US 2930722 A
Description (OCR text may contain errors)
March 29, 1960 J. R. LIGENZA 2,930,722
METHOD OF TREATING SILICON Filed Feb. 3. 1959 2 Sheets-Sheet 1 I E H 11? J2 WASH RINSE //v WASH m RINSE //v wAsl-l //v DE/ONIZED xmws BOILING //v H/VOa HF- HNO; WATER Ar 90%: WATER Ar IOOC. 1 v 1 RINSE //v RINSE [NI-I07 STEAM OXIDATION Ar Loam/1250 $22??? I I 7 500masoc. AND
WATER ROOM I SURFACE I 25-475 ATMOSPHERES DOP/NG 11 E sxpaszo IN J E i HF VAPOR 1 JZZZ' F/G 5 I 600 k FIG. 2
PRESSURE (ATMOsPHERES) TEMPERATURE (c INVENTOR J. R. L/GENZA BY ATTORNEY March 29, 1960 J. R. LIGENZA 2,930,722
METHOD OF TREATING SILICON Filed Feb. 3, 1959 2 Sheets-Sheet 2 FIG. 4
/NVENTOR By J. R. L/GENZA ATTORNEY United States Patent 2,930,722 7 METHOD TREATING SILICON Joseph R. Ligenza, Westfield, N.J., assignor to Bali Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Application February a, 1959, Serial No. 790,848
' Claims. c1. 148-15) ICC should preferably-first be rendered hydrophilic, clean, and slightly oxidized.- This condition is achieved by methods comprising cleaning with an'organic solvent to remove waxes or other organic contaminants, rinsing in clean water, slightly oxidizing the surface with an oxidizing agent, and rinsing again. A number of variations on this skeleton process may be devised. The treatment most advantageously used comprises etching the surface ofa silicon element in a mixture of hydrofluoric and nitric acids, rinsing, chemically cleaning by immersion irr a hydrocarbon solvent, rinsing, treating in hot nitric acid,'and rinsing. the element'once more before sealing into autoclaves for steam oxidation; Other aspects of the invention will become apparent from the accom panying drawings.
Fig. l-is a flow sheet of the preferred preparatory proeess and steam oxidation process herein described;
Fig. 2- is a'front elevation in section; greatly-enlarged, of a diodedevice comprising asingle'crystal body of silicon having" thereon a silicon oxide stabilizing film produced by the methods of this invention;
surfaces by a preparatory technique culminating in an f;
oxygen oxidation of the silicon at 900. centigrade to give a stable oxide thereon. The methods of the present invention teach apreferredpreparatory technique similar to that in they patent mentioned, followed by an oxidation of the clean silicon surfaces by clean steam under high pressures' and at lower temperatures than those heretofore used. The modified process has several significant advantages over the process as formerly practiced.
v The use of steam under pressure as an oxidizing medium results in much higher rates of film growth than are possible by oxygen oxidation; Oxidation with oxygen of a silicon surface is rate limited by the speed of thermal dilfusion of silicon ionsffrom the bulk of the material through the layers of oxide already formed on the surface. The more rapid growth rates observed when pressurized steam is employed are believed due to an attractive force exerted on silicon ions .in the bulk of the body by a high concentration of oxygen ions (from the Water molecule) accumulated on the silicon surface. The attractive electrostatic force of these oxygen ions is believed responsible for pulling silicon ions to the surface. Thermal diffusion is no longer rate limiting, and lower temperatures can be used' to accomplish the oxidation. The high growth rates and low temperatures possible in the present method permit formation of oxide films 5000 angstroms thick, or thicker, within periods which save considerable time over oxygen oxidation.
' Because lower temperatures are used for shorter periods 7 v of time in forming an oxide coating by the new process herein described, there is less danger of shifting of junctions in devices having regions of difiering conduc- FigJB is a front elevation, partly in section, of a simple autoclave in which a number of: silicon semiconducting ;elements'can simultaneously be oxidized with steam un- 'der high pressure; 1 1
Fig. 4 is a front elevation, partlyfi'insection of an autoclave system for the oxidation of silicon semiconducting elements in which'steam is generated at a source removed from the portions of the oxidation occurs; and j l r Fig. 5 is a pressuretemperature diagram defining those conditions of temperatureand pressure mostTsatisfactory for carrying out the steam oxidations hereimdescribed. The flow diagram of Fig. shows-various steps involved in the preferred process of the. present invention; In step'I, a single crystal body of silicon is etched at room temperature in a mixture of nitric and hydrofiuoric acids, conveniently comprising six parts by. volume of concentrated nitric acid. to one part by volumelof 48 percent hydrofluoric acid. Otheracid mixtures rang-.- ing in concentration from greater than 20 to 1 to less thanl to 1 can be used successfully, as known in the art. The etchant used is widely known and is used in.
the art in all proportions. This step may be followedby an optical quench in concentrated nitricacidtnot shown;
.on the flow sheet) which avoids the formation of surfacestains, possibly of silicon monoxide, which may other-,
wise form after the etching step.
Step II is a rinsing of the etched body in deionized water, which may be characterized in having aconducg tivity of less than 0.05 micromho.
In step III, the washed body is treated by rinsing a.-
. continuously flowing distillate of a hydrocarbon solvent,-
tivity types. Lower temperatures and high growth rates also permit a greater variety of contacts tobe made to the silicon body before treatment without danger of loosening the contacts under the influence ofrprolonged heating. Also lower temperatures reduce or eliminate difiusion of contact metals into the silicon substrate, which difiulsion could otherwise lead to changes in electrical properties of the silicon. The avoidance of ele-. vated temperatures made possible by steam oxidation also minimizes the danger of devitrification of the amorphous silica coatings produced. Devitrification can occur if silica films are heated to high temperatures in the presence of traces of certain inorganic materials.
It is to be understood that the steam oxidations here described have been relatively ineffectivej'alone in producing stable surfaces on silicon without a preparatory cleaning of the surface. The surface to be oxidized of which benzene and xylene are exemplary. This;rinsing, continued for about fifteen minutes, aids in removing any organlc residue on the surface of the semiconducting j body.
Traces of the solvent used for rinsing are removed by next rinsing the washed element in boiling deionized water for about fifteen'minutes, as indicated forstep I-V of the flow chart in Fig. 1. if further cleaning of; the.
Patented/Mar. 29,, 1960* system in which silicon rinsing steps I and II before I 3 tigrade, followed in step VII by a further rinse in a similar bath at room temperature. The steps to this stage have produced an exceptionally good silicon surface which is clean, hydrophillic, wet, slightly oxidized.
Depending on the characteristics finally desired in the silicon element, two alternative further treatments can be used. The treatment through step VII used on high purity, high resistivity, silicon produces a silicon surface which is lightly oxidized and almost perfectly hydrophilic. If induced p-type conductivity is desired in the surface regions of such high purity silicon elements, the treated elements are immediately subjected to a steam oxidation, as indicated in step IX. To inhibit possible contamination or deterioration of the cleaned and preparedsurfaces, this oxidizingstep should immediately follow step VII. The oxide induces a p-type conductivity in surfaceportions of high resistivity intrinsic silicon covered with the oxide.
However, if an n-type oxide induced conductivity region is desired in surface portions of a silicon element of high resistivity, additional step VIII may be practiced before the'oxidation of step IX. As indicated in the flow chart, an n-type surface may be produced by exposing the silicon surface to vapors of hydrogen fluoride, conveniently to hydrofluoric acid vapors, for a short time (for example, one to fifteen seconds) prior to oxi-- dation. This surface doping may be accomplished with various other vapors, for example chlorine, and various salt solutions.
Another technique, not shown in Fig. I, for. producing an n-type surface region comprises diffusing certain significant impurities, for example gold or iron, into the silicon body before any surface treatment is begun. During the oxidation, these impurities will be drawn from the bulk' of the material into the oxide film giving an ntypeconductivity surface layer.
The oxidation of low resistivity silicon bodies, such as those priorly doped for device uses, has no significant effect on the conductivity type-of the underlying body. The effect of the oxide in inducing a surface conductivity type is slight, and becomessignificant only for intrinsic, high resistivity, silicon substrates.
Step IX, the oxidation by steam under pressure, is discussed in greater detail later in this specification.
Fig. 2 is a greatly enlarged sectional view of a silicon diode device fabricated in part by the methods of the present invention. The element comprises a single crystal body of silicon having p-type region 20, n-type region 21, p-n junction 22 between them, and strongly n-type or n -type region 23. The p-n junction may be produced within the body by diffusion techniques known to the art. For example, boron, which is a doping impurity for silicon, may be diffused into one side of an n-type wafer to produce a p layer and a p-n junction therebetween. Phosphorus, another significant impurity for silicon, may then be diffused into the other surface of the n-type wafer to give a three layered n+-n-p structure like that in Fig. 2.
Metallic layers 24, of a material such as platinum, for example, maythen be affixed to both sides of the wafer by means known to the art such as in a paste, or by evaporating, sputtering, or plating, to serve as low resistance contacts for the device. Finally a central raised portion or mesa" is formed by known cutting or etching techniques, and the device is then processed as described in Fig. 1. Thin stabilizing protective oxide film 25 is thereby formed on the silicon surfaces. Film formation is particularly desirable on the mesa edges in the vicinity of-exposed p-n junction 22. As mentioned earlier, formation of the oxide has no significant doping efi'ect on the already doped silicon substrate.
Fig. 3 is a sectional view of a simple autoclave for carrying out the steam oxidations of step 9 of Fig. 1. The outer casing of the autoclave or bomb, comprising main portion of casing30 and threaded screw cap 31, is of a strong metal. The alloy Inconel X (73 percent nickel, 15 percent chromium, 7 percent iron, 2.5 percent titanium, 1 percent columbium or niobium, balance small mounts of aluminum, silicon, manganese and carbon) has proved particularly successful as a material for the outer casing, but other strong materials are equally good. For example, an alloy of percent platinum and 20 percent rhodium has been used with particularly good results. Close fitting thin inner liner 32 of an inert metal, conveniently gold, is within casing 30 to preclude possible contamination of the samples from the metals of casing 30. Cylindrical disc 33 having a smaller disc 34 of inert metal, preferably gold, faced thereon, fits into screw cap"31. When cap 31 is tightly joined to casing 30, disc 34 seals inner liner 32. Particularly if disc 34 is made of a deformable metal such as gold, a tight joint to liner 32 can be made. In use, the silicon elements are placed in the autoclaves with suflicient water to produce a desired pressure at the temperature to be used in heating the bombs. To as sure that surface characteristics be uniform in any single element, it is important that thermal gradients, which favor differential growth rates of the oxide films and variation in surface properties, be avoided. For this reason, the bombs of Fig. 2 advantageously are kept small in size, the longest dimension of the casing being about two and one-half inches and that of the gold liner about one inch. The water used in the oxidation step to produce steam is high purity deionized water as is employed in the silicon cleaning process. In addition, the autoclaves are cleaned prior to use by washing in nitric acid at centigrade, rinsing in hot deionized water, and then rinsing again in deionized water at room temperature.
A convenient autoclave systemfor oxidizing a large number of samples simultaneously is shown in Fig. 4. The system comprises double walled cabinet 41 with insulation 42, for example glass wool, between the walls thereof. Strip heaters 43, comprising metal clad resistance heaters, are mounted in cabinet 41 as a convenient heat source for the cabinet interior. Steam generating unit 44 is a thick walled autoclave having deionized water therein, and sealed to withstand high interior pressures. Autoclave 44 shown is a commercial item, a product of the American Instrument Company, Silver Spring, Maryland. Other similar apparatus could be used equally successfully, however. Tube 45 leads through heated valve 46, also a commercial product, to tubular con-. tainer 47, in which the silicon elements to be oxidized are placed. Container 47 is ofa chemically'inert, structurally strong material. Though noble metals can be used, or a metal casing having a noble metal lining, container 47 is conveniently made of silica with a wall thickness of about /8 inch. A pressure-tight seal of container 47 to cabinet 41 can be made by leading tube 45 to steel ring 48 having a raised inner portion fitting into container 47. Second steel ring 49, fitting around container 47, presses lip 50 of container 47 against ring 48. Gold washer 51, deformable under pressure, aids in forming a gas-tight seal. Bolts 52 are provided on ring 48 so that ring 50 can be clamped tightly thereto, compressing lip 50 and washer 51. Furnace 53, conveniently comprises a coil of resistance wire, surrounds container 47 to give uniform heating of all portions of container 47, minimizing thermal gradients.
The advantage of the autoclave system of Fig. 4 is the generation of steam at a pressure determined by the temperature of steam generating unit 44. Steam at this pressure is fed through tube 45 and valve 46 to container 47, where it is used at the temperature of furnace 53. Prior to use, all portions of the system contacting either the silicon to be oxidized or the deionized water used for the process are cleaned with hot nitric acid and,
, rinses of deionized water as earlier described.
Oxidation of silicon surfaces under conditions of too TABLE 1 Maximum Temperature (Degrees Centigrade) Pressures (Atmospheres) Although oxidation of silicon by steam takes place at even very low pressures, a minimum'steam pressure of 25 atmospheres, through the temperature range used, is
advantageously used so that thick. films can be formedf in feasibly short time periods. As shown in Fig. 5, steam pressures up to 475 atmospherescan be used with good results in the lower temperature range. Temperatures between 500 centigrade and 850 centigrade are preferred for the oxidations as shown in Fig. 5. Temperatures between 600 centigrade and 700 centigrade have given particularly good oxide coatings, and a temperature of 650 centigrade has been found to yield optimum results in many instances. For the smaller temperature range mentioned above, maximum pressures, asshown in Fig. 5, range between about 465 atmospheres and 250 atmospheres. At 650 centigrade the maximum pressure is 375 atmospheres. At this temperature and pressure, the growth rate of oxide films is about 120 angstroms per minute, and a film 3000 angstroms thick can be grown in about 25 minutes. For'comparison, a film-of comparable thickness produced by prior art heatings in oxygen would require one hour at an elevated temperature of l250 centigrade. k
In the steam oxidation, increases in either temperature or pressure within the limits specified will increase the growth rate of oxide. I
At a given temperature and pressure, oxidation is continued until an oxide coating of desired thickness is .produced. Oxide fihns as thin as 300 angstroms have been found useful in stabilizing surface properties, but thicker films up to 10,000 angstroms, and particularly between 5000 angstroms and 10,000 angstroms, are usually preferred.
A specific example of the practice of the invention herein described follows below.
Example Silicon diode devices like those shown in Fig. 2 were made by doping a sheet of n-type silicon by exposure to gaseous boron to form a structure having a p-n junction about 0.00l5 inch below the semiconductor surface.
. One face of the wafer was then doped with phosphorus p-n junctions, the metal coated'portions 'of "the water: I
being protected by "wax. The wax was removedffby solvents, and the wafers rinsed in deionized water. The
wafers were then washed in xylene and rinsed again in The wafers were then re-etched for deionized water. 7 five secondsin the 6:1 HNO HF'mixture, quenched in HNO and rinsed in deionized water. They were next soaked for fifteen minutes in nitric acid at 100 centigrade, rinsed in deionized water for fifteen minutes, and
The samples were oxidized in small bombs like those shown in Fig. 3, made of an 80 percent platinum-20percent rhodium alloy, with gold liners. The diodes were oxidized at 650 centigrade under a pressure of 50 atmospheres for two hours, and had an oxide coatingv about 3000 angstroms thick.
Table 2 presents the electrical characteristics of some of the diodes treated as described above.
TABLE 2 Reverse current at breakdown voltage less two volts Reverse current 10 volts (in amperes) 3 (IO- 4 (10 2.2 (IO- 3 (10") 2.5 (10- 1.2.(10-
V 1.0 (10" r 4.6 (10' 3.0 (Ml- 2.6 (10" which method comprises washing a body of single crystal silicon in a mixture of nitric acid and hydrofluoric acid,
rinsing said body in deionized water, washing said body-1' 'in a flowing hot hydrocarbonv solvent, washing said body in boiling deionized water, immersing said body in hot nitric acid, rinsing said body in hot deionzed water, rinsing said body in deionized water at room tempera-'- ture, and immediately thereafter oxidizing the surface of saidbodyfwith steamat a temperature between. 500 centigrade'and 850 centigrade and below a maximum pressure of between 475 atmospheres at 500 centigrade and atmospheres at 850 centigrade fora period. sufiicient to produce an oxide layer having a thickness of at least 300 angstroms.
2. The method substantially as described in claim 1 which includes the step of exposing said body of silicon to vapors of hydrofluoric acid just prior to oxidizing the surface of said body with steam.
3; The method of fabricating a semiconductor device, which method comprises diffusing at least one significant impurity into a single crystal body of silicon, washing said body in a mixture of nitricv acid and hydrofluoric acid,
rinsing said body in deionized water, washing said body in a hot hydrocarbon solvent, washing said body in hot deionized water, immersing said body in hot nitric acid, rinsing said body in hot deionized water and then in deionized water at a lower temperature, and then oxidizing said body in steam at a temperature between 500 centigrade and 850 centigrade and below a maximum pressure 300 angstroms.
4. The method of fabricating a semiconductor device, which method comprises preparing a clean hydrophilic surface on a body of single crystal silicon, and then oxidizing said body in steam at a temperature between 500 centigrade and 850 centigrade and below a inaxlmum pressure of between 475 atmospheres at 500 centigrade, and 105fatmosphc'res 'at'850" centigrade for a period sufficient to produce an oxide layer having a thickness of at least 300 angstroms.
5. The method of fabricating a semiconductor device, which method comprises washing a silicon body with an organic solvent, rinsing'in water, slightly oxidizing the surface of said body, rinsing again in water, and then oxidizing said body in steam at a temperature'between 500 centigrade'and -850 Centigrade and below a maximum pressure of between 475 atmospheres at 500 centigrade and 105 atmospheres at 850 centigrade for a period sufiicient to produce an oxide layer having a thickness of at least 300 angstroms.
References Cited in the file of this patent UNITED STATES PATENTS