US3622365A

US3622365A - Process of forming an arsenic sulfide mask

Info

Publication number: US3622365A
Application number: US722281A
Authority: US
Inventors: William I Lehrer
Original assignee: Fairchild Camera and Instrument Corp
Current assignee: Fairchild Semiconductor Corp
Priority date: 1968-04-18
Filing date: 1968-04-18
Publication date: 1971-11-23
Anticipated expiration: 1988-11-23

Abstract

A process for forming masks employed in the manufacture of devices by photofabrication wherein arsenic sulfide is employed as one of the masking or lifting materials. Arsenic sulfide is soluble in solutions such as ammonia water while other masking or lifing materials which may be employed in such processes are soluble in acids or organic solutions. The arsenic sulfide when melted forms a razor sharp edge that can be superior to that of the original master mask.

Description

United States Patent William 1. Lehrer Los Altos, Calif.

Apr. 18, 1968 Nov. 23, 1971 Fairchild Camera and Instrument Corporation Syosset, Long Island, N.Y.

Inventor Appl. No. Filed Patented Assignee PROCESS OF FORMING AN ARSENlC SULFlDE MASK 9 Claims, 7 Drawing Figs.

US. Cl ll7/5.5, 96/383, 117/8, 117/8.5, 117/37 R, 117/38, 117/124A, 117/1243 Int. Cl B44d l/52, B44d 1/02 Field olSearch 117/38,

212,106, 5.5, 8.5, 8,106, 37 R, 124 A, 124 B; 96/36, 36.2, 38.3, 38.4; 156/17 [56] References Cited UNITED STATES PATENTS 2,923,624 2/1960 Hensler 1 17/5.5 X 2,995,461 8/1961 Boicey et a1. 1 17/5.5 3,115,423 12/1963 Ashworth 1 17/5.5 X 3,376,139 2/1968 Giangualano et al. 96/362 3,423,237 1/1969 Hutchinson 1 17/106 X OTHER REFERENCES M. C. Sneed and R. C. Brasted Comprehensive inorganic Chemistry Vol. 5, D. Van Norstrano Co., N.Y., 1956, p. 151

Primary Examiner-Ralph S. Kendall Assistant ExaminerC. K. Weiffenbach Altorney- Roger S. Borovoy PROCESS OF FORMING AN ARSENIC SULFIDE MASK BACKGROUND OF THE INVENTION This invention relates to a process for forming devices by photofabrication and to a process for forming masks employed in such processes.

In many arts, such as the electronic art and semiconductor art, a film or layer of material is fonned into a predetermined configuration on a selected surface. For example, after forming a monolithic circuit, it is necessary to make electrical contact with the numerous elements or regions of the device. This may be accomplished by forming a predetermined configuration of contacting material over the surface of a monolithic device. In other instances, it is desired that a predetermined configuration in the form of a narrow cut in a passivating layer of material (e.g., silicon monoxide) be made incident to forming a device in or on a substrate. In other instances, independent masks (that is, masks not on the surface of an actual device) are formed for employment in exposing and processing of an actual device. All of these various applications may involve the formation of a film or layer of material into a predetermined configuration.

Predetermined configurations of a film of material have been formed by etching, by lifting, or by a combination of these techniques. Etching involves the selective chemical dissolution of undesired regions of the film material from the surface. Certain materials are impractical or very difficult to etch, (e.g., Ba Ti Nichrome). In addition, etching, being a diffusion limited process, always undercuts to some degree the film, thereby reducing the accuracy of the process. For example, a one micron diameter opening in a deposited oxide film, or a two by two micron aluminum square, cannot be readily formed by etching. Also, the reliability of the etching process in part depends upon the adherence of a photoresist layer to the film material being etched. This adherence cannot be readily checked so that the application of the etchant to the photoresist can be a hit and miss operation.

The lifting process on the other hand involves depositing the film of material partially upon a surface and partially upon a lifting material which is formed in a pattern on the surface. The lifting material is then removed along with the film material deposited thereover leaving a pattern of the film material on the surface desired. Organic lifting materials and metal film lifting materials are commonly used. The organic materials have limited temperature application since they generally decompose at elevated temperatures, that is, temperatures greater than 300 C. Organic lifting materials also have the shortcoming of being limited in the thickness of the layer that may be lifted, that is, thickness in excess of several thousand angstroms are very difiicult to lift. Organic lifting materials also pose a contamination problem when employed in vacuum systems. The metal film lifting materials usually have the shortcoming of microalloying or diffusing into the substrate at temperatures greater than 400 C., which alters the property of the film and in addition may alter the property of the semiconductor junction, if employed in the particular device.

One recent significant improvement in the lifting processes involves the use of halides, calcium flouride in particular, as a lifting material. This procedure is described in US. Pat. application Ser. No. 509,825, now abandoned, filed on an invention of William I. Lehrer, and assigned to the same assignee as this application. As described in that application, Lehrers procedure comprises forming a pattern of photoresist material on a surface of a device, forming a layer of a Group II (A) fluoride over the photoresist; removing the photoresist pattern thereby to leave a pattern on the device surface outlined by fluoride; depositing a metal film over both the fluoride and the pattern; and removing the fluoride to leave the metal film on the surface of the device in the pattern of the photoresist material. In one embodiment, Lehrer discloses the use of calcium fluoride as the Group II (A) fluoride lifting material. The use of materials such as calcium fluoride overcomes many of the disadvantages of previous metal and organic lifting materials. For example, the metal halides, such as calcium fluoride, decompose at high temperatures (e.g., temperatures in excess of I,300 C.), and do not afiect the electrical and integrated circuit arts, and they do adhere to common substrate materials. Most importantly with respect to the present invention, the halides may be lifted by dilute acids and they are not affected by solutions commonly employed with photoresist images (alcohols, ketones, etc.

As effective as the halides are for lifting materials, they do have the disadvantages that the lifting technique results in edge definition that limits the state of the art, and they are acid soluble; therefore an acid cleaning dip prior to further depositions cannot be performed. The invented process solves these problems and has all of the advantages incident to the lifting process employing the halides and in addition, possesses many other advantages over the prior art processes.

SUMMARY OF THE INVENTION In a process for forming devices by photofabrication, a layer of arsenic sulfide is formed on a surface and is formed into a pattern. The layer of arsenic sulfide may be melted. The formed pattern of arsenic sulfide may be used as a masking or lifting material.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. la illustrates a device or substrate surface;

FIG. lb illustrates a surface with a layer of material such as a photoresist;

FIG. 10 illustrates the developed photoresist;

FIG. ld illustrates the developed photoresist with a layer of arsenic sulfide thereover;

FIG. 1e illustrates FIG. M with the photoresist material removed and the pattern of arsenic sulfide formed;

FIG. If illustrates the pattern of arsenic sulfide of FIG. 16 with an additional film of material thereover;

FIG. lg illustrates the product of FIG. If with the arsenic sulfide material removed and has a film of material formed into a pattern;

Detailed Description of an Embodiment of the Invention At the outset, it should be understood that the various steps as described hereinafter may be arranged in a number of different sequences and various steps may be omitted consistent with employment of the invented process. For example, if the process employs a photoresist etching step, a halide material lifting process, and the invented process, the three may be combined in at least nine (9) different ways and considering that all three are not absolutely necessary, numerous other processes may be derived therefrom. In addition, while the invented process is described with reference to electronic devices and semiconductor technology, in its broadest aspects it has general application to the photofabrication of devices regardless of the end use of the particular device. The term mask, as used herein, refers to masks actually formed on the device as a result of physical or chemical alteration of the device being formed as well as masks which are independent of the device and employed to expose a photoresist or other similar material.

FIG. Ia shows a device or substrate 10 having a surface 12. The finally formed device 10 may be a mask, a semiconductor device, such as an integrated circuit, a metal part, a plastic part, or a glass surface or other similar surface. In accordance with the invention, a layer of arsenic sulfide is to be formed into a pattern on surface 12. This may be accomplished by etching or lifting material or a photoresist. If lifting is selected, it is desirable to employ a lifting method such as shown in pending patent application Ser. No. 509,825, now abandoned invented by William I. Lehrer, and assigned to the same assignee of this invention. In that patent application, a halide such as calcium fluoride is employed to perform the lifting process. It should be noted that calcium fluoride is soluble in dilute acids such as dilute nitric acid, dilute sulfuric acid or dilute hydrochloric acid. In the embodiment shown in FIG. lb, a layer of photoresist I4 is first formed on surface 12. The photoresist material 14 may be a photoresist such as AZI350 (autopositive) marketed by the Shipley Corporation. The photoresist is applied in the form of a continuous layer by standard photoresist coating apparatus. The photoresist is then exposed and developed according to well-known photoengraving techniques leaving an image 18 of the photoresist material such as is shown in FIG. 1c.

The surface 12 and photoresist image I8 are then covered with a layer of arsenic sulfide (e.g., arsenic trisulfide, arsenic disulfide and/or arsenic pentasulfide). The arsenic sulfide material 20 shown in FIG. 1d may be deposited by standard vacuum deposition techniques employing ordinary vacuum deposition chambers having sources which operate at temperatures of about 700 C. The arsenic sulfide has excellent deposition characteristics, particularly since it does not tend to spatter before the deposition process begins which results in the forming of a uniform regular layer of material. The formed layer of arsenic sulfide is a relatively porous structure. In addition, it has the following characteristics and properties.

1. It does not affect the electrical characteristic of the semiconductor junctions;

2. It does not alloy with metals commonly employed in the electrical and integrated circuit arts;

3. It may be lifted or etched by basic solutions which do not affect the other components or materials associated with an integrated circuit or electrical devices;

4. It adheres to common substrate materials such as silicon,

glass, ceramics, etc.;

. It is not affected by solutions employed to remove photoresist images such as organic solutions;

6. It is not affected by solutions commonly employed to remove lifting materials such as the alkaline earth halides; and,

. It may be melted at 200 C. sealing pin holes and providing a very sharp edge resulting in excellent edge definition.

From the above properties, it should be noted that the photoresist, halides, and the arsenic sulfide films are etched or soluble in different solutions and consequently may be used together in a process with the removal of one material not affecting the removal of the other material. This provides a powerful versatility in processing.

Following the deposition of the arsenic sulfide material 20 (FIG. 1d the photoresist image 18 is removed by submerging the substrate 10 or submerging the portion of substrate 10 having photoresist image 18 and arsenic sulfide material 20 thereon in a solvent such as acetone. The application of the solvent removes the photoresist image 18 and the portion of the sulfide thereon leaving the mask or pattern 22 (FIG. 1e of arsenic sulfide material 20. This removal is possible notwithstanding the fact that the photoresist image is covered with arsenic sulfide. The porous structure of arsenic sulfide enables the solvent to penetrate it and thereby remove the undesired photoresist and the arsenic sulfide covering it. The mask 22 while shown as a simple geometric form may take any of the more complex forms commonly employed in the integrated circuit masking art or in the photofabrication of devices.

In the above process, the photoresist material 14 (FIG. 1b) was placed on surface 12 prior to the deposition of the arsenic sulfide material 20. It should be recognized that it is consistent and within the scope of the invention to first deposit the arsenic sulfide material 20 and then cover that surface with a layer of photoresist. The photoresist could then be exposed and developed to uncover certain areas of the arsenic sulfide material 20. The uncovered arsenic sulfide material would then be removed by the application of a basic solution such as ammonia water. Following the application of the basic solution, the remaining photoresist material would be dissolved by the application of an appropriate organic solution such as acetone. In either instance, either the first described steps illustrated in FIGS. 1a-ld, or the steps just described, the resulting structure would be substantially as shown in FIG. 1e, that is, a pattern of arsenic sulfide material on surface 12.

The mask 22 (FIG. 12 may then be melted by heating to approximately 200 C. This heating would in essence result in the formation of a glasslike substance having the desirable characteristics of a very sharp, smooth edge that provides excellent edge definition. The glass forms at a relatively low temperature and any semiconductor devices which may reside in substrate 10 would be substantially unaffected. The melting would also seal any pinholes in mask 22 (FIG. 1e), thereby providing a layer of material substantially impervious to acids, such as hydrofluoric acid. This layer may be employed advantageously in semiconductor technology as an oxide etching mask or as any other mask. It may also be desirable to form another layer of photoresist over the mask 22 and again etch the melted material by application of a basic solution such as dilute NH.,OI-I.

The final step in this embodiment of the invented method is to use the formed arsenic sulfide mask 22 in the process of forming a device. This may involve the use of the mask 22 as an independent mask to expose photoresist coated over another surface or as shown in FIGS. lf-lg, wherein the predetermined configuration of arsenic sulfide 20 is employed to lift another material and thereby form this material. As shown in FIG. If, a layer of material 26 is formed or deposited over arsenic sulfide 20. The arsenic sulfide (FIG. 1e), is then removed by lifting it with a basic solution such as ammonia water. Such a solution readily removes arsenic sulfide 20 and the overlying material 26, leaving a predetermined configuration 28 (FIG. lg) of film material 26 intact. The predetermined configuration 28 has a form determined by mask 22 (FIG. 1e).

In the embodiment shown in FIGS. la-Ig, the arsenic sulfide functions as a lifting mask or material. It is readily apparent that it could equally function as an etching mask or a mask for the exposure ofa photoresist by merely altering the steps shown in FIG. 1 (d-g).

In another embodiment of the invention, the material employed to form the arsenic sulfide in a predetermined configuration would be a halide lifting material such as calcium fluoride rather than the photoresist. The halide lifting material is first formed into a predetermined configuration by the method shown in aforementioned patent application Ser. No. 509,825, heretofore referred to. Following the forming of the arsenic sulfide by such a lifting procedure, the formed mask is employed to form an oxide etch. Subsequently, the arsenic sulfide may be formed into different configurations by applying another layer of photoresist over the first formed pattern, exposing and developing the photoresist and removing the arsenic sulfide not covered by photoresist by the application of a basic solution, such as ammonia water. Thus, it is possible by employing the instant invention to use alkaline earth halides, photoresists, and arsenic sulfide in a combination of steps to form a device. This is possible since the photoresists, alkaline earth halides, and the arsenic sulfides are all soluble in different classes of solutions. Thus, the use of arsenic sulfide as a masking material or a lifting material provides a powerful tool in the fabrication of devices and provides a new dimension in versatility in such fabrication. This advantage along with excellent edge definition and other heretofore listed advantages are a few of the more important benefits which flow from this invention.

Although this invention has been disclosed and illustrated with reference to particular applications, the principles involved are susceptible of numerous other applications which will be apparent to persons skilled in the art.

Iclaim:

I. In a process for forming devices by photofabrication, the steps comprising;

forming a layer of arsenic sulfide on a surface; and

shaping said layer into a pattern by removing selected portions of said layer from said surface.

2. In a process for forming masks that are employed in the manufacture of devices by photofabrication the steps comprismg:

a. forming a first pattern of lifting material on a substrate,

leaving exposed a portion of the surface of said substrate;

b. depositing a layer of arsenic sulfide over said pattern and the exposed portion of said surface; and,

c. removing said lifting material and the overlying arsenic sulfide while leaving arsenic sulfide on the surface of said substrate thereby forming said arsenic sulfide into a second pattern.

3. The method defined in claim 2 wherein said lifting material is a photoresist.

4. The process defined in claim 2 wherein said lifting material is calcium fluoride.

5. The method of claim 2, including the additional step of melting said layer of arsenic sulfide subsequent to the forming of said arsenic sulfide into said second pattern;

6. In a process for forming masks by photofabrication, the steps comprising:

depositing a layer of arsenic sulfide over the surface of a substrate;

forming a pattern of masking material on said arsenic sulfide layer leaving exposed portions of said arsenic sulfide layer;

removing the exposed portions of said arsenic sulfide layer;

and

removing the pattern of material overlying the remaining arsenic sulfide layer thereby to leave a portion of said arsenic sulfide layer on said surface in the form of said pattern.

8. The process defined in claim 6 wherein said arsenic sulfide is formed into said pattern by dissolving the exposed portions of said arsenic sulfide in a basic solution.

8. The method of claim 7 wherein said basic solution is dilute ammonia.

9. The process of claim 6, including the additional step of melting said layer of arsenic sulfide after removing said pattern of material;

Claims

2. In a process for forming masks that are employed in the manufacture of devices by photofabrication the steps comprising: a. forming a first pattern of lifting material on a substrate, leaving exposed a portion of the surface of said substrate; b. depositing a layer of arsenic sulfide over said pattern and the exposed portion of said surface; and, c. removing said lifting material and the overlying arsenic sulfide while leaving arsenic sulfide on the surface of said substrate thereby forming said arsenic sulfide into a second pattern.
3. The method defined in claim 2 wherein said lifting material is a photoresist.
4. The process defined in claim 2 wherein said lifting material is calcium fluoride.
5. The method of claim 2, including the additional step of melting said layer of arsenic sulfide subsequent to the forming of said arsenic sulfide into said second pattern;
6. In a process for forming masks by photofabrication, the steps comprising: depositing a layer of arsenic sulfide over the surface of a substrate; forming a pattern of masking material on said arsenic sulfide layer leaving exposed portions of said arsenic sulfide layer; removing the exposed portions of said arsenic sulfide layer; and removing the pattern of material overlying the remaining arsenic sulfide layer thereby to leave a portion of said arsenic sulfide layer on said surface in the form of said pattern.
8. The process defined in claim 6 wherein said arsenic sulfide is formed into said pattern by dissolving the exposed portions of said arsenic sulfide in a basic solution.
8. The method of claim 7 wherein said basic solution is dilute ammonia.
9. The process of claim 6, incluDing the additional step of melting said layer of arsenic sulfide after removing said pattern of material;