US3721584A - Silicon coated substrates and objects fabricated therefrom - Google Patents

Abstract

Silicon coated substrates for such uses as optical filters, photo masks, passive circuits and as an information storage means.

Description

'Unuaa State's P5115617 3,721,584

Diem 1March 20, 1973 [541 SILICON COATED SUBSTRATES AND 2,344,250 3/1944 OBJECTS FABRICATED THEREFROM 212 1 3.??? 2; [76] Inventor: Albert R. Diem, 1181 Lake Street, 314431915 5,1969

Salt Lake City, Utah 3,476,617 11/1969 Robinson 3,009,834 11/1961 Hanlet [221 1970 3,410,626 11/1968 Magrath ..350/l66 [21] 271744 OTHER PUBLICATIONS Baumeister Multilayer Filters Institute of Optics, [52] US. Cl. ..1l7/212, 117/55, 117/333, Uni ofRochesmr 1964 pp 20 14to 117/355, 117/217 [51] Int. Cl. ..B44d l/l8 Primary Examiner A|f1-'ed L Leavitt [58] Field of Search.....l 17/212, 5.5, 38, 124 A, 217, Assistant Examiner ])avid Simmons 117/45, 33.3; 96/362; 156/17; 350/164, 165, s 1 d H ABSTRACT [56] References cued Silicon coated substrates for such uses as optical fil- UMTED STATES PATENTS ters, photo masks, passive circuits and as an information storage means. 3,170,859 2/1965 Boudart et al ..1 17/124X 2,999,034 9/1961 Heidenhain ..l17/5.5 13 Claims, 9 Drawing Figures PATENTEUHARZOIBYS SHEET 1 or 2 jll )9 22 I9 I" "1 I I l 21 I Fig 4 10 ,/II///// fl l Afro/W4? ys SILICON COATED SUBSTRATES AND OBJECTS FABRICATED THEREFROM BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the fields of silicon coatings and integrated circuit manufacture.

2. Prior Art In the prior art, masks used for semiconductor manufacture have been primarily emulsion masks or chrome masks. The emulsion masks consist of a photographic gelatin coating containing silver halide particles on one side of a glass plate. Transparent and opaque areas are formed by the conversion of the silver halide to metallic silver. These developed emulsions are relatively soft and because of this are subject to rapid wear and deterioration in use.

As an alternative, chrome masks are now being made, wherein chromium is evaporated onto a glass substrate and then a photo-resist is put over the chrome. Subsequent exposure and development of the photo resist exposes areas of the chrome which can be etched away by a suitable etchant. After etching and removing the photo resist, a chrome image is left on the glass plate. The chrome is considerably harder than a photographic emulsion and, therefore, is subject to wear and deterioration at a much slower rate than the emulsion masks. However, the process for making such masks is much more involved than the simple photographic process for making the emulsion masks, thereby resulting in a much higher cost for chrome masks than for emulsion masks. Therefore, at the present time, the choice is between a relatively inexpensive mask which has a short useful life and a relatively expensive mask which has a longer useful life.

The opaque areas of both types of masks are opaque to visible light as well as ultraviolet light. Consequently, when the mask is in place over a semiconductor wafer,

the areas of the wafer beneath the opaque portion of the mask are not visible to the naked eye. In addition, the chrome masks have the undesirable characteristic of being highly reflective. Such reflective qualities tend to degrade the definition that may be reproduced by chrome masks.

Another art involving a transparent substrate, with opaque films thereon, is the art of optical filters for use with neon or glow discharge tube read-out devices (e.g. as a window for the enclosure containing the read-out device). At the present time, a plastic filter is used which consists of a layer of polarizing material laminated with a layer of phase rotating material. In operation, the ambient light (e.g. light originating externally to the read-out device enclosure) is polarized as it passes through the filter and then phase rotated by the phase rotating material. On reflecting from the inner surfaces of the enclosure back to the filter, it is rotated an additional 45 in the same direction as the first rotation to complete a full 90 rotation. Consequently, the polarizing filter blocks this reflected ing information. In the prior art, reflective masks have generally not been used. Also, the closest equivalents for the permanent storage of information are such things as punched cards and preprogrammed diode matrices; neither of which is sufficiently similar to the present invention to draw extensive comparisons.

Heretofore, in the integrated circuit art, semiconductor resistors have been created by doping a portion of an existing semiconductor substrate. However, such constructions have only been employed for very short conductor paths because of their relatively high resistance. The interconnections for integrated circuits have been previously made by vacuum deposition of various metals in a pattern over the substrate. Consequently, conduction path crossovers could not be created by the direct crossing of either two semiconductor conductors or two deposited conductors, but had to be created by the combination of a semiconductor conductor in the substrate, or a deposited conductor on the substrate, an intermediate silicon oxide or dielectric coating, and a subsequent metal deposition. By this method, crossovers have been obtained. In addition, though most semiconductor processing consists of various heating cycles for doping and/or oxidizing the surface of the semiconductor substrate, the vacuum deposition of a metal lead requires separate and distinct equipment and is a relatively expensive and slow process.

Similarly, capacitors have been previously created either by using the characteristic capacitance of a back biased P-N junction, thereby requiring the proper bias be applied to assure the junction is back biased at all times, or by the vacuum deposition of a metal to a nonconducting surface of the substrate, followed by the application of a thin film of dielectric such as silicon oxide, and a subsequent second vacuum deposition to create the second capacitor plate. In the P-N junction capacitor, the P-N junction must be back biased and the value of capacitance that can be obtained is low and is sensitive to both temperature and voltageslt is used, however, because it is easy to fabricate and requires no processing steps other than those normally used for creating P-N junctions in semiconductor devices.

At the present time, integrated networks of resistors and capacitors are not widely used, and when they are used, are constructed on a semiconductor substrate by using the methods described above. Such circuits have the limitations described above including the requirement of the vacuum deposition processing step.

in many other applications, it is desired to make the electrical connections to electronic devices which are deposited on, or created within, the surface of a substrate. By way of example, precision carbon resistors are commonly made by heating a non-conducting base or substrate in the presence of methane, which deposits a thin layer of carbon to the surface of the substrate. Electrical connections are conventionally made to these carbon resistors by crimped metal caps or by applying a conductive paste to the appropriate areas which is subsequently fired in an inert atmosphere. The crimped metal cap, relying only on mechanical pressure to assure electrical contact, is relatively unreliable, and neither the crimped metal cap nor the paste and firing method lend themselves to extremely small dimensional control and fabrication.

BRIEF SUMMARY OF THE INVENTION An object comprised of a substrate with a thin film of patterned silicon may be used for many new purposes and to achieve many new results, depending on the nature of the substrate and the thickness of the silicon film. The word substrate, as used herein, is used in the general sense to indicate some formof base substance or substances. By way of example, one embodiment of the present invention makes use of a glass or transparent plate as a substrate. Other embodiments may use ceramics, polished metals or other such materials. In its broadest sense, the word substrate may indicate a composite base structure, such as an integrated circuit chip comprised of various doped areas, platings and coatings. The term silicon as employed herein refers to a silicon containing various impurities such as oxygen, or silicon oxides up to percent by weight. In most instances these impurities are not intentionally added but result from the nature or type of process employed. Certain impurities may be added intentionally.

A transparent substrate with a thin film of silicon on one surface may be used as a filter over such things as glow discharge data readout devices. In such an application, the silicon film thickness would be chosen to allow the light from the device (normally a reddish light) to be easily seen, while greatly attenuating the two way reflection of lower wave length ambient light, thereby allowing the device to operate in a substantially dark background. Similarly, a transparent substrate with a coating of silicon may be used as a photomask in certain applications (e.g. manufacture of integrated circuits) by etching away portions of the silicon coating to form a pattern or image in the remaining silicon. In such a mask the silicon, by thickness control, could be made transparent to normal ambient light, but opaque to shorter wave lengths light which are operative to perform a function such as exposing a photoresist or emulsion. A glass substrate having a pyrolytically deposited silicon film might also provide an article for use in the fabrication of an improved photomask.

A reflective opaque substrate with a patterned coating of silicon is useful as a reflective mask or as a means of permanently storing information.

A substrate containing non-conducting areas, which are coated with silicon, can be used as a basis for fabricating a multitude of devices. By way of example, if the silicon coated areas are in a pattern located on a silicon oxide coating over an integrated circuit, such areas can be used as resistors and for circuit interconnections, by either doping the silicon so as to create a semiconductor resistor or by plating the silicon with a metal. Circuit cross-overs may be created by a further coating of silicon oxide over the plated metal, followed by another subsequent coating of silicon and metal. Capacitive elements may be fabricated as cross-overs, as previously described, having a substantial area, thereby creating a significant capacitance at the area of cross-over. Thus, using ordinary plating techniques and the deposition of silicon by such processes as pyrolytic deposition, additional circuits and circuit connections may be added to the surface of an integrated transistor circuits without having to use the vacuum processes required by the prior art. In addition, resistors, capacitors and networks of these components may be created on other substrates, such as a ceramic chip, to create hybrid passive networks.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a transparent substrate with a thin coating of silicon on one surface.

FIG. 2 shows a cross-section of a transparent substrate with a patterned coating of silicon on one surface, which may be used as a photo mask.

FIG. 3 shows a cross-section of a transparent substrate with a coating of silicon and an intermediate coating to substantially eliminate the reflection encountered at the interface of the two transparent materials.

FIG. 4 shows the location of the mask, the semiconductor wafer and the light sources prior to contact printing a semiconductor device.

FIG. 5 shows a substrate with a resistor, a capacitor and a circuit cross-over placed on a surface of the substrate.

FIG. 6 shows a cross-section of the resistor of the circuit shown in FIG. 5 taken along line 6-6.

FIG. 7 is a cross-section of the capacitor of FIG. 5 taken along line 7-7.

FIG. 8 is a cross-section of the circuit cross-over of FIG. 5 also taken along line 7-7.

FIG. 9 shows a carbon film resistor with electrical contact with the film made by a plated silicon film.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 is a cross-section of a flat transparent substrate 10 with a thin coating of silicon 12 on one surface. The substrate material may be a relatively inexpensive material and, if pyrolytic deposition techniques are used to deposit the silicon film, should be capable of surviving, without detriment, processing temperatures of 500 to 550C. Many glasses are well suited to such an application.

The thin silicon film 12 may be created by any of the common techniques, such as pyrolytic deposition, vapor deposition or chemical deposition. If either vapor deposition or chemical deposition techniques are employed, the substrate need not have the temperature capability previously mentioned. However, of particular interest for some applications are silicon films which are pyrolytically deposited to various substrates. Such films are created by heating the substrate in either a silane environment or an environment composed of a mixture of silane and an inert gas. When so heated, the silane decomposes in the presence of the substrate and silicon is deposited on the surface thereof. For a pure silane atmosphere, the rate of deposit is entirely dependent on the temperature of the substrate and time employed unless a second order control, such as control of the amount of silane-available, is employed. If a mixture of silane and an inert gas is employed, then by controlling the percentage of silane in the inert gas, controlling the flow so that all surfaces for deposition are exposed to the same amount of controlled mix, and by having the surface temperature high enough to cause rapid deposition, the amount of deposition will be at the same rate over all surfaces, even though there are some variations in surface temperature. Thus, controlling the mix and flow and also the temperature gives a very fine overall control of the rate of silicon film buildup, resulting in the deposit of a very uniform film of the desired thickness.

A thin (e.g. in the order of 50 to 1,500 Angstroms) film of silicon is relatively transparent to visible light of the longer wave lengths, such as red, orange and yellow light, but is essentially opaque to the shorter wave lengths radiation such as ultraviolet light. However, the amount of light of any particular wave length that will be transmitted through the silicon film depends on the thickness of the film. By way of example, films of some thicknesses will exhibit a maximum reflectivity of incident light and will transmit a minimum amount of the incident light (of a particular wave length) while films of other thicknesses will exhibit a minimum reflectivity and will transmit a maximum amount of the incident light. These films are referred to herein as one-quarter wave length and one-half wave length films and are described in some detail below.

When' light passes from a medium of one index of refraction to a medium of another index of refraction, a portion of the incident light is reflected from the interface between the two mediums. For instance, when light is directed through air perpendicular to the surface of a transparent material, a fraction of the incident light will be reflected in accordance with the following equation:

where r equalsthe fraction of incident light that is reflected and n is the index of refraction of the transparent material.

When substantially monochromatic light passes through a medium of one index of refraction and is directed normal to the surface of a thin film of transparent material of a second index of refraction (such as a silicon film) on the surface of a medium of a third index of refraction, the net reflected light depends not only on the index of refraction of the mediums, but also on the thickness of the thin film. If the film thickness is substantially equal to one-quarter of a wave length for the light being considered (one-quarter of a wave length for the light in the film, not in air or free space) then the following will occur. Some of the light passing through the first medium will be reflected upon first striking the thin film (e.g. the second medium) and some of the light will be transmitted through the thin film. Of the light passing through the film, part will be reflected from the interface between the film and the third medium and will again pass through the film to be partially reflected by the interface between the first medium and the film and partially to pass into the first medium. Though an exact analysis of the total reflected light would require a consideration of the endless reflections that occur at the interfaces of the material (as well as the light absorption characteristics of the materials) generally the major components of reflected light in the first medium is that that was initially reflected from the interface between the first medium and the second medium and that that was initially reflected from the interface between the second medium and the third medium. This second component has traveled twice through the one-quarter wave length film (once through and once in being reflected back) and thus is 180 out of phase (e.g. shifted one-half a wave length) with the light that was initially reflected from the interface between the first medium and the second medium. Consequently, to the extent that the two reflected components are equal, destructive interference results, thereby substantially reducing the 'net amount of reflected light in the first medium. It can be also shown that such a result is obtained if the film thickness is any odd multiple of one-quarter of a wave length. Such films are referred to herein as one-quarter wave length films and are well known in the field of optics.

In addition, since the reflection from either interface depends on the index of refraction of the mediums, the two reflected components of light in the first medium may be made to substantially equal to each other if the index of refraction of the one-quarter wave length film is properly chosen. ln such a case, the net reflection is essentially reduced to zero. These films are also well known in the field of optics and are referred to herein as one-quarter wave length non-reflective films.

One further type of film is referred to herein and, for purposes of clarity, is presently described. If the film thickness is made substantially equal to one-half the wave length of the light used (or odd multiples of one half the wave length) then the two reflected waves in the first medium will be in phase and will reinforce each other. Consequently, the net reflection from such a film is maximized. Also, if the absorption of the film is low, then the sum of the reflected light and the transmitted light must be substantially equal to the incident light. Therefore, since the net percentage of light'that is reflected from such a film is maximized, the percentage of light that is transmitted through the film is minimized (e.g. minimized for that wave length light). Such films are also well known in the field of optics and are referred to herein as one-half wave length reflective films.

Returning now to FIG. 1, the first embodiment of the present invention may be further described. It was previously stated that theamount of light of any particular wave length that will be transmitted through the silicon film depends on the thickness of the film. For a silicon film thickness 12 of, say, a hundred Angstroms, the transmission though the film is limited by the reflections that occur at the interfaces and the absorption within the film and substrate. Therefore, the transmission of longer wave length light may be greatly enhanced by using a silicon film thickness equal to onequarter of the wave length of the light that is desired to be transmitted (e.g. a one-quarter wave length film as previously defined and described), since the amount of light of approximately that wave length that is reflected from the surface of such a silicon film is greatly reduced. Consequently, though some of the light that would have been reflected will be absorbed within the silicon film itself, much of it will be transmitted through the film. For instance, a silicon film thickness characterized by a green interference color" (believed to be as described above, is useful as a low pass light filter,

that is, as a filter which will pass light of the desired wave length but will not pass substantial amounts of light of the shorter wave lengths as described above. Such a filter might be used as a covering for an enclosure containing neon glow discharge tubes or solid state electroluminescent data readout devices which emit radiation of a wave length approximately equal to that of red light. In such an application, the data readout device will be easily visible through the transparent substrate and silicon film. However, the majority of ambient light, either natural or artificial, is normally of a shorter wave length. Consequently, much of this light will not pass through the silicon coated substrate (most of that which does pass through will be absorbed by the walls of the enclosure and much of that which is reflected from the walls will be re-reflected from the coated substrate rather than to pass to the eyes of a person viewing the readout device) and thus, the data readout device will operate in an essentially black background.

Such a filter has advantages over the presently used filters in that the silicon coating is extremely hard and wear resistant. In addition, these filters are relatively rigid, strong and suitable for use where a hermetic seal is to be made between the filter and the other parts of the enclosure. One method of creating such a hermetic seal is to selectively metal plate the silicon surface around the edges of the filter and then to create the hermetic seal by such means as soft soldering or brazing (e.g. commonly referred to as hard soldering) the metal plated surface to the metal enclosure. The silicon surface may be plated by any of various means, such as the electroless plating of nickel to the desired surfaces of the silicon, or by doping the silicon in selected areas by conventional semiconductor manufacturing techniques so as to lower the resistance of the silicon coating and thereafter electroplating with one or more metals to the doped silicon surface. If pyrolytic deposition techniques are used, doping may be accomplished during the deposition by using a silane atmosphere containing the desired amounts of gaseous dopants or compounds of such dopants. Peeling of the plating may be prevented by a subsequent heating so as to diffuse some of the metal into the silicon coating. It is to be understood that the partial diffusion of the metal into the silicon film includes the diffusion of the metal so completely into the silicon that the result has the appearance of a homogenous material.

It is to be understood that the method of creating a hermetic seal as described above is not limited to light filters. Such hermetic seals might also be used for other purposes, such as to seal electrical feed throughs or covers in the manufacture of precision instruments. In 7 such applications, the materials constituting the substrate (the covers, feed throughs, etc.) need not be transparent, but may be essentially any material of a reasonable thermal expansion rate and to which the silicon coating will adequately adhere, such as most metals, glasses and ceramics. The advantage of such seals over conventional fused glass to metal seals is that the ultimate heremetic seal may be created by any metal joining process such as a relatively low temperature soft soldering process rather than a high temperature glass fusing process.

FIG. 2 is a cross-section ofa transparent substrate 10 with a patterned silicon coating 14 on one surface. The patterned coating on the transparent substrate may be created from the silicon coated substrate of FIG. 1 by such techniques as conventional photoctching techniques. To obtain the pattern, the silicon coating would be first coated with a photosensitive film and then exposed to a light source through an appropriate photomask, so as to expose portions of the photosensitive surface. Such exposure fixes the exposed areas of the photosensitive film, and a subsequent immersion in an appropriate developing solution will dissolve away the unexposed areas of the photosensitive film. Immersion of the part (with the developed photosensitive material thereon) into a suitable etchant will remove the areas of silicon which are not coated with the fixed photosensitive material. (One etchant that has been used forthis purpose is a solution comprised of 1 part HF (reagent), 6 parts nitric acid (reagent), 2 parts H O (deionized) and 4 parts of a solution comprised of 84.1 grams FeCl;,6H O in c.c. solution with H O (deionized). Finally, immersion in appropriate solvent will remove the fixed photosensitive material, leaving the substrate with a pattern of silicon on the surface as shown in FIG. 2. The resulting coating is referred to herein as a patterned silicon coating.

Such a transparent substrate and a patterned coating of silicon is useful as a photomask in processes where the photosensitive material is sensitive to short wave length radiation, such as ultraviolet light, and where the silicon coating is essentially opaque to such radiation. This type of mask has advantages over both emulsion masks and chrome masks in that the silicon pattern is exceptionally hard, durable and wear resistant, and is much cheaper to fabricate than are chrome masks.

In addition, a silicon film mask as shown in FIG. 2 has a characteristic unobtainable with either the emulsion masks or the chrome masks. The silicon film, though being essentially opaque to shorter wave length radiation such as ultraviolet radiation, is transparent to much of the light in the visible range. Consequently, a person viewing the photosensitive surface through the silicon film mask may see the entire photosensitive surface, including that part of the surface lying beneath the silicon.

It has been found in practice that two factors detract from the clarity of the photosensitive surface as viewed by a person under light in the visible spectrum. In use as a photomask, the mask of FIG. 2 would be placed over the photosensitive surface with the silicon pattern 14 resting on the photosensitive surface. To view the photosensitive surface, visible light would be directed to as to pass from the surrounding air, through glass substrate 10, through silicon pattern 14, through a thin film of air to reflect off the photosensitive surface, thereupon returning through air, the silicon pattern 14, the glass substrate 10 and through the air to the viewer. However, when light passes from a medium of one index refraction to a medium of a second index refraction, acertain percentage of the light is reflected at the interface between the two mediums. By way of example, approximately 4 percent of the initial light impinging on the glass substrate 10 is reflected at the air-glass interface. Similarly, approximately I5 percent of the light passing through the glass is reflected from the glass-silicon interface and approximately 30 percent of the light passing through the silicon is reflected from the subsequent silicon-air interface. These reflections detract from the clarity and contrast of the photosensitive surface as viewed under visible light for two reasons. First, normally it is desirable to have a silicon film thickness which is approximately equal to a quarter of a wave length for orange or yellow light, since the photosensitive surface will normally be viewed under such light. Such a film is approximately 830 Angstroms thick and appears plum colored with reflected lights. This one-quarter wave length thick silicon film effectively performs as a non-reflecting film as described before and will substantially reduce the net reflection of light in the lower wave length region of the visible spectrum, such as red, orange and yellow light. However, for shorter length visible light, such as blue light, the components of reflected light from the two film surfaces are not substantially out of phase, consequently there is less cancellation by interference, with a substantial net reflection of these wave lengths. Consequently, most of the reflected light from such a film, under ordinary lighting conditions, will be of the shorter wave lengths such as blue light and will appear as a hazy overcast to the photosensitive surface. (In addition, the absorption of the silicon for these shorter wave lengths further detracts from the effectiveness of the film as a nonreflective film for blue light since the component of reflection that passes twice through the film is greatly attenuated and emerges too small to accomplish substantial destructive interference). Since this color is not within the range of wave lengths needed to be used for viewing the wafer, it can easily be removed by a filter that passes only the 'red-orange-yellow spectrum, or by a multilayered interference filter that passes all visible wave lengths longer than the blue region. This latter filter is more effective than the colored filters as it allows more useful light to be used for viewing. This filter could be incorporated in the mask construction by depositing the appropriate multiple films. However, it is more practical to use one filter per alignment machine or viewer than to reproduce the filters on each mask.

The reflection at the air-glass interface can be eliminated in the red-yellow wave length range by coating the back of the glass substrate 10 with a similar onequarter wave length anti-reflective coating for that wave length range. However, since this reflection was only approximately 4 percent, it would not be a significant enough improvement to warrant the expense in most cases. Also, in some applications and/or with some light sources, it may be desirable to use an antihalation backing to prevent fogging due to light reflection and dispersion within the silicon film. Such backings are well known in the field of photography and will not be described herein.

The l5 percent reflection at the interface of the glass-silicon interface could be significant enough to be undesirable, and if so, this reflection may be substantially eliminated by an intermediate interface of either of two types. One type is to place a one-quarter wave length film between the glass and silicon, this film having a refractive index approximately equal to the mean difference between refractive indexes for glass and silicon (e.g. equal to the square root of the product of the refractive indexes of glass and silicon). With such a film, essentially complete destructive interference results between the reflection from the two interfaces. The other type is an inhomogenous film which is applied to the glass to which the silicon is subsequently deposited. As this intermediate film is first being applied, it is composed of materials of a refractive index closely matching glass. As the intermediate film builds up, the refractive index is then linearly, or in multiple discrete steps, changed so that on completion of the film the refractive index of the last deposited material closely matches that of silicon. This may be done by compounding two materials in a changing mix ratio as the film is deposited; one material closely matching the index of refraction of glass and the other material closely matching the index of refraction of silicon. By way of example, one method of creating such a film is to first deposit silicon trioxide on the glass, then silicon dioxide, then silicon monoxide and finally just silicon. Such materials may be deposited by conventional vapor deposition techniques.

in addition, the silicon-air interface can be made anti-reflective by the same technique (that is by the application of a suitable quarter wave length thick coating to the surface of the silicon), but such a film would cause some loss of resolution in the printed image due to the spacing it would introduce between the silicon pattern and the photosensitive surface. Normally, this distance is made as small as possible by employing a vacuum between the mating surfaces of the mask and the photosensitive surface and/or the use of pressure on the face of the mask. Therefore, the gain from such a film would be offset somewhat, depending on image sizes, and the extent of collimation of the light source by the reduction in image resolution. Such anti-reflective coatings are well known in the field of optics and may be applied to the various reflective surfaces as the particular application allows or requires.

If the mask is intended for a projection type of use wherein the pattern is projected onto a surface by a lens system, the one-quarter wave length antireflective coating could be of significant value. In such an application, the silicon mask has the additional advantage of high resistance to the elevated mask temperature caused by high intensity illumination systems, as compared with conventional emulsion masks. A mask with various anti-reflecting films is fabricated starting with a suitable substrate coated with the appropriate films which are subsequently etched in a pattern to create the mask. By way of example, FIG. 3 shows a substrate 10 with a silicon coating 12 and an intermediate anti-reflective coating 16. The subsequent photo etching of the surface will remove portions of the silicon coating 12 and, if desired, corresponding portions of the anti-reflective coating 16, thereby creating a patterned silicon coating, beneath which lies an antireflective coating. in some instances a different etchant may be required for each coating.

In the preceding explanation, the advantages ofa silicon coating that is transparent to visible light and particularly the methods of achieving and utilizing this result were described in detail. However, it is to be understood that the hardness, wear resistance, and use of fabrication of silicon masks give these masks substantial advantages over the prior art masks even without obtaining (or making use of) the transparency of the silicon film to visible light. By way of example, a silicon mask which does not have any of the various antireflective coatings and its transparency to visible light is not utilized is useful because of its advantages of long life,

low cost and low pin hole density in the silicon coating. Similarly, a silicon mask in which the silicon is substantially opaquebecause of such things as the presence of impurities in the silicon will also have the above advantages over the prior art masks. (In some applications such impurities might intentionally be added so that the mask would have a visual appearance similar to prior art masks).

FIG. 4 is a schematic diagram of a contact printing set up, such as used in semiconductor device manufacture, using a silicon mask of the present invention. The specific silicon mask shown has an intermediate antireflective coating as was shown in FIG. 3 (details of the pattern are omitted for clarity). The photosensitive surface 20 is a thin film of photo resist applied to the surface of a semiconductor wafer 18. The-photo resist 20 is sensitive to ultraviolet light but is relatively insensitive to light in the visible range. The photo mask is positioned over the photosensitive surface 20 with the silicon image 12 in contact with the photo resist. To assure a high quality printed image, normally a mask is held firmly against the semiconductor wafer by a vacuum between the wafer and the mask and/or by applying pressure to the back surface of the mask. Because of these pressures, conventional emulsion masks are frequently damaged due to the softness of the emulsion. However, the silicon is well suited to such use since it is extremely hard and wear resistant, and may be deposited and etched by relatively inexpensive processes to yield exceedingly fine patterns of material having a low pin hole density.

The mask and photo sensitive surface may be illuminated by either visible light or by ultraviolet light. Light bulbs 19 are sources of visible light and filters 21 are used to filter this light so as to substantially eliminate the wave lengths that cause the hazy overcast previously described. This light source allows an operator to view the entire semiconductor wafer, for pur poses of alignment and inspection.

Light source 22 is a source of ultraviolet light and is then used to expose the photosensitive surface 20 when the mask and wafer are properly aligned. Such a silicon mask may be repeatedly used without substantial wear and deterioration because of its hardness and resistance to wear. In addition, such masks may be made to be essentially free of pin holes since the pyrolytic deposition of silicon by the decomposition of silane results in an extremely uniform and pin hole free coating.

As an alternative embodiment of the present invention, the patterned silicon film may be applied to a reflective substrate and used as a reflective mask (for the wave lengths of light for which the light absorption of the silicon is high such as ultraviolet light.) In such an application, the reflection from the surface of the silicon pattern can be substantially eliminated by applying a one-quarter wave length non-reflective coating to the surfacepfthe silicon. If the pattern is created by the conventional photoetching technique described earlier, the etchant and the etching times should be selected so as to not unduly etch the substrate and substantially detract from its reflective qualities. Such a reflective mask could be used where optical enlargement or reduction of the image was desired by passing the light reflected from the mask through a suitable lens system.

In addition, objects of the construction of the transparent mask or the reflective mask may be usedl'for the permanent storage of information or as a key or identification means. The storage means may be accomplished by various schemes, one scheme being the storage of data bit information as is done on punched cards in the prior art.

As a key or identification means, the code or identification would not be visible. to the eye under normal lighting but could be optically read by exposure to ultraviolet light and the viewing the pattern by electrooptical means or by visually viewing the pattern through an appropriate apparatus.

FIG. 5 shows a top view of a substrate 24 with a resistor 26, a capacitor 28 and a circuit crossover 30 created by the deposition (and processing) of various materials to the surface of the substrate 24. Though the particular circuit shown is a very simple circuit, it is to be understood that, by the techniques of the present invention, extremely small and complex circuits containing resistors, capacitors and/or circuit connections and crossovers may be created on a small non-conducting substrate or on a non-conducting surface of a composite substrate such as an integrated semiconductor circuit.

FIG. 6 shows a cross-section of the resistor 26 of FIG. 5 taken along line 6-6 of that figure. In this figure a dashed line 34 near the top surface of substrate 24 is to indicate that the substrate may be of a composite structure, provided that at least the surface of the substrate 24 is non-conducting (e.g., is an insulator such as a silicon oxide). To the surface of substrate 24 is first applied a silicon coating 38. This coating may be applied by any of the conventional techniques, such as vapor deposition. However, of particular interest, for

reasons which will subsequently become apparent, is

the pyrolytic deposition of silicon to create the coating 38. This is achieved by the heating of the substratein the presence of silane, thereby decomposing the silane and depositing the silicon from the silane so decomposed. This process is presently commonly used in semiconductor device manufacture. As employed herein, this process is used to deposit a silicon film, generally polycrystalline in form, in the non-conducting (insulative) surface of a substrate. Generally, this deposit will not be a crystal for crystal extension of the substrate lattice, and, in general, the substrate itself may be polycrystalline in form, such as an ordinary ceramic. If the substrate is a silicon semiconductor device, what is contemplated is not a crystal for crystal extension of the substrate lattice but rather the deposit of a silicon film upon an insulated coating, such as a silicon oxide coating, over the semiconductor.

After the silicon coating has been deposited, a selected impurity may then be diffused 'into the central portion of silicon film 38 so as to form the semiconductor resistor 26. Such diffusion techniques are well know to the art of semiconductor manufacture. In other selected areas, metal is plated over the surface of the silicon film 38, so as to form the circuit connections-40. This metal may be plated to the silicon by electroless plating techniques or by doping the silicon as was done to form resistor 26 and electroplating the selected areas. It has been found that by subsequent heating, a plating on the surface of the silicon film may be partially diffused into the silicon thereby making it virtually impossible to peel off. The net result is a semiconductor resistor with electrical connections that has been created on an insulator, rather than within a semiconductor substrate, and which has been created without the use of a vacuum process. The precision of the resistor is largely determined by the precision of the etching process and the control of the diffusion process. The largest single advantage of this new technique is that the processes involved are substan-' tially the same as those presently used to manufacture semiconductor devices, and without additional equipment, complex passive circuits may be deposited on the surface of the insulative coating on a semiconductor device and connected to and made part of the semicon ductor circuit by the same techniques.

FIG. 7 is a cross-section of a capacitor of FIG. taken along line 77 of that figure. The construction and many of the construction techniques are the same as those for the resistor of FIG. 6. The first step is to deposit a patterned silicon layer 42 to the nonconductive coating of the substrate 24. To this coating a metal layer is plated which may, if desired, be partially diffused into the silicon. Then an insulating layer 46 is deposited, such as silicon oxide or silicon nitride. These insulating layers may also be pyrolytically deposited as are the silicon films. Then a further silicon film 48 is deposited and finally metal film 50, which forms the top plate of the capacitor and the electrical connection to this top plate. By keeping silicon film 48 relatively thin and/or by diffusing part of the metal film 50 into the silicon film, the thickness of dielectric is principally determined by the thickness of the insulating film 46.

It is common practice in the semiconductor fabrication art to provide a final layer on otherwise exposed silicon surfaces to serve as an environmental protection layer. Layers of silicon monoxide and silicon dioxide are commonly used for the purpose and may be created by heating the device in an oxidizing atmosphere. It is to be understood that any of various protective layers, well known in the prior art, can be used in conjunction with the devices of the present invention, but such coatings have not been otherwise described or pictured in the drawings for clarity only.

FIG. 8 is a cross-section of a circuit cross-over of FIG. 5, also taken along line 7-7 of that figure. The construction of the cross-over may be essentially identical to the construction of the capacitor shown in FIG. 7, with the exception that the area of the crossover is minimized for a circuit cross-over whereas this area is maximized if a capacitor is desired. The films shown in FIG. 8 are a first silicon film 52, a second film of metal 54, a third film of insulating material 56, a fourth film of silicon 58 and finally a top film of metal 60 forming the cross-over conduction path. Each of these films, of course, is formed in an appropriate pattern to create the desired end result. Also, it is to be understood that circuit crossovers consisting ofa conduction path crossing a resistor or resistor crossing a conductor may be readily fabricated by the techniques previously described.

FIG. 9 is a cross-section of a carbon film resistor. Such resistors usually consist ofa thin film of carbon 70 pyrolytically deposited to a non-conducting substrate 72 by the high temperature decomposition of methane.

A high reliability electrical contact can be made to such deposited films by applying a silicon film 74 to each end of the non-conducting substrate 72 (preferably by pyrolytic decomposition of silane, thereby allowing the use of the same deposition equipment as is used for the deposition of the carbon film). To the silicon films 74 is applied one or more metal platings 76 in the same manner as previously described (in the preferred embodiment the first is partially diffused into the silicon film as previously described.) Now the deposition of a carbon film on the center portion of the substrate 72, in a manner so as to overlap the metalized silicon ends, forms a positive electrical contact between the film 72 and the metalized ends. This electrical contact may be established by the physical contact of the carbon film 70 and the metalizedsilicon ends, or may result from the partial diffusion of the silicon, metal and/or carbon films. Permanent leads 78 may be readily attached to these metalized ends using conventional thermal compression, ultrasonic or soldered connections. An insulative coating 80, such as an epoxy coating, provides the desired physical protection and electrical insulation of the device. As with the prior devices fabricated by the use of a deposited silicon coating, an advantage of the above described method of making electrical contact, as opposed to the prior art methods, is that a high strength and high reliability connection can be made to the resistive film using substantially the same processes as are used to create the resistive film itself, thereby reducing the equipment and simplifying the production processes for such devices. In alternative embodiments, the carbon film can be first deposited and then the silicon and metal deposited over the carbon, or the carbon film may be an intermediate film between films of silicon and/or metal. Electrical contact between the carbon film and the metal films may be achieved by physical contact between the carbon film and the metal film, or in the alternative may be achieved by the partial diffusion of the metal through the silicon film to effectively make electrical contact to the carbon film.

Iclaim:

1. A photomask comprised of a transparent substrate and a pyrolytically deposited patterned silicon film on one surface of said transparent substrate, said silicon film being of a thickness between approximately 50 angstroms and 1,500 angstroms.

2. The photomask of claim 1 including a layer of photosensitive material over said silicon film.

3. The photomask of claim 1 wherein said photomask is also comprised of an additional film, said additional film being located between said transparent substrate and said silicon film and further being a onequarter wave length film for a wave length of light within the visible light spectrum.

4. The photomask of claim 3 wherein said photomask is also comprised of one or more onequarter wave length films, for a wave length of light within the visible spectrum, on one or' both surfaces of said photomask.

5. The photomask of claim 4, wherein said onequarter wave length films are substantially one-quarter wave length non-reflecting films.

6. A photomask comprised of a glass substrate with a patterned film of pyrolytically deposited silicon on the surface thereof, said silicon film being of a thickness which is substantially plum colored when viewed under normal lighting conditions.

7. A light filter comprised of a transparent substrate, a silicon film on one surface of said substrate, and a layer of metal on selected areas of said silicon film partially diffused into said silicon film, mounted to an enclosure having metal surfaces disposed adjacent at least a part of said layer of metal, said metal surfaces being attached to the adjacent said layer of metal by an intermediate layer of solder.

8. The light filter of claim 7 wherein said solder between said metal layer and said metal surfaces create a hermetic seal.

9. The light filter of claim 7 wherein said layer of metal is a layer of electroless nickel. 7

10. The light filter of claim 9 wherein said silicon film is a pyrolytically deposited silicon film.

ll. The light filter of claim 10 wherein said electroless nickel layer is partially diffused into said silicon 12. The light filter of claim 11 mounted to an enclo- ,Z sure having metal surfaces disposed adjacent at least a C 6 part of said layer of electroless nickel, said metal sur- I faces being attached to the adjacent said electroless nickel layer by an inten'nediate layer of solder.

13. An article for use in the fabrication of an improved photomask comprising; a glass substrate having a a pyrolytically deposited silicon film on one surface thereof, said silicon film being of a thickness which is substantially plum colored when viewed under normal lighting conditions.

Claims (12)

1. 2. The photomask of claim 1 including a layer of photosensitive material over said silicon film.
2. 3. The photomask of claim 1 wherein said photomask is also comprised of an additional film, said additional film being located between said transparent substrate and said silicon film and further being a one-quarter wave length film for a wave length of light within the visible light spectrum.
3. 4. The photomask of claim 3 wherein said photomask is also comprised of one or more one-quarter wave length films, for a wave length of light within the visible spectrum, on one or both surfaces of said photomask.
4. 5. The photomask of claim 4, wherein said one-quarter wave length films are substantially one-quarter wave length non-reflecting films.
5. 6. A photomask comprised of a glass substrate with a patterned film of pyrolytically deposited silicon on the surface thereof, said silicon film being of a thickness which is substantially plum colored when viewed under normal lighting conditions.
6. 7. A light filter comprised of a transparent substrate, a silicon film on one surface of said substrate, and a layer of metal on selected areas of said silicon film partially diffused into said silicon film, mounted to an enclosure having metal surfaces disposed adjacent at least a part of said layer of metal, said metal surfaces being attached to the adjacent said layer of metal by an intermediate layer of solder.
7. 8. The light filter of claim 7 wherein said solder between said metal layer and said metal surfaces create a hermetic seal.
8. 9. The light filter of claim 7 wherein said layer of metal is a layer of electroless nickel.
9. 10. The light filter of claim 9 wherein said silicon film is a pyrolytically deposited silicon film.
10. 11. The light filter of claim 10 wherein said electroless nickel layer is partially diffused into said silicon film.
11. 12. The light filter of claim 11 mounted to an enclosure having metal surfaces disposed adjacent at least a part of said layer of electroless nickel, said metal surfaces being attached to the adjacent said electroless nickel layer by an intermediate layer of solder.
12. 13. An article for use in the fabrication of an improved photomask comprising; a glass substrate having a a pyrolytically deposited silicon film on one surface thereof, said silicon film being of a thickness which is substantially plum colored when viewed under normal lighting conditions.

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