United States Patent [19J
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`[ 11] 3,864,819
`[45J Feb. 11, 1975
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`[54] METHOD FOR FABRICATING
`SEMICONDUCTOR DEVICES
`
`[ 7 5) Inventor: Robert S. Ying, Torrance, Calif.
`
`[73] Assignee: Hughes Aircraft Co., Culver City,
`Calif.
`
`[22] Filed:
`
`Sept. 29, 1971
`
`[21) Appl. No.: 184,767
`
`Related U.S. Application Data
`[ 63] Continuation-irトpart of Ser. No. 95,652, Dec. 7,
`1970, abandoned.
`
`[52] U.S. CI ....・ H ・H ・H ・.....・ H ・.....・ H ・−−….29/583, 29/580
`[51] Int. Cl .........・ H ・−…...・ H ・...・ H ・H ・H ・...・ H ・..BOlj 17/00
`[ 5 8] Field of Search ......・ H ・−…...・ H ・.29/580, 590, 583
`
`[ 5 6]
`
`2司444,255
`2,865,082
`3,193,418
`3,333,324
`
`References Cited
`UNITED STATES PATENTS
`6/1948 Hewlett ..........…・−−…..29/583
`I 2/1958 Gates. ・…・・ −−……・・・・・H ・...29/580
`7/1965 Coop巴ret al.……−−・……・… 29/580
`8/1967 Roswell .......・ H ・・・…・・ ・・“ 29/576J
`
`20 22 26
`
`8 6 4 2
`
`24
`
`Primary Eλaminer-Roy Lake
`Assistant Eλw11iner-W. Tupman
`
`[57)
`
`ABSTRACT
`
`A plurality of individual semiconductor devices are
`simultaneously produced by thinning a doped semi-
`conductor wafer to a desired uniform thickness, ther-
`mocompression bonding the thinned semiconductor
`wafer to a metal support plate, etching the wafer into
`many tiny discrete members, and punching out small
`individual sections of the support plate, each section
`forming a heat sink and having one of the bonded
`semiconductor members thereon. Alternately, the
`semiconductor wafer may be thinned after the bond-
`ing step, has been effected, although prior thinning is
`preferred.
`
`15 Claims, 12 Drawing Figures
`
`001
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`PATENTEOfEB t I 1975
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`3.864,819
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`3,864,819
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`1
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`METHOD FOR FABRICATING SEMICONDUCTOR vices each including a tiny, thin semiconductor element
`DEVICES
`bonded to a small metal support member.
`These and other objects and advantages of the pres・
`This is a continuation-in-part of copending U.S. Pat. ent invention will become apparent from the following
`application, Ser. No. 95,652, filed Dec. 7, 1970, for 5 description and drawings of illustrative embodiment,
`"Semiconductor Devices And Method Of Making wherein:
`Same", and now abandoned.
`FIG. 1 depicts an exemplary doped silicon wafer pre-
`The present invention relates to the fabrication of paratory to the processing steps of the present inven-
`small semiconductor devices and, more particularly, to tion;
`an inexpensive method for simultaneously fabricating 10 FIG. 2 depicts the wafer of FIG. I metallized on its
`a plurality of such devices.
`upper doped surface;
`FIGS. 3a and 3b respectively depict the wafer of FIG.
`Small semiconductor devices are useful for a variety
`of functions, such as for use as PN diodes, PIN diodes, 2 in partial cross-section having a thinned substrate and
`varactor diodes, IMPATT (impact avalanche ioniza- the apparatus for thinning the wafer;
`tion transit time) and TRAPATT (trapped plasma ava・ 15 FIGS. 4a and 4b illustrate the step of bonding the
`lanche trigger transit) diodes, triodes, etc., and transis- thinned semiconductor wafer to a metal support plate
`tors. The primary function of IM PATT and TRAP A TT in accordance with the present invention, FIG. 4b
`diodes, when employed as avalanche diodes, ts to gen- showing an enlarged view of the wafer, plate and block
`erate and/or amplify microwave and millimeter wave portion of FIG. 4a;
`signals. Several factors must be considered for the fab-20 FIG. 5 shows the bonded wafer-metal support plate
`rication of these devices, namely, low cost, efficiency assembly with a portion of the wafer removed after the
`and power output, reliability, ease of packaging, high bonding operation of FIG. 4;
`FIGS.6 and 7 illustrate successive steps in fabricating
`frequency operation, adaptability to various applica圃
`tions and to various circuits, e.g., coaxial, waveguide an array of a plurality of tiny semiconductor elements
`and microstrip.
`25 disposed on a common face of the metal support plate;
`The purpose of this invention is to provide a process
`FIG. 8 depicts the assembly of FIG. 7, positioned in
`for fabrication of such devices wherein all the above a punch press prior to the punching out of separate sec・
`features are combined so that a high power and high tions from the metal support plate, each having a semi-
`efficiency packaged device can be produced in an ecか
`conductorelement thereon;
`nomical fashion. As is known, such devices comprise a 30 FIG. 9 is a plan view of a single tiny semiconductor
`tiny, thin, semiconductor element affixed to a small device assembly; and
`metal plate. Because of their small size, they have been
`FIG. 10 is a side elevational view of a hermetically
`relatively difficult to fabricate and several techniques packaged semiconductor device.
`have been devised therefor. The two most common
`In order to produce various kinds of devices such as
`methods are infrared alignment to form a pill structure 35 PN diodes, PIN diodes, varactor diodes, IM PATT and
`diode and double mask alignment to form a mesa struc- TRAPATT diodes, triodes, etc., a wafer, for example,
`ture diode which, in both cases, is thermocompression of silicon, gallium arsenide, and germanium, may at the
`bonded with other individual diodes into a package. In outset contain regions of predetermined conductivity
`both techniques, a small section is punched out of a type. Although the wafer described herein comprises a
`copper plate and the tiny, thin, pre-shaped silicon semi- 40 specific p十一n-n+layer configuration, it is to be under-
`conductor element is then thermocompression bonded stood that other, differently doped wafers may be uti-
`to the small section. These techniques require separate lized in the inventive process. However, for purposes of
`assembling and bonding operations and, consequently, clarity and simplication in explaining the process of the
`are tedious, time consuming, cumbersome, and there- present invention, a specifically P+-n-n+ doped wafer
`45
`fore, expensive. In addition, because of handling diffi”
`will be utilized in the description of the preferred em-
`culties, the diodes must be more than 50 microns thick. bodiment of the process.
`Since the active region of the device is only about 1 to
`Accordingly, with reference to FIG. 1, a wafer 10 in-
`5 microns thick, the excess silicon thickness adds series eludes a thick n+ substrate 12 almost as thick as the
`loss and reduces the diodes rf performance. The pack- wafer, a thin n layer 14, and a thin p+ layer 16. For pur-
`50
`ages used are also frequency limited because of high poses of illustration, layers 14 and 16 are shown greatly
`loss at higher frequencies and, therefore, they are not thickened. However, as examples of thicknesses uti-
`suitable to inte~rated microstrip circuit application.
`lized, substrate 12 is approximately 6 mils in thickness,
`The present invention avoids these and other prob- layer 14 is I micron in thickness, and layer 16 is 12 mi-
`lems by mass producing many such devices in a single 55 crons in thickness. Alternatively, in a 7 mil thick wafer,
`operation. Briefly, a semiconductor wafer is thinned to then and p+ layers may be extremely thin, the n layer
`a desired thickness and secured to a metal support being typically about 1.4 microns in thickness and the
`plate. Alternatively, the wafer may be thinned after p+ layer being typically about 0.4 microns in thickness.
`being secured to the plate. Thereafter, the wafer is The wafer is, for example, of general circular cross-
`etched into a plurality of discrete devices. The plate 60 section having a diameter of approximately 1.25 inches
`with each device is then subjected to a multiple, single and one edge may be flattened for alignment purposes,
`step punching or dicing operation to form a plurality of although other cross-sectional configurations can be
`individual devices.
`used. Such as-doped wafers are commercially obtain-
`It is, therefore, an object of the present invention to able, or may be otherwise doped by well-known pro-
`provide a method for mass producing small semicon-65 cesses, such as by diffusion and ion implantation.
`ductor devices.
`As shown in FIG. 2, layer 16 is coated with a metal
`Another object of the present invention is to provide film comprising, for example, coatings 18, 20 and 22 of
`a method for producing many small semiconductor de- gold, platinum, and chromium or, if desired, platinum
`
`004
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`3,864,819
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`3
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`Motor32 is caused to alternately rotate holder 27
`coating 20 may be replaced with a chromium”gold in・
`terface. Examples of thicknesses for the former are and wafer IO within solution 34 while the solution is
`magneticallystirred to obtain washing-machine-like ag-
`I 0,000 A gold, 1,000 A platinum, and 800 A chrか
`mium. For the latter, gold layer 18 may be approxi- itation, in order to obtain a smooth, ungrooved, fine
`mately 15 microns thick, platinum layer 20 may be ap- 5 etch on the exposed portion of surface 24 of substrate
`proximately 200 A thick, and a chromium layer 22 12. The speed of rotation and the period of reversing
`about 600 A thick. Coatings 18, 20 and 22 are applied the direction of rotation of the motor is determined ex-
`by standard evaporation techniques except that, after perimentally in accordance with the depth of etching
`some of the gold of layer 22 has been evaporated in and the quality thereof .. In the etching of silicon, for ex-
`situ, further gold is added by electroplating to build up IO ample, motor 32 is caused to turn at 1,000 rpm with a
`its thickness.
`reversal in direction of rotation every I minute for the
`Wafer 10, as built and metallized, is then thinned at first 5 mil etch. Thereafter, reversal is every 5 seconds
`its unmetallized end surface 24 to form the configura- for final control of etch. The water jacket cools the so・
`tion shown in FIG. 3a by a process utilizing the appara- lution to maintain a constant temperature in order to
`tus depicted in FIG. 3b. Wafer 10 is first affixed, by a 15 overcome the heating of the solution by the etching
`wax, such as bees wax, to a disc 25, such as sapphire, process.
`which is not capable of being attacked by an etching
`After etching, the holder and wafer are removed
`solution utilized in the thinning process. The disc is af- from the solution and rinsed in deionized water to re-
`fixed to surface 26 at the metallized side of the wafer move remnants of the etchant. The disc and wafer are
`containing coating 22. Such affixation may be effected 20 then removed from the holder and separated from one
`by placing the wax between the wafer and the disc and another. Any remaining wax on the wafer is removed
`by heating the two for a time and at a temperature suffi- with tetrachloroethylene and methyl alcohol. After
`cient to enable the wax to melt and the two to stick to- drying, the etched wafer had the appearance as that ii-
`gether upon cooling after the wax has melted. The se” lustrated in FIG. 3a, showing a thinned wafer 10 having
`cured together disc and wafer are then hand pressed 25 an etched portion 48 and a rim 50. The purpose of the
`into a holder 27, such as of “Teflon”（ trademark of E. rim was to facilitate ease in further handling of the
`I. Du Pont de Nemours & Co.), the disc side being wafer since, at this time, the wafer had an approximate
`within the holder within an end recess 28 thereof. For thickness of 10 microns± 2 microns.
`As shown in FIGS. 4a and 4b, wafer 10, as thinned,
`this purpose, recess 28 is provided with width and
`depth dimensions which are substantially the same as 30 is then disposed in face-to-face relation on a copper
`those of the disc and wafer to permit substrate side 24 plate 52, with metallized side 26 in contact therewith.
`of the wafer to be flush with an end 29 of the holder. The copper plate has a thickness of several mils and a
`A black wax, such as “Apiezon”（ trademark of diameter smaller than the wafer diameter between the
`James G. Biddle Co.), (see also “The Condensed periphery of rim 50. Typically, copper plate 52 is about
`Chemical Dictionary，＇’ A. and E. Rose, Reinhold Pub- 35 10 mils thick. It is to be understood that support plate
`lishing Corporation, 1969, 7th Ed.) is placed around 52 need not be made of copper but may be made of
`the intersecting edges of the holder and the wafer sub- various suitable metals; but it is preferred to utilize
`strate so as to produce a rim 30 which extends slightly commercially available; oxygen-free copper material
`onto the surface of the wafer substrate to form an ex- for the plate in view of its excellent thermal and electri-
`posed interior portion of surface 24.
`40 cal conductivity.
`The holder is then secured to the apparatus depicted
`The assembled wafer and plate are placed between
`in FIG. 3b by attaching it to a motor shaft 31 which is opposed plates 54 and 56 of a manually operated hy-
`capable of being alternately rotated by a reversible draulic laboratory bench press 58 having heater fila-
`motor 32. The end of the holder containing the waxed ments and water coolant lines within the plates. A
`45
`wafer is then placed within an etchant solution 34 con- stainless steel block 62, having a diameter Jess than
`tained within a receptacle 36. A plurality of “Teflon” wafer 10 to fit within rim 50, is placed on the substrate.
`baffles 38 are symmetrically placed around the holder. Blocks 56 includes portions 64 and 66 having a ball
`Below the receptacle is placed a magnetic stirr巴r40. In bearing 67 arranged within spherical recesses 68 in the
`order to maintain constant temperature of solution 34, block portions to provide an adjustment for non-
`50
`a coolant 42, such as water, is placed around the recep- parallelism of block portion 64 and plate 54 and non-
`tacle and held within a container 44. The whole appa- alignment of plate 52 and wafer 10. Lower press plate
`ratus is supported on a base 46.
`54 is moved upward (as indicated by arrows 70) toward
`Solution 34 is an etching solution for removal of a upper block 56 to press wafer IO and plate 52 together.
`portion of substrate 12 not masked by the black wax. 55 The wafer and copper plate are heated to a tempera-
`The type of etchant utilized will, of course, depend ture less than the silicon-gold eutectic temperature
`upon the material of the substrate, all of which are well- while the wafer-plate assembly is under compression.
`Inthis way thermocompression bonding of wafer 10 to
`known in the prior art. However, for purposes of illus司
`tration, the etching solution for silicon comprises 3 plate 52 is accomplished. Illustratively, the wafer-plate
`parts by volume of hydrogen fluoride, 5 parts by vol- 60 ?ssembly can be thermocompression bonded by heat-
`ume of nitric acid, and 3 parts by volume of acetic acid. mg it to 200。Cfor about 20 to 30 minutes while exert”
`For gallium arsenide, the etching solution may com- ing a pressure on the assembly of about 40,000 psi. It
`prise 3 parts by volume sulphuric acid, I part by vol- is preferred that the pressure not exceed I 00,000 psi to
`ume water, and one part by volume hydrogen peroxide. prevent wafer damage. In practice, the temperature,
`Germanium is etchable by a solution comprising 3 p.arヤ65 pressure and time of the ti悶 mocompreぉionstep are
`by volume hydrogen fluoride, 5 parts by volume mtnc variable to a considerable extent with continued satis-
`acid, 6 parts by volume of acetic acid, and 0.3 percent factory results. It is to be understood, however, that,
`of the foregoing combination of bromine.
`although a particular thermocompression bonding step
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`005
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`3,864,819
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`5
`6
`has been described, other bonding methods are as ap- from the copper plate in a single punching operation.
`plicable as will become apparent to those skilled in the Generally, the FIG. 7 assembly is disposed upside down
`between opposed apertured plates 86 and 88 of the
`art. Examples are by ultrasonic bonding and plating.
`After the wafer and plate have been bonded together, punch press. Copper plate 52 is provided with a pair of
`the periphery of the wafer including rim 50 is cracked 5 alignment holes 90 which are used to align it to press
`off or otherwise removed to make the wafer diameter plates 86 and 88 by registering alignment holes 90 with
`alignment bosses 92 formed on the press plate目 Each
`equal that of the plate, as depicted in FIG. 5.
`Alternately, if wafer 10 is not pre-thinned, it is hand press plate has a matching array of holes 94 and 96 cor-
`lapped and machine and chemically polished by well- responding in number and arrangement to the array of
`known methods after affixation to plate 52 to remove 10 semiconductor elements 82 affixed to copper plate 52,
`most of n + substrate layer 12 and to reduc巴 thewafer except that holes 94 and 96 are typically of a diameter
`thickness to less than 50 microns. Illustratively, the about I 0 times as great as the diameter of the semicon-
`wafer thickness is reduced to less than I 0 microns with ductor elements.
`Thepunch press also includes an additional plate 98
`a minimum thickness somewhat greater than the com・
`bined thickness of n and p layers 14 and 16. The thus 15 having an array of cylindrical holes 100 in its lower-
`thinned wafer itself is quite fragile but, since it is affixed most face into which an array of cylindrical pins 102 is
`to plate 52, the combination presents no handling prob- fitted. The array of pins 102 project downwardly from
`plate 98 into holes 96 of plate 88. Each pin 102 is of
`lems.
`As illustrated in FIG. 6, a composite metal film is a diameter slightly less than hole 96 into which it
`next deposited on the wafer in a suitable manner, such 20 projects. Three compression springs 104, of which two
`as evaporation and electro-deposition. Such a metal are shown, are arranged symmetrically about the verti-
`film comprises a layer 72 of chromium about 600 A cal axis of the press apparatus between plates 88 and
`thick, a layer 74 of chromium and gold about 200 A 98. A receptacle 106 is disposed beneath the plate 86
`thick, and a layer 76 of gold about 10 microns thick.
`for receiving parts punch巴dfrom copper plate 52.
`The resultant film is covered with a film of photore- 25 The punch press is operated by moving plate 98 verti-
`sist, such as by "KTFR”（ trademark of Eastman Kodak cally downward through an approximate vertical dis-
`Co.). Such photoresist is selectively exposed to ultravi- tance S, thereby causing pins 102 to punch out circular
`olet light through a suitably patterned exposure mask sections of copper from copper plate 52. The circular
`for a time sufficient to completely expose the un- sections pass through holes 94 of plate 86 and are col-
`masked photoresist. The exposed portions are devel- 30 lected in receptacle 106. As a result, many identical
`oped in the “KTFR”developer and the undeveloped small semiconductor device assemblies are simulta-
`portions are removed by a“KTFR”rinse and xylene to neously produced, each of which having the general ap-
`provide an array of circular dots 78, about 3 to 6 mils pearance of the device assembly depicted in FIG. 9. In
`diameter, as shown in FIG. 6. The diameter of dots 78 this figure there is shown a plan view of a tiny silicon
`depends upon the desired size of the metal on element 35 element 82 which is centrally disposed on and bonded
`10 to be retained, which, in turn, depends upon the fre- to a small copper plate member 52’which has been
`qucncy band at which the end product is to function. punched out of the much larger copper plate 52.
`The dots are spaced from center to center by about 50
`Instead of using a punch press apparatus in the man-
`mils. The array of photoresist dots 78 constitutes an ner described, copper plate 52 with the silicon ele-
`etching mask for the underlying metal film on the wa-40 ments thereon may be sawed or diced into many device
`fer. Thereafter, the metal film, except those portions assemblies, but the use of the punch press apparatus af『
`under the dots 78, is etched away, using the dots to pro- fords superior results in simply and economically pro-
`tect the underlying portions of the metal film against viding circular copper plates 52’free of rough edges
`etching, to remove all of the metal film. In this etching with a minimum of handling.
`45
`Hermeticallypacked silicon elements 82 may be pro-
`process, the gold and the platinum are etched by a po・
`tassium iodide solution and the chromium is removed vided by producing devices such as that illustrated in
`by hydrochloric acid at room temperature. The times side sectional view of FIG. 10. In FIG. 10, a cylindrical
`for removal depend upon the thickness of the film. The quartz or ceramic ring 110, having suitably metallized
`dots are then removed by “Jぺ00(trademark of Indust・ upperand lower rims, is soldered to copper disc 52’to
`50
`Ri-Chem Lab), a solvent, leaving behind metal film encircle element 82. The ring is typically about I 0 mils
`dots 80 on the wafer.
`high, is metallized on each rim with successive evapo・
`Next, portions of the silicon wafer, not protected by rated layers of chromium, molybdenum, copper, and
`the metal dots previously formed, are etched away with gold, for example, and is soldered to disc 52’by a ring-
`an etchant, such as a' mixture of 3 parts hydrofluo.ric 55 shaped solder preform (e.g., of tin and gold) interposed
`acid, 5 parts nitric acid, and 6 parts acetic acid, usmg therebetween which is solderable at a temperature
`metal dots 80 as masks against the etching away of the below the eutectic temperature of silicon-gold. A gold
`silicon portions directly therebeneath so as to form ribbon 112, about 3 mils wide, is compression bonded
`many, typically upward of a hundred, disc-shaped tiny at each end to the upper rim of the quartz ring 110 and
`silicon section or members 82 as is generally depicted 60 at its middle to the metal film on the silicon element 82.
`in FIG. 7. It is possible to provide silicon sections of A gold-clad molybdenum cap 114 is bonded to the
`mesa type configuration by performing other routine quartz ring 110.
`procedures starting with the thinned wafer 10 already
`Alternately, a quartz standoff metallized on both
`sides can be soldered to copper section 52’and a gold
`bonded to the plate 52.
`The resultant assembly is next placed in a punch 65 ribbon bonded to the diode and the quartz standoff.
`press apparatus 84, as indicated in FIG. 8. The punch During operat10n, the copper chip is soldered to a
`press is designed for simultaneously punching out many larger heat sink in any type of circuit and contact is
`copper discs, typically about 40 to 50 mils diameter, made on the standoff rather than on the diode so that
`
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`007
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`
`3,864,819
`
`9
`film formed on the waffer’s face is selected from a layer
`consisting of the combination of chromium and gold,
`and gold and the combination of chromium, platinum,
`and gold.
`12. A method as in claim 2 wherein said step of si- 5
`multaneously separating the plate into a plurality of
`sections about each of the semiconductor elements
`comprises the step of punching out the sections of the
`plate.
`13. A method as in claim 12 wherein said punching 10
`out step comprises the steps of placing the elements on
`the plate in a punch press with the elements aligned in
`and with means defining holes in a punch plate and si・
`multaneously moving a plurality of punch pins against
`the plate having the elements therein to cause the sec- 15
`tions to be formed as the pins force portions of the
`plate having the elements thereon through the hole
`means.
`14. A method for simultaneously fabricating a plural-
`ity of semiconductor devices comprising the steps of: 20
`A. fabricating a plurality of semiconductor elements
`on a plate by
`I thinning a semiconductor wafer by
`a. affixing the wafer to a disc of substantially the
`
`10
`same circumferential configuration as that of
`the wafer to completely seal the wafer and the
`disc together at their matching surfaces.
`b. fitting the wafer and the discs in one end of a
`holder,
`c. sealing the wafer to the holder to expose only
`an interior portion of one surface of the wafer,
`the interior portion having a surface area less
`than that of the wafer, and
`d. uniformly etching the exposed interior portion
`in an etching solution to thin the interior por-
`tion and to form a rim of substrate material;
`and
`2. affixing the thinned wafer to the plate; and
`3. removing portions of the thinned wafer to form
`the plurality of semiconductor elements; and
`B. simultaneously separating the plate into a plurality
`of sections about each of the semiconductor ele-
`men ts.
`15. A method as in claim 2 further including the step
`of removing the unthinned portion after said affixing
`step.
`
`＊ ＊ ＊ ＊ ＊
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`008