Faraday)129 in a state of minute division emanating from the wire, which becomes black and rough as if corroded. The minute particles of platinum are deposited in a lateral direction from the negative wire, and consequently in a different manner from what is described as occurring in the voltaic arc.&#;128 &#;The platinum coating is deposited on the portion of the tube surrounding the negative wire, but none at or near the positive.&#;129 Gassiot described sputter deposition from the platinum target. He noted that &#;&#;when this deposit is examined by transmitted light, it is translucent, presenting to the eye an extremely thin bluish-black film; but by reflected light, either on the outside or inside (i.e., viewed either from the glass or the film side), it has the appearance of highly polished silver, reflecting the light as from the finest mirror.&#;129

Historical footnote: Gassiot, in addition to being a successful businessman, was a very enthusiastic amateur scientist interested in electricity. He maintained a well-equipped laboratory and library in his home where young James Maxwell (&#;), a Scottish mathematical physicist who developed electromagnetic theory and presented a unified model of electricity, magnetism, and light,130 did much of his own scientific work during the s. Gassiot was a founder of the London Electrical Society in and the Chemical Society in and was elected as a Fellow of the Royal Society in . He was also a close associate of Grove.

Practical applications of sputter-deposited single and multilayer metal films as mirrors and optical coatings on telescope lenses and eyepieces were discussed in papers published in by Arthur Wright (American physicist, &#;, Yale University).131,132 The first of the two articles reported the growth of adherent noble-metal films sputter-deposited from wire targets onto glass microscope slides.131 Unfortunately, the films had large lateral thickness variations since the target was the tip of a wire whose length was encased in a glass tube. However, an ingenious solution was presented in Wright's next paper.132 He designed a deposition system, evacuated with a Sprengel mercury pump (see Sec. III&#;A), in which the substrate was mounted on a pendulum to provide motion in two orthogonal directions with respect to the target such that films of uniform thickness could be &#;painted&#; onto the substrate (Fig. 10). In Wright's words: &#;The perfect control of the process obtained by the use of the movable electrode will even make it possible to apply the method of local correction for the improvement of a defective figure, or to parabolize a spherical mirror by depositing the metal in a layer increasing in thickness toward the center.&#;

Historical footnote: Wright was a member of the first Ph.D. graduating class in the United States. The class consisted of three scholars at Yale University. Wright's doctoral dissertation was on satellite mechanics.133,134 As an undergraduate and graduate student, he studied mathematics, mineralogy, botany, and modern languages, in addition to physics. He also studied law and was admitted to the bar. Following teaching positions and postdoctoral research programs at Heidelberg and Berlin, Wright became Professor of Molecular Physics and Chemistry at Yale University in and later Professor of Experimental Physics. His sputter-deposited thin films were used extensively in the first studies of polarized light emitted from the solar corona (plasma surrounding the sun, most easily observed during a solar eclipse). Wright became a member of the U.S. National Academy of Sciences and a Fellow of the UK Royal Astronomical Society.

Wright characterized the sputtering process spectroscopically using optical emission from gas, and ejected target atoms, which were excited in the discharge (following earlier work by Faraday).105 As-deposited platinum films, some with thicknesses <350&#;Å (estimated using a combination of weight change, to within 10&#;μg, for thicker films, deposition rate calibrations, and optical interference rings for thinner layers), were analyzed using optical transmission as a function of wavelength. Mirror-like sputter-deposited films were found to be more adherent than solution-grown layers and less sensitive to local delamination caused by water penetration to the film/glass interface. By the late s, sputter-deposition was routinely used in manufacturing commercial mirrors.

Wright described his films as &#;&#;surfaces of exquisite perfection and the most brilliant polish. They can only be compared to the surface of clean liquid mercury, far surpassing in luster anything that can be obtained by the ordinary methods of polishing.&#; Wright tuned the reflectivity of his mirrors based upon interference effects to obtain brilliant &#;white light&#; by depositing multilayer films with predetermined layer thicknesses.

In the late s, William Crookes (&#;), a British chemist, developed what are now referred to as Crookes discharge tubes [Fig. 11(a)]136 based on the earlier Geissler tubes.111 Crookes also took advantage of the Sprengel mercury pump to obtain better vacuum, and hence longer mean-free paths, in his tubes which he used to promote research on gas-discharge electronics. Crookes tubes were instrumental in the discoveries of x-rays137 (Wilhelm Röntgen, , Nobel Prize in Physics, ), electrons138 (J. J. Thomson, , Nobel Prize in Physics, ), and thermionic emission139,140 (Owen Richardson, , Nobel Prize in Physics, ), which enabled vacuum-tube electronics.141,142

. 11.

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(Color online) Reproduction of a Crookes tube, containing a Maltese cross between the cathode and the far end of the glass envelope, used to investigate the electronic characteristics of, and the optical emission from, glow discharges. (Left panel) No power applied to the tube. (Middle panel) Power applied to the cathode gives rise to green fluorescence emanating from the glass behind the Maltese cross which casts a shadow by blocking &#;cathode rays (radiant matter)&#; from the target. (Right panel) A magnet was used to rotate the shadow image. Photographs are attributed to Zátonyi Sándor. Licensed under a Creative Commons Attribution-Share Alike 3.0 Unported license. The labels were added by the present author.

. 11.

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Crookes published a very significant paper on sputter deposition of thin films in .143 During a long series of experiments, he employed a Sprengel-type mercury pump53 to evacuate his discharge tube to pressures of the order of 7&#;×&#;10&#;4&#;Torr. He then used the residual air to sputter Ag, Al, Au, Cd, Cu, Fe, Ir, Mg, Ni, Pb, Pd, Pt, and Sn targets as well as AlAu and CuZn (brass) alloys and measure the erosion rates via target weight loss.

Although the experimental details (sputtering pressure, voltage, and ion current densities) were not well specified, Crookes, in order to obtain comparable results, designed a multitarget sputtering system with indexed motorized external electrical contacts as illustrated in Fig. 12.143 Every experiment was carried out using four wire targets, 0.8&#;mm in diameter and 20&#;mm in length, in which one of the four was always a gold reference electrode. Power was alternately applied to each target in succession, using a revolving commutator, for the same length of time (typically 6&#;s) over periods of several hours. By this means, variations in current and sputtering pressure were accounted for in order to obtain a set of metal sputtering rates, all referenced to that of gold. Aluminum and magnesium targets were reported to be &#;practically nonvolatile.&#; Today, we know that this was due to the formation of strongly-bonded oxynitride (primarily oxide) dielectric layers at the target surfaces due to the use of air (approximately 78% nitrogen and 21% oxygen by volume) as the sputtering gas.

. 12.

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Four-target sputtering system used by Crookes to measure the sputtering rates of different metals. The targets were 0.8-mm-diameter metal wires. Reproduced with permission from Crookes, Proc. R. Soc. London 50, 88 (). Copyright by Royal Society.

. 12.

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The &#;rich purple color&#; of the aluminum-gold alloy target turned to the &#;dull white color of aluminum&#; as gold was preferentially sputter removed. This was the first mention in the literature of preferential sputtering from alloy targets. While this occurs for all alloys, the target surface composition during sputtering in rare gases rapidly (depending on the ion energy, current density, and relative ion and target-atom masses) reaches a steady-state value as the surface coverage of the low-sputtering-rate component increases to compensate for the difference in elemental removal rates as shown experimentally by Tarng and Wehner144 and theoretically by Eltoukhy and Greene.145 In Crookes' aluminum-gold alloy experiments, oxidation further decreased the aluminum sputtering rate.

Since the targets in Crookes' experiments were uncooled, low-melting-point metals such as tin, cadmium, and lead quickly melted. For these materials, he devised a holder to sputter liquid metals. It is likely that a significant part of their measured weight loss, especially for the high-vapor-pressure element cadmium, was due to evaporation, in addition to sputtering.

In his April 4, Bakerian Lecture,146 Crookes, like Faraday105 and Gassiot128,129 before him, discussed the use of optical spectroscopy to characterize plasmas, in many discharge-tube configurations, at working pressures from &#;0.08&#;mm Hg (80 mTorr) down to a low of 0.&#;mm Hg (0.01 mTorr).&#;147 He often specified the sputtering pressure not in pressure units but as the thickness of the cathode dark space, which came to be known as the Crookes dark space, adjacent to the very bright glowing region in front of the target. He noted that during long sputtering runs, it was necessary to periodically bleed some air into the discharge tube &#;to reduce the vacuum.&#; After examining metal films deposited on the inside of the discharge tube and finding them to be porous with rough surfaces, he concluded that sputtering gas (residual air in these experiments) was &#;occluded&#; in the growing films. While some gas was likely trapped in the growing films, most was captured (adsorbed) via reactions with the fresh metal layers, deposited on surfaces throughout the system, which acted as a getter pump.

Francis Aston (&#;, English chemist and physicist, Nobel Prize in Physics, , for discovery of isotopes in nonradioactive elements) also made important contributions to the modern understanding of glow discharges. After three years working as a chemist in a brewery, Aston joined Birmingham University with a scholarship in to study glow discharges. In , he discovered a very narrow region in gas discharges, now called the Aston dark space,148 immediately adjacent to the target and preceding the bright cathode-glow region. The area is &#;dark&#; (less luminous) since the average energy of electrons emitted from the target due to ion bombardment (see discussion in Sec. IV&#;D) is less than that required for the excitation of gas atoms. However, the electrons are rapidly accelerated by the applied electric field to produce the cathode glow.

Aston carried out his own glass blowing to fabricate gas-discharge tubes with movable aluminum electrodes in order to measure the distance from the target to the end of the &#;Crookes dark space&#; and, hence, provide an estimate of the width of the &#;cathode fall&#; over which ions are accelerated to the target. He showed, for a wide range of gases, that the cathode fall distance d is given by

d = A P + B J T 0.5 ,

&#;

(1)

in which P is the gas pressure, JT is the target current density, and A and B are functions of the gas and target material.149&#;151 This empirical relationship is now referred to as the Aston equation. Aston was appointed lecturer at the University of Birmingham in but moved to the Cavendish Laboratory in Cambridge in , on the invitation of Thomson, to continue working on gaseous electronics.

In , Thomas Edison (&#;), an American inventor and businessman, patented a very early forerunner to copper contact technology in modern microelectronic device fabrication.152,153 Edison's U.S. patent 713,863 (&#;Process of coating phonograph records&#;) describes the use of dc sputtering for the deposition of metal films on wax phonograph masters as &#;seed&#; (and adhesion) layers for electroplated overlayers.154 This follows an earlier Edison patent in which the seed layers were deposited by vacuum-arc deposition.155 Edison claimed in the patent that the arc process was too slow and that sputter-deposited films had much more uniform thickness distributions.

Historical footnote: Thomas Edison, a prolific inventor who was issued 1,093 U.S. patents (phonograph, motion picture camera, sound recording, etc.) and many patents in other countries, is often credited with the invention of the light bulb. While Edison was issued a U.S. patent for an &#;Electric lamp&#; in ,156 he did not &#;invent&#; the light bulb. Rather, he took advantage of the availability of better vacuum due to the development of the mercury momentum-transfer pump53 by Sprengel in (see discussion in Sec. III&#;A) to develop a much longer-lived bulb which was commercially viable. In fact, a year before Edison was born, Grove, who published the earliest recorded description of sputter deposition and ion etching,25 as discussed above, used a platinum-filament electric light to illuminate the lecture theater157 during his first Bakerian Lecture before the Royal Society on November 19, , as he described the use of his improved voltaic dc battery to dissociate water:158,159 &#;On Certain Phenomena of Voltaic Ignition and the Decomposition of Water into its Constituent Gases by Heat.&#; The history of the light bulb is rich and interesting, stretches back to at least , and involves many previous researchers as chronicled in Ref. 160.

Grove's and Crookes' research on sputtering attracted the attention of scientists worldwide. A review paper, entitled &#;Cathode Sputtering, a Commercial Application,&#; published in by Fruth,161 of Western Electric Company (Chicago), lists 113 references published in the field between the time of Grove's pioneering article25 and . Fruth described commercial equipment (Fig. 13, left panel) and procedures for sputter-depositing gold electrodes, from six gold cathodes, onto multiple radio-broadcasting microphone diaphragms. A photograph of the deposition chamber, which contains a rotating McLeod gauge58 and a &#;bleeder&#; valve in order to maintain constant pressure, with diaphragms ready to be coated, is shown in the right panel of Fig. 13. Fruth described the system operation as follows.

. 13.

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(Left panel) Commercial sputter-deposition unit, with six gold targets, for depositing metal electrodes on microphone diaphragms. (Right panel) A closer view of the deposition chamber, showing the diaphragms. Reproduced with permission from Fruth, Physics 2, 280 (). Copyright by American Institue of Phyics; labels were added by the present author.

. 13.

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&#;In order to maintain a constant residual gas pressure, the pump is operated continuously and air is allowed to leak in slowly through the bleeder valve which is located near the pump. This practice was found necessary in order to overcome variations in pressure due to the early evolution of gases and the later cleanup usually accompanying electrical discharges in vacuo. A pressure of 0.100&#;mm (100 mTorr) is readily maintained by this method. After a new charge has been placed in the bell jar, the bleeder valve is temporarily cut off by closing a stopcock so that the required vacuum can be more quickly obtained. By this means, sputtering can be started in about 4&#;min after the bell jar has been placed in position.&#;

Fruth demonstrated that dc sputter-deposited gold films, <1&#;μm thick, offer substantial lifetime advantages over previous electroplated films which developed &#;blisters,&#; peeling, and pinholes after three months of continuous use, while the sputter-deposited films exhibited no sign of wear or degradation.

An early ion-beam source was developed by Louis Maxwell in .162 A hydrogen discharge at pressures of 3.5&#;120 mTorr, with a liquid-air cold trap to remove water vapor and minimize mercury contamination due to back-streaming from the pump, was established in a small brass vacuum vessel. The ion current was controlled by thermionic electron emission from a hot, low-work-function filament. Large magnetic fields, &#;12&#;17&#;×&#;103 G, parallel to the positive ion beam were used to minimize ion losses to the wall and provide 0.1&#;3&#;mA through a 1-mm-diameter circular extraction electrode to a collector electrode in a small attached chamber maintained at 3.3&#;×&#;10&#;3 to 1.5&#;×&#;10&#;4&#;Torr.

The first recorded description of a dc glow discharge ion-beam sputtering system was given by Seeliger and Sommermeyer in .163 They drilled a 2-mm-diameter hole in the cathode of their discharge tube to &#;collimate&#; an Ar+ ion beam (the beam was actually divergent) to strike solid silver or liquid gallium targets at energies of 5&#;10&#;keV and observed that sputtered-atom emission can be approximated by a cosine distribution.

In , Wehner and Rosenberg,164 using a mercury-pool-supported glow discharge (see Sec. IV&#;E) to sputter polycrystalline metal targets with normally incident 100 to &#;eV Hg+ ions, showed that sputtered-atom angular ejection distributions ranged from under-cosine at lower energies toward cosine at higher energies and noted that the crystalline orientation (texture) of the target was important. Much later (), Matsuda et al.165 reported, based on normally incident Ar+ ion-beam experiments, that the angular distribution of sputtered iron atoms varied from cosine at 600&#;eV to slightly over-cosine at &#;eV and increasingly over-cosine at 2 and 3&#;keV. With a simple cosine emission distribution, often used as a first approximation in sputter deposition (see Sec. IV&#;F&#;2), the sputtered flux ejected from a given point on the target surface along any angle φ is just the flux at normal incidence multiplied by the cosine of φ.166

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Sputter deposition

Sputter deposition

Equipment

Endeavor M1 Aluminum Nitride Sputter Tool

Lab 18-1

Lab 18-2

PVD 75 Sputter Tool

Materials

Method of operation

Applications

Figures of merit

Responses of figures of merit with parameter changes

Deposition Rate, Stress, Resistivity and Step Coverage

Uniformity

See also

Further reading

Review Article: Tracing the recorded history of thin-film ...