GB1583459A

GB1583459A - Masks their manufacture and the manufacture of microminiature solid-state devices using such masks

Info

Publication number: GB1583459A
Application number: GB52934/77A
Authority: GB
Original assignee: Philips Electronic and Associated Industries Ltd
Current assignee: Philips Electronics UK Ltd
Priority date: 1977-12-20
Filing date: 1977-12-20
Publication date: 1981-01-28

Description

(54) MASKS, THEIR MANUFACTURE, AND THE MANUFACTURE OF MICROMINIATURE SOLID-STATE DEVICES USING SUCH MASKS (71) We, PHILIPS ELECTRONIC AND ASSOCIATED INDUSTRIES LIMITED, of Abacus House, 33 Gutter Lane, London, EC2V 8AH, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to masks for use in the manufacture of microminiature solidstate devices, particularly but not exclusively a semiconductor device, and to methods of manufacturing such a mask. The invention further relates to methods of manufacturing microminiature solid-state devices including a step using such a mask, and to microminiature solid-state devices manufactured by such a method.

Microminiature solid-state devices are solid-state devices in which the accuracy with which some part (for example, a part of a layer pattern), is provided is of the order of microns (,am) or in some cases even fractions of a micron. Well known examples of such devices are semiconductor devices, both integrated circuits and discrete circuit elements. Other known microminiature solidstate devices are for example surface acoustic wave devices and so-called magnetic "bubble" devices.

In the manufacture of microminiature solid-state devices device patterns are generally defined photolithographically using a mask; the mask has at least one opaque area and at least one window for defining a radiation pattern from radiation incident on the mask. Expressions such as "opaque", "transparent", "window", and "optical density" as used in the present Specification and Claims should be understood as being relative to the radiation used with the mask.

Although not essential. this radiation is generally ultra-violet; the mask is generally termed a photomask regardless of the wavelength of the radiation used.

A new type of photomask is proposed in R.C.A. Technical Note TN 1156 entitled "Abrasive Resistance Photomask" by W.W.

Cook and issued by R.C.A. Princeton, U.S.A. in 1976, and in the paper entitled "A New Photomask with Ion-implanted Resist" by T. Hashimoto et al of Nippon Electric Company Limited which is published in pages 198 to 201 of the Technical Digest of the International Electron Devices Meeting held at Washington, U.S.A., in December 1976. These new photomasks comprise a transparent substrate, and a resist pattern hardened by ion implantation and opaque to at least ultra-violet radiation. The resist pattern is present directly on a major surface of the substrate and defines the opaque areas of the mask; the boundary between the windows and opaque areas of the mask is defined by the side walls of the resist pattern. Cook and Hashimoto describe implantationhardening of ultra-violet sensitive resists; however the expression "photoresist" as used in the present Specification and Claims should be understood as also including within its scope resists sensitive to other radiations and wavelengths, for example X-ray sensitive resist, and even electron-sensitive resist.

Photomasks of which the opaque areas are formed by a metal layer (in particular chromium) on a transparent substrate are widely used in the manufacture of semiconductor devices. Such chromium photomasks are described, in, for example, British Patent Specification 1,057,105 and the articlesenti- tled "Chromium Masks" by R.E. Szupillo in Solid State Technology, July 1969, pages 49 to 52 and August 1969, pages 58 to 61.

Compared with chromium photomasks the proposed implantation-hardened resist masks have both advantages and disadvantages. Chromium is an expensive material so that the proposed resist masks should be cheaper to manufacture. If a defect is found in the mask pattern during manufacture, it is cheaper to remove and replace a resist pattern than a chromium pattern. Although the dimensions of a chromium mask pattern are defined photolithographically, they are determined by a critical etching step, whereas no etching is involved in determining the dimensions of the proposed resist mark patterns. Implantation-hardened resist areas are generally non-reflective whereas chromium areas are reflective unless treated in a special way; the reflective chromium can cause multiple rcflections resulting in undesirable exposure under the edges of the chromium mask pattern.

The adhesion of the resist mask pattern to a mask substrate may however be poorer and less uniform than that of chromium, and this may affect the durability of the mask. Furthermore implantation-hardened resist may have a lower optical density than an opaque area of metal such as chromium; in general the optical density of the resist areas increases with increasing implanted-ion dose and with increasing thickness of the photoresist; however increasing the ion dose can cause electrostatic charging problems which can result in cracking and disintegration of the mask pattern, particularly for large areas of resist. In order to obtain a hard scratchresistance mask it is generally desirable to reduce the thickness of the resist so as to facilitate implantation-hardening throughout its thickness. If the resist thickness is increased a higher ion energy is required for the ions to penetrate throughout its thickness; however, the use of a high dose of high energy ions may cause undesirable heating of the resist which in very bad cases may distort and change the dimensions of the desired mask pattern. The Applicants have also experienced problems with pin-holes occurring in some large resist areas.

According to a first aspect of the present invention there is provided a mask for use in the manufacture of a microminiature solidstate device and having at least one opaque area and at least one window. which mask comprises a transparent substrate. having on a major surface a resist pattern formed from a radiation-sensitive material which defines the at least one opaque area of the mask which resist pattern is hardened by ion implantation and is opaque to at least ultraviolet radiation. the boundary between the at least one window and the at least one opaque area being defined by a side wall of the resist pattern, and the resist pattern being present on a metal layer on the said major surface of the substrate.

Such masks having an implantationhardened resist pattern on a metal layer on the transparent substrate can rctain some of the advantages of implantation-hardened resist masks and metal photomasks whilst avoiding or reducing some of their disadvantages. If during the manufacture of the mask a defect is found in the mask pattern defined in the resist, the mask pattern can be changed without wasting the metal layer; thus the resist pattern can be removed before implantation-hardening and replaced with a further layer of resist provided on the same metal layer on the transparent substrate.

Since the mask pattern is defined in the implantation-hardened resist, the metal layer may be, if desired, less opaque and thus less thick than in, for example, a chromium mask so saving material; the metal layer can help however to increase the optical density of the opaque area or areas of the mask and to reduce the pin-hole effects. The nature of the metal layer can be chosen to provide good adhesion of the implantation-hardened resist to the substrate. As will be described hereinafter the presence of the metal layer can be advantageous during the manufacture of the mask in reducing charging problems.

The implantation-hardened resist can act as an antireflection coating on the surface of the metal layer.

Preferably the metal layer is in the form of a pattern which underlies the resist pattern but is absent from the window or windows of the mask, and in this case both the metal layer pattern and the resist pattern are preferably opaque to the radiation used with the mask in the manufacture of the microminiature solid-state device. In this manner a double-layer mask is obtained, the opaque area or areas of which have an increased optical density without affecting the transmission through the window or windows of the mask. The metal pattern can be formed readily by etching using the implantationhardened resist as an etchant mask, and the etching process is not critical because the boundary between the window(s) and opaque area(s) of the mask is defined by the side wall(s) of the resist pattern.

The actual metal used as the underlying layer is generally not critical. although the applicants have found chromium to be particularly advantageous; when using chromium, highly durable double-layer masks were formed and the incidence of pin-holes was very low. The applicants have however obtained satisfactory masks also when using a cheaper material such as aluminium. instead of the chromium.

According to a second aspect of the invention there is provided a method of manufacturing a mask in accordance with the first aspect including the steps of depositing a continuous metal layer on a major surface of a transparent substrate, coating the continuous metal layer with a layer of photoresist material. exposing selectively and developing the photoresist layer to leave on the con tinuous metal layer a resist pattern having at least one window, implanting ions in the photoresist pattern by bombarding the pattern with ions of such mass, dose and energy as to harden the resist pattern throughout its thickness, and retaining this implantationhardened photoresist pattern in the finished mask as at least one opaque area of the mask.

According to a third aspect of the invention there is provided a method of manufacturing a microminiature solid-state device, including the steps of selectively exposing a layer of radiation-sensitive material at a surface of a device body using a radiation pattern from a mask which is in accordance with the first aspect of the invention, and developing said exposed material to define a device pattern.

In order to illustrate the realisation of these and other features in accordance with the invention and their advantages, embodiments of the various aspects of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figures 1 to 3 are cross-sectional views of part of a photomask in accordance with the first aspect of the invention, at successive stages of its manufacture by a method in accordance with the second aspect of the invention, and Figure 4 is a cross-sectional view of part of a device body and part of a photomask at a stage in the manufacture of a microminiature solid-state device by a method in accordance with the third aspect of the invention.

It should be understood that the drawings are diagrammtic and not drawn to scale: in particular the thickness and proportions of the different layers have been shown exaggerated for the sake of clarity.

Figure 3 shows part of a photomask which is in accordance with the invention and which is intended for use in the manufacture of a microminiature solid-state device. The mask pattern consists of opaque areas (1,2) and windows 3 which define a pattern to be formed photolithographically at a surface of the device body. The mask comprises a substrate 4 which is transparent to the radiation (for example, ultra-violet radiation) to be used with the mask for forming the device pattern. The substrate 4 may be, for example. of bdrosilicate glass, although a wide choice of suitable materials is available. The dimensions of the substrate 4 are not important. but in a typical case, the substrate may have a thickness of. for example. 200 to 300 microns (Cim) m) and a diameter determined by the area of the device body to be processed.

for example. 75 or 100 mm.

An opaque pattern of implantationhardened resist l present on a metal layer 2 on a major surface of the substrate 4 and defines the opaque areas (1,2) of the mask.

In general, such a mask pattern corresponds to an array of identical device patterns to be formed from the mask. The boundary between the windows 3 and the opaque areas (1,2) is defined by side walls of the resist pattern 1 rather than by the side walls of the metal layer 2.

In this embodiment the metal layer 2 is a hard metal such as chromium and is in the form of a pattern which underlies and corresponds substantially to the resist pattern 1.

The chromium pattern 2 is thus absent from the windows 3 of the mask and so does not affect the transmission of these windows; this feature is important since in this embodiment the chromium layer thickness is such that both the chromium pattern 2 and the resist pattern 1 are opaque to ultra-violet radiation. A double layer masking pattern (1,2) is thus formed.

The inclusion of the chromium pattern 2 below the implantation-hardened resist pattern 1 results in several advantages. The chromium can have very good adhesion to a mask substrate 2 of, for example, borosilicate glass. The chromium pattern 2 increases the optical density of the opaque areas (1,2) and helps to reduce the effect of pin-holes.

Since both the chromium and hardenedresist contribute to the optical density of the opaque areas (1,2), an excessive thickness of resist can be avoided which facilitates implantation-hardening throughout its thickness, and the chromium layer can be, if desired, less opaque and thus lesF thick than in a chromium mask. The resulting mask pattern (1,2) can thus have a very high optical density and be very hard and durable. The implantation-hardened resist 1 also acts as an antireflection coating on the surface of the chromium layer 2. Other advantages arise during the manufacture of the mask which will now be described with reference to Figures 1 and 2.

A major surface of the mask substrate 4 is cleaned in known manner in preparation for the deposition of a continuous layer 12 of chromium. The chromium is preferably deposited by sputtering, although evaporation may be used instead. The thickness of the layer 12 may be in the range of, for example, 700 (0.07 micron) to 1,000 (0.1 micron). However, if desired, a larger thickness may be used. although this increases the' cost of the chromium used.

The continuous metal layer 12 is coated in known manner with a layer 11 of photoresist material having a thickness of, for example, 0.3 to 0.5 microns (clam) The photoresist 11 may be of the positive or negative. type; examples of suitable resists are the positive resist AZ l3SOH of Shipley Chemicals Limited, and the negative Micro resist type 747 of Eastman Kodak Company. Both these examples are ultra-violet sensitive resists. However, for example, electrpnsensitive and X-ray-sensitive resists may be used instead; in these cases by using X-ray or electron exposure smaller dimensions can be obtained in the resist pattern, and the electron exposure may be effected by deflecting an electron beam in a pattern generator, instead of using a mask.

After providing the photoresist layer 11, the layer 11 is selectively exposed and developed. Figure 1 illustrates the case of a positive resist 11 exposed to a radiation pattern 13 which may be, for example, ultraviolet radiation projected onto the layer 11 from a larger-scale mask (not shown). The exposure pattern corresponds to the desired pattern of windows 3 in the final photomask.

After developing the photoresist in the usual way, a pattern 1 of resist is left on the continuous metal layer 12. At this stage, the pattern 3 may be carefully inspected to ensure it corresponds to the desired mask pattern of the mask of Figure 3. If a defect is found in this pattern, the resist 1 can be removed easily from the layer 12 and replaced with a further layer of photoresist provided on the same metal layer 12; in this way, the mask pattern can be changed during manufacture, without wanting the metal layer 12.

As indicated by arrows 15 in Figure 2, ions are then implanted in the resist pattern 1 by bombarding the pattern 2 with ions 15 of such mass, dose and energy as to further polymerise and harden the photoresist material. The implantation conditions may be as described in, for example, the two previously-mentioned publications by W.W.

Cook and T. Hashimoto et al. The Applicants have obtained good results with both phosphorus and arsenic ions, with an implanted ion dose of 1016 ions per sq. cm.

and with an ion beam energy of 150 keV. It is important to choose the ion energy sufficiently high to ensure penetration and hence hardening throughout the thickness of the resist pattern 1. The photoresist material is preferably polymerised as completely as possible to increase its scratch resistance, durability, and optical density.

During the implantation, the continuous metal layer 12 serves to reduce electrostatic charging of the resist pattern by the high dose of high energy ions. This can be effected by contacting the metal layer 12 around its periphery with an annular metal clip (not shown in the drawings) which is connected to a source of suitable electrical potential, for example an earthing point on the ion implantation machine. The metal layer 12 can thus maintain the whole of the overlying resist pattern l at a common potential to prevent cracking and disintegration of the mask pattern caused by electrostatic charging. A small amount of heat flow from the pattern l can also occur through the metal layer 12 and the metal clip.

The thickness of the resist 11 is found to have shrunk as a result of the implantationhardening. The dimensions of the pattern otherwise appear to be unchanged.

While using the implantation-hardened resist pattern 1 as an etchant mask, the metal layer 12 is then etched away at the areas exposed at the windows in the resist pattern 1 so as to leave the metal pattern 2 underlying the pattern 1. This etching process is not critical, and in order to ensure complete removal of the metal at the windows some underetching of the metal below the edges of the resist pattern 1 can be allowed; this is because the boundary between the windows and the opaque areas of the mask is defined by the side walls of the pattern 1 rather than those of the pattern 2. For a chromium layer 12 the applicants have used an etchant comprising ceric ammonium nitrate in perchloride acid. However, other etchant solutions and even gas plasma etching may be used; by using a gas plasma etching process the extent of lateral etching of the chromium under the edge of the resist areas can be reduced.

Instead of chromium the applicants have also fabricated double-layer masks using a 0.2 micron (clam) thick aluminium layer to form the metal pattern 2. An advantage of aluminium is that it is considerably cheaper than chromium. A suitable etchant for aluminium is, for example, a solution of orthophosphoric acid containing both acetic and nitric acids, although gas plasma etching may again be used instead.

Figure 4 illustrates a photolitographic step in the manufacture of a microminiature solid-state device using such a double-layer mask in accordance with the invention. The step involves defining a device pattern in a layer 21 of radiation-sensitive material at a surface of a device body 25 using a radiation pattern 26 from the double-layer mask (4, 2, 1) to selectively expose the layer 21. Generally, the mask pattern will consist of an array of device patterns so that the device manufacture results in the fabrication of an array of identical devices on a common body 25 which is subsequently divided to form individual bodies for each device. As shown in Figure 4 the mask (4. 2, 1) and the device body 25 are positioned so that the masksubstrate surface having the resist pattern 1 faces towards the device body 25, and the opposite surface of the mask substrate 4 is exposed to radiation 27 (for example ultraviolet radiation) from which the radiation pattern 26 is formed.

In the embodiment of Figure 4, the ultraviolet pattern 26 selectively exposes a photoresist layer 21 on a layer 22 on the device body 25. after which the resist layer 21 is developed to form an etchant mask for selectively etching the layer 22; depending on the type of device and the stage in its manufacture, the layer 22 may be for example a metal layer or an insulating layer. The photolithographic step may be performed with the mask (1 2, 4) in contact with or close to the device body layer 21, or the radiation pattern 26 may be projected onto the layer 21 through one or more lenses which are not shown in the Figure.

Many modifications are of course possible within the scope of the invention. Instead of ultra-violet photolithography, masks in accordance with the invention may be used in, for example, electron beam image projection which is described in for example the article entitled "An Electron Image Projector with Automatic Alignment" by J.P. Scott in I.E.E.E. Transactions on Electron Devices, ED-22, No. 7, July 1975, pages 409 to 413; in this case, a layer of photoemissive material, for example palladium or caesium iodide is provided over the windows 3 and resist pattern 1 is an additional final step in the manufacture of the mask (1, 2, 4); the ultra-violet light pattern 26 formed at the windows 3 then generates a corresponding electron beam pattern from the photoemissive material, and using electron-optics the image of this electron beam pattern is then projected onto the device body 25 to selectively expose an electron-sensitive resist layer 21.

In the case of caesium iodide a continuous thin metal layer is needed to maintain the photoemissive layer at a common potential.

Such a continuous metal layer can be provided over the photoemissive layer. However if desired a much thinner metal layer than that previously described may be used as the layer 12 in forming a mask in accordance with the invention; such a metal layer 12 which may be, for example, 100 to 200 A (0.01 to 0.02 microns) thick is semitransparent and may be retained in the final mask as a continuous layer which covers also the windows 3 of the mask (1, 12, 4) and provides the necessary continuous metal layer for the photoemissive caesium iodide; with such a mask (1. 12, 4) some of the advantages of the mask structure (1,2,4) of Figure 3 are reduced; thus, for example the semi-transparent layer 12 increases the optical density of the opaque areas of the mask (1. 12. 4) only slightly and decreases the transmission of the windows 3.

WHAT WE CLAIM IS: 1. A mask for use in the manufacture of a microminiature solid-state device and having at least one opaque area and at least one window, which mask comprises a transparent substrate having on a major surface a resist pattern formed from a radiation sensitive material which defines the at least one opaque area of.the mask, which resist pattern is hardened by ion-implantation and opaque to at least ultra-violet radiation, the boundary between the at least one window and the at least one opaque area being defined by a side-wall of the resist pattern, characterised in that the resist pattern is present on a metal layer on the said major surface of the substrate.

2. A mask as claimed in Claim 1, in which the. metal layer is in the form of a pattern which underlies the resist pattern but is absent from the window or windows of the mask.

3. A mask as claimed in Claim 2, in which both the metal layer pattern and the resist pattern are opaque.

4. A mask as claimed in any of the preceding Claims, in which the metal layer is of chromium.

5. A method of manufacturing a mask claimed in Claim 1, including the steps of depositing a continuous metal layer on a major surface of a transparent substrate, coating the continuous metal layer with a layer of photoresist material, exposing selectively and developing the photoresist layer to leave on the continuous metal layer a resist pattern having at least one window, implanting ions in the resist pattern by bombarding, the patterns with ions of such mass, dose and energy as to harden the resist pattern throughout its thickness, and retaining this implantation-hardened resist pattern in the finished mask as at least one opaque area of the mask.

6. A method as claimed in Claim 5, in which while using the implantationhardened resist pattern as an etchant' mask the metal layer is etched away at the area or areas exposed at the window or windows in the resist pattern so as to leave a metal layer pattern underlying the resist pattern.

7. A method as claimed in Claim 6, in which the metal layer pattern is deposited in a sufficient thickness as to be opaque.

8. A method as claimed in any of Claims 5 to 7, in which the continuous metal layer is formed by depositing chromium on the said major surface of the substrate.

9. A method of manufacturing a mask substantially as described with reference to Figures 1 and 2 of the accompanying drawings.

10. A mask manufactured by a method claims in any of Claims 5 to 9.

Il. A mask substantially as described with reference to Figure 4 of the accompanying drawings.

12. A method of manufacturing a microminiature solid-state device, including the steps of selectively exposing a layer of radiation-sensitive material at a surface of a device body using a radiation pattern from a mask claimed in any of Claims 1,2,3,4. 10 or

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Claims

**WARNING** start of CLMS field may overlap end of DESC **.

developed to form an etchant mask for selectively etching the layer 22; depending on the type of device and the stage in its manufacture, the layer 22 may be for example a metal layer or an insulating layer. The photolithographic step may be performed with the mask (1 2, 4) in contact with or close to the device body layer 21, or the radiation pattern 26 may be projected onto the layer 21 through one or more lenses which are not shown in the Figure.

Many modifications are of course possible within the scope of the invention. Instead of ultra-violet photolithography, masks in accordance with the invention may be used in, for example, electron beam image projection which is described in for example the article entitled "An Electron Image Projector with Automatic Alignment" by J.P. Scott in I.E.E.E. Transactions on Electron Devices, ED-22, No. 7, July 1975, pages 409 to 413; in this case, a layer of photoemissive material, for example palladium or caesium iodide is provided over the windows 3 and resist pattern 1 is an additional final step in the manufacture of the mask (1, 2, 4); the ultra-violet light pattern 26 formed at the windows 3 then generates a corresponding electron beam pattern from the photoemissive material, and using electron-optics the image of this electron beam pattern is then projected onto the device body 25 to selectively expose an electron-sensitive resist layer 21.

In the case of caesium iodide a continuous thin metal layer is needed to maintain the photoemissive layer at a common potential.

Such a continuous metal layer can be provided over the photoemissive layer. However if desired a much thinner metal layer than that previously described may be used as the layer 12 in forming a mask in accordance with the invention; such a metal layer 12 which may be, for example, 100 to 200 A (0.01 to 0.02 microns) thick is semitransparent and may be retained in the final mask as a continuous layer which covers also the windows 3 of the mask (1, 12, 4) and provides the necessary continuous metal layer for the photoemissive caesium iodide; with such a mask (1. 12, 4) some of the advantages of the mask structure (1,2,4) of Figure 3 are reduced; thus, for example the semi-transparent layer 12 increases the optical density of the opaque areas of the mask (1. 12. 4) only slightly and decreases the transmission of the windows 3.

WHAT WE CLAIM IS: 1. A mask for use in the manufacture of a microminiature solid-state device and having at least one opaque area and at least one window, which mask comprises a transparent substrate having on a major surface a resist pattern formed from a radiation sensitive material which defines the at least one opaque area of.the mask, which resist pattern is hardened by ion-implantation and opaque to at least ultra-violet radiation, the boundary between the at least one window and the at least one opaque area being defined by a side-wall of the resist pattern, characterised in that the resist pattern is present on a metal layer on the said major surface of the substrate.
2. A mask as claimed in Claim 1, in which the. metal layer is in the form of a pattern which underlies the resist pattern but is absent from the window or windows of the mask.
3. A mask as claimed in Claim 2, in which both the metal layer pattern and the resist pattern are opaque.
4. A mask as claimed in any of the preceding Claims, in which the metal layer is of chromium.
5. A method of manufacturing a mask claimed in Claim 1, including the steps of depositing a continuous metal layer on a major surface of a transparent substrate, coating the continuous metal layer with a layer of photoresist material, exposing selectively and developing the photoresist layer to leave on the continuous metal layer a resist pattern having at least one window, implanting ions in the resist pattern by bombarding, the patterns with ions of such mass, dose and energy as to harden the resist pattern throughout its thickness, and retaining this implantation-hardened resist pattern in the finished mask as at least one opaque area of the mask.
6. A method as claimed in Claim 5, in which while using the implantationhardened resist pattern as an etchant' mask the metal layer is etched away at the area or areas exposed at the window or windows in the resist pattern so as to leave a metal layer pattern underlying the resist pattern.
7. A method as claimed in Claim 6, in which the metal layer pattern is deposited in a sufficient thickness as to be opaque.
8. A method as claimed in any of Claims 5 to 7, in which the continuous metal layer is formed by depositing chromium on the said major surface of the substrate.
9. A method of manufacturing a mask substantially as described with reference to Figures 1 and 2 of the accompanying drawings.
10. A mask manufactured by a method claims in any of Claims 5 to 9.
Il. A mask substantially as described with reference to Figure 4 of the accompanying drawings.
12. A method of manufacturing a microminiature solid-state device, including the steps of selectively exposing a layer of radiation-sensitive material at a surface of a device body using a radiation pattern from a mask claimed in any of Claims 1,2,3,4. 10 or

11, and developing said exposed material to define a device pattern.
13. A method of manufacturing a micrpminiature solid-state device, substantially as described with reference to Figure 4 of the accompanying drawings.
14. A microminiature solid-state device manufactured by a method claimed in Claim 12 or Claim 13.