Process for the removal of resist material

The present invention relates to a process for the removal of an unnecessary resist material from an object such as semiconductor wafer at a step of forming a fine pattern during the preparation of semiconductors, circuits, printed circuit boards, liquid crystal panels, etc.

In the process for the preparation of a semiconductor device, for example, a resist material is applied to a wafer such as silicon. The resist material is then subjected to ordinary photographic process to form an image made of resist pattern. With this image as a mask, the wafer is then etched. Unnecessary resist material is then removed. This procedure is then repeated. Unnecessary resist material is removed also in the case where a circuit is formed on various circuit boards.

With the recent tendency towards the enhancement of the density and integration of LSI and the density and size of liquid crystal panel, it has been important more and more from the standpoint of yield and reliability of product to simply and certainly remove unnecessary resist material from semiconductor wafer or glass substrate. Heretofore, a dry removal process using an asher (carbonization apparatus) or a wet removal process using a resist removing solvent has been normally employed at the step of removing unnecessary resist material.

However, the removal process using an asher is disadvantageous in that it takes much time to remove a resist material which has been doped with ions at a high dose. If plasma ashing is effected, the semiconductor substrate can be damaged by plasma. The wet removal process using a resist removing solvent is disadvantageous in that it deteriorates the working atmosphere and discharges waste liquid that pollutes the global atmosphere. This wet removal process is also disadvantageous in that the resist material which has once been removed could be again attached to the wafer.

In order to solve these problems, a process for the removal of a resist material has been proposed involving the use of a sheet-like or tape-like pressure-sensitive adhesive sheet. This removal process comprises applying a pressure-sensitive adhesive sheet to an object having a resist material provided thereon so that the resist material is fixed to the pressure-sensitive adhesive layer, and then peeling the pressure-sensitive adhesive sheet off the object together with the resist material to remove the resist material from the object. This removal process is free from the problems inherent to the conventional removal processes using an asher or solvent, contributing to the enhancement of the yield of product.

However, the foregoing removal process using a pressure-sensitive adhesive sheet is disadvantageous in that, in cases where some kinds of resist material are used or some kinds of treatment on resist material are effected, the resist material can not be removed completely from an object such as semiconductor substrate. In particular, resist materials doped with ions at a dose as high as not less than 1 × 10¹⁵ ions/cm² can be hardly peeled off the substrate in most cases.

US-A-5466325, JP-A-06-151671 and JP-A-10-158602 disclose processes for the removal of a resist material formed on an article, said processes comprising the steps of applying a pressure-sensitive adhesive tape to the surface of the resist material and peeling off the resist material together with the adhesive tape.

It is an object of the present invention to provide a process for the peeling of a resist material with a pressure-sensitive adhesive sheet which involves the improvement in the removal of resist material or the enhancement of peelability of resist material to certainly remove the resist material from the object regardless of the properties or treated state of the resist material.

The inventors made extensive studies of the foregoing object of the present invention. As a result, it was found that, by cooling the pressure-sensitive adhesive sheet together with the resist material and the object or heating them and then cooling, prior to peeling of a pressure-sensitive adhesive sheet applied to the upper surface of a resist material on an object such as semiconductor substrate, the shrinkage or expansion of the pressure-sensitive adhesive sheet is caused to impart a stress to the interface between the resist material and the object, thereby enhancing the peelability of the resist material and hence making it possible to simply and certainly remove the resist material from the object regardless of the properties of the resist material or other factors.

The present invention provides a process for the removal of a resist material which comprises applying a pressure-sensitive adhesive sheet comprising a curing type pressure-sensitive adhesive layer to the upper surface of a resist material present on an object, effecting a curing treatment for the curing type pressure-sensitive adhesive layer, effecting a stress-imparting treatment which causes shrinkage or expansion of said pressure-sensitive adhesive sheet so that a stress develops at the interface between said resist material and said object, said stress-imparting treatment comprising (1) cooling said pressure-sensitive adhesive sheet, together with said resist material and object, to a temperature of not higher than 0°C or (2) heating said pressure-sensitive adhesive sheet to a temperature of not lower than 100°C and then allowing said pressure-sensitive adhesive sheet, together with said resist material and object, to cool to room temperature, and then peeling said pressure-sensitive adhesive sheet together with said resist material to remove said resist material from said object.

According to a preferred embodiment of the invention the ratio of the linear expansion coefficient of the pressure-sensitive adhesive layer in said pressure-sensitive adhesive sheet to the linear expansion coefficient of said object is not less than 2.

The term "object having a resist material provided thereon" as used herein is meant to indicate, e.g., one obtained by a process which comprises applying a known resist material to an object such as semiconductor substrate and glass substrate, subjecting the substrate to ordinary photographic process to form a predetermined resist pattern (resist film image) thereon, injecting ions such as As⁺, P⁺ and B⁺ into the substrate at the open side thereof with the resist material as a mask, and then subjecting the substrate to any other suitable treatments such as etching. The thickness of the resist material, if used as a mask with which ions are injected into the substrate, is normally in a range of from 1 to 5 µm, but is not specifically limited.

In the present invention, a pressure-sensitive adhesive sheet is applied to the upper surface of the resist material present on the object. The application of the pressure-sensitive adhesive sheet may be effected at ordinary temperature. However, the pressure-sensitive adhesive sheet is preferably thermally contact-bonded to the resist material to allow the resist material and the pressure-sensitive adhesive layer to come in close contact with each other. This thermal contact bonding is accompanied by some expansion or shrinkage. However, the expansion or shrinkage is not so strong enough to cause such a stress at the interface between the resist material and the object that a good peelability imparts to the resist material.

The pressure-sensitive adhesive sheet to be used herein is a sheet-like or tape-like film substrate having formed thereon a pressure-sensitive adhesive layer generally having a thickness of from 20 to 150 µm. Examples of the film substrate include plastic films made of various synthetic resins and generally having a thickness of from 10 to 1,000 µm. Examples of the synthetic resins include polyester, polycarbonate, polyethylene, polypropylene, ethylene-propylene copolymer, ethylene-vinyl acetate copolymer and ethylene-ethyl acrylate copolymer.

The pressure-sensitive adhesive layer is of curing type in terms of complete removal of the resist material. The curing type pressure-sensitive adhesive layer may be made of a pressure-sensitive adhesive which cures (solidifies) when cooled from hot state, such as hot melt pressure-sensitive adhesive, or a polymerization-curable pressure-sensitive adhesive comprising a curable compound and a polymerization catalyst incorporated in a pressure-sensitive adhesive polymer such as acrylic polymer. Preferred among these pressure-sensitive adhesives is polymerization-curable pressure-sensitive adhesive, particularly an ultraviolet-curing pressure-sensitive adhesive.

The linear expansion coefficient of such a pressure-sensitive adhesive layer is preferably not less than 1 x 10^-4/°C to transmit more effectively a stress to the interface of the resist material with the object such as semiconductor substrate. The ratio of the linear expansion coefficient of the pressure-sensitive adhesive layer to the linear expansion coefficient of the object such as semiconductor substrate is preferably 2 or more, more preferably 10 or more. The linear expansion coefficient of the silicon wafer is normally about 3 x 10^-6/°C. If the linear expansion coefficient of the pressure-sensitive adhesive layer is as specified above, the ratio of the linear expansion coefficient of the two components satisfies the above-specified range sufficiently.

The term "linear expansion coefficient" as used herein means the amount indicating the ratio of the change in thermal expansion of a solid under a predetermined pressure with temperature. The linear expansion coefficient can be represented by the following equation. For the measurement of the thermal expansion of the pressure-sensitive adhesive layer used in the present invention, TMA (thermal mechanical analysis) apparatus may be used.<maths id="math0001" num=""><math display="block"><mrow><msup><mrow><mtext>Linear expansion coefficient α = (1/L</mtext></mrow><mrow><mtext>0</mtext></mrow></msup><mtext>)•(dL/dt)</mtext></mrow></math><img file="EP0971270B1_D0001.tif" /></maths> wherein L⁰ represents the length of the object at 0°C; and L represents the length of the object at t°C.

In the present invention, after the pressure-sensitive adhesive sheet is applied to the upper surface of the resist material provided on the object, a predetermined curing treatment is effected, and a stress-imparting treatment is then effected. The stress-imparting treatment comprises (1) cooling the pressure-sensitive adhesive sheet together with the resist material and the object to a temperature of not higher than 0°C or (2) heating them to a temperature of not higher than 100°C and then allowing them to cool to room temperature. For the former cooling process, the material may be directly cooled with liquefied nitrogen or dry ice. Alternatively, the material may be allowed to come in contact with a liquid or metallic fixture which has been cooled with liquefied nitrogen or dry ice.

The foregoing application of stress causes the pressure-sensitive adhesive sheet to shrink or expand/shrink. This accompanies the generation of a shearing stress at the interface of the resist material with the object that reduces the adhesion of the resist material with the object. In the present invention, the pressure-sensitive adhesive sheet can be easily peeled off the object together with the resist material under these circumstances. In this manner, the resist material cannot be left behind even if the resist material has been modified by ions injected to form a hard surface layer, i.e., the resist material exhibits any properties. Thus, a desired good peelability can be realized with a good reproducibility.

The present invention is not limited to production of semiconductor devices. The application of the present invention is not limited so far as the object has a patterned resist material formed thereon. In practice, however, it goes without saying that the present invention can be applied to various objects.

The present invention will be further described in the following examples. The term "parts" as used herein is meant to indicate "parts by weight".

REFERENCE EXAMPLE 1

A positive-working resist was applied to a silicon wafer having formed thereon an oxide layer having a thickness of 10 nm according to a CVD method to form a resist layer having a thickness of 1µm, heated, exposed to light, and then subjected to development to form a resist film image thereon. P⁺ ions were then injected into the silicon wafer at an injection energy of 80 keV and an injection density of 1 x 10¹⁶ ions/cm² with the resist film image as a mask.

EXAMPLE 1-1

80 parts of n-butyl acrylate, 15 parts of ethyl acrylate and 5 parts of acrylic acid were allowed to undergo solution polymerization with 150 parts of ethyl acrylate and 0.1 parts of azobisisobutyronitrile at a temperature of 60°C in a stream of nitrogen for 12 hours to obtain a solution of an acrylic polymer having a weight-average molecular weight of 500,000. The solution thus obtained was then uniformly mixed with a polyethylene glycol diacrylate as a curable compound, a urethane acrylate ("U-N-01", produced by Shinnakamura Chemical Co., Ltd.), a polyfunctional isocyanate compound ("Coronate L", produced by Nippon Polyurethane Industry Co., Ltd.) and dimethoxy(phenyl)methyl phenyl ketone as a photopolymerization initiator in an amount of 50 parts, 50 parts, 3 parts and 3 parts, respectively, based on 100 parts of the acrylic polymer to prepare a solution of ultraviolet-curing pressure-sensitive adhesive.

The pressure-sensitive adhesive solution thus prepared was applied to a polyester film having a thickness of 50 µm to provide a dry thickness of 35 µm, and then dried at a temperature of 130°C for 3 minutes to prepare a pressure-sensitive adhesive sheet having an ultraviolet-curing pressure-sensitive adhesive layer. The pressure-sensitive adhesive sheet thus prepared exhibits a very small adhesion to silicon wafer before as well as after curing by irradiation with ultraviolet rays. In some detail, the pressure-sensitive adhesive sheet exhibited a 180° peel strength (at 20°C) of 13 g/10 mm width with respect to silicon wafer before curing by irradiation with ultraviolet rays. The pressure-sensitive adhesive sheet exhibited a 180° peel strength (at 20°C) of 8 g/10 mm width with respect to silicon wafer after curing by irradiation with ultraviolet rays. 20°C The pressure-sensitive adhesive layer thus ultraviolet-cured exhibited a linear expansion coefficient of 2.4 x 10^-4/°C.

Subsequently, the foregoing ultraviolet-curing pressure-sensitive adhesive sheet was contact-bonded to the upper surface of the resist film image on the silicon wafer prepared by the method of Reference Example 1 over a 130°C hot plate for 30 seconds. Thereafter, the silicon wafer was irradiated with ultraviolet rays from a high pressure mercury vapor lamp at a dose of 900 mJ/cm² so that the pressure-sensitive adhesive was cured. The whole silicon wafer was then cooled to -196°C with liquefied nitrogen. Thereafter, when the pressure-sensitive adhesive sheet was peeled, the resist material, too, was removed from the silicon wafer. The silicon wafer from which the resist material had been thus removed was then observed under microscope. As a result, no resist material was observed left behind on the silicon wafer.

EXAMPLE 1-2

The same pressure-sensitive adhesive sheet as used in Example 1-1 was applied to the upper surface of the resist film image on the silicon wafer prepared by the method of Reference Example 1 over a 130°C hot plate for 30 seconds and then irradiated with ultraviolet rays in the same manner as in Example 1-1. The silicon wafer was then heated to a temperature of 180°C for 30 seconds. The silicon wafer was then allowed to cool to room temperature. When the pressure-sensitive adhesive sheet was peeled, the resist material, too, was removed from the silicon wafer. The silicon wafer from which the resist material had been thus removed was then observed under microscope. As a result, no resist material was observed left behind on the silicon wafer.

COMPARATIVE EXAMPLE 1-1

The same pressure-sensitive adhesive sheet as used in Example 1-1 was applied to the upper surface of the resist film image on the silicon wafer prepared by the method of Reference Example 1 and then irradiated with ultraviolet rays in the same manner as in Example 1-1. The pressure-sensitive adhesive sheet was then immediately peeled. As a result, a large portion of the resist material was removed. When observed under microscope, however, the silicon wafer had a slight amount of resist material left behind thereon (30 µm□, i.e., in the area of 30 µm × 30 (µm).

As mentioned above, the present invention can provide a process for the removal of a resist material which comprises applying a pressure-sensitive adhesive sheet to the upper surface of a resist material on an object such as semiconductor substrate, and then peeling the pressure-sensitive adhesive sheet together with the resist material, wherein, after the application of the pressure-sensitive adhesive sheet, effecting a stress-imparting treatment which causes shrinkage or expansion of the pressure-sensitive adhesive sheet so that a stress develops at the interface between the resist material and the object, whereby the resist material can be removed away from the object easily and certainly regardless of the properties thereof, i.e., the resist material can be completely peeled off the object with a good reproducibility, making it possible to enhance the reliability in peeling.