X-ray exposure process using an electrically conductive x-ray mask

Abstract: An X-ray mask support member, an X-ray mask, and an X-ray exposure process using the X-ray mask, that can effectively prevent the phenomenon of electrostatic attraction or contact of X-ray masks by appropriately disposing an X-ray mask support membrane in an exposure apparatus that employs soft X-rays, by which a high precision alignment can be performed,are provided.

Description: The present invention relates to an X-ray proximity exposure process for manufacturing semiconductor devices such as IC and ULSI. More particularly it is concerned with a process that uses soft X-rays having wavelengths of about 2 Å to about 150 Å. In recent years, in exposure apparatus for manufacturing semiconductor devices such as IC and LSI, many kinds of exposure apparatus utilising soft X-rays and capable of obtaining prints with a higher resolution have been proposed as the semiconductor devices have become more highly integrated. In general, the X-ray mask used in this soft X-ray apparatus comprises a support frame of ring shape, and a membranous member stretched thereacross having X-ray transmitting portions and X-ray non-transmitting portions. The non-transmitting portions are formed of an X-ray opaque material (including an absorber) having a goeometrical pattern and provided on a support membrane (a mask membrane) that constitute a substrate in said membranous member. The X-ray opaque material is so constituted that patterns are transferred on a wafer surface accurately in the size of submicrometre. On the other hand, the transmitting portions are made of the mask support membrane itself corresponding to the part on which the X-ray opaque material is not provided. In the exposure apparatus using soft X-rays, both the space in which the X-ray mask is disposed and the upper space of a wafer on the side on which a resist is coated are, in many instances, put under vacuum, a reduced pressure atmosphere, or a low-pressure helium atmosphere, in order to prevent the absorption loss of energy of the light irradiated from a soft-X-ray source for exposure. If exposure is carried out using soft X-rays having wavelengths, for example, of approximately from 2 to 150 Å, then under such conditions, the X-ray mask support membrane, the mask absorber and ambient gas atoms absorb the soft X-ray and emit photoelectrons by virtue of the photoelectric effect. In particular, when the support membrane comprises an insulative thin membrane, such exposure causes not only electrostatic charging owing to the emission of photoelectrons from the constituent atoms of the support membrane but also electrostatic charging owing to the emission of photoelectrons from the absorber having a large number of electrons, resulting in a support membrane having a high positive potential. In general, the distance between the support membrane and the resist surface on the wafer is so small that the run-out error caused by the divergent light from the source can be negligible, and is set, for example, in the range of from 10 µm to 100 µm. For this reason, the electrically charged support membrane and wafer may electrostatically act on each other, attracting the mask towards the wafer, resulting in deformation of the support membrane, so that it sometimes occurs that the exposed pattern precision is lowered because of a flexure of the support membrane or because the support membrane comes into contact with the wafer when the attraction force is strong. In exposure apparatus aiming at obtaining a high resolution such that the size of a pattern to be transferred is 0.5 µm or less, the support membrane commonly is so constituted that it may have a thickness of about 2 µm, using an inorganic material (ceramics in particular) having a small thermal expansion coefficient and large Young's modulus so that the mis-registration due to its thermal expansion or the distortion of absorbers due to residual stress can be suppressed. In general, many of the materials for this purpose are brittle and hard. Hence, the support membrane is easily broken by an excessive deformation or an application of nonuniform pressure. As a means of solving this latter problem, there is, for example, a method in which a metal having a small absorption to X-rays, as exemplified by aluminium, is deposited in the hundreds Å thickness on the surface. This method can achieve a good electric conductance and good soft X-ray transmittance, but has involved problems that the transmission of visible or infrared light in making alignment with the wafer is so extremely low that the precision of alignment may be extremely lowered. It is also known to use a layer of magnesium as a means of reinforcement - see EP-A-0104685. Hitherto, problems have been known to arise due to the photogeneration and emission of electrons during X-ray exposure. It is known that such emission can give rise to electrostatic charging and that this can attract dust particles. In US-A-3949131 this is remedied by applying a conductive coating to the patterned surface of the mask and grounding the mask. It is also known that such electron emission and secondary electron generation produced by emitted electrons in transit between the mask and the wafer can give rise to degradation of pattern precision. In Patent Abstracts of Japan, Vol. 8, No. 81 (E-238)(1518) 13th April, 1984 and corresponding JP-A-59-2324, a remedy is provided by using a mask having a conductive film, e.g. of Al, between the support membrane and the X-ray opaque patterned film. Electrodes are placed in the neighbourhood of the mask and these are maintained at a high positive potential relative to the mask in order to attract and divert those emitted and secondary electrons. The X-ray proximity exposure process disclosed in the last mentioned reference, has in common with the present invention the following process steps of: arranging an X-ray mask adjacent in close proximity to and spaced from a wafer having an X-ray sensitive member provided thereon, said X-ray mask mask being electrically conductive and including a support frame, an X-ray transmissive support membrane held thereon, and an X-ray absorber provided in a patterned form on the surface of said support membrane; and exposing said X-ray sensitive member on said wafer to X-rays directed through said X-ray mask. An object of the present invention is to provide an X-ray proximity exposure process that effectively can prevent the phenomenon of electrostatic attraction between X-ray mask and wafer, and thus contribute to improvement in pattern precision. To meet the aforesaid object, the X-ray proximity exposure of the present invention is characterised by: cancelling potential difference between said X-ray mask and said wafer by employing one of the following measures: (a) grounding both said support membrane (20) and said wafer; or (b) electrically connecting said support membrane (20) to said wafer; or (c) applying a common voltage to said support membrane and said wafer; in each case said support membrane having an electrical resistivity of 10 Ω.cm or less. In the accompanying drawings: Fig. 1 is a schematic cross section of an X-ray mask; Fig. 2 is a schematic cross section of an X-ray mask; Fig. 3 is a schematic cross-section to describe a method of preparing an X-ray mask; and Fig. 4 is a schematic cross-section of X-ray apparatus, including mask and wafer, to describe the X-ray exposure process of the present invention. A description of a preferred embodiment and example now follows. It is given by way of example only. Fig. 1 is a schematic cross section of an X-ray mask that can be used in performing the present invention. In Fig. 1, the numeral 10 denotes a support membrane, comprising a single transmissive layer having a given transmittance to soft X-rays. The numeral 1 denotes a support frame of ring shape, which stretchingly holds the support membrane 10. The numeral 2 denotes a non-transmissive layer comprising an absorber to the soft X-rays, which is geometrically drawn on the support membrane 10 to form a pattern. The support membrane 10 is constituted of a material having a small electrical resistivity of 10 Ω.cm or less. Thus the support membrane 10 is endowed with electrical conductance on its surface or the whole, and the support membrane 10 is connected to ground to maintain the potential thereof at the ground potential so that no electric charges may be accumulated on the surface of the support membrane 10 by the irradiation of X-rays. The higher the X-ray transmittance of the support membrane 10 is, the better. For this purpose it may preferably be constituted as thinly as possible when the same kind of material is used, but is required also to ensure its mechanical strength to a certain degree, and it is necessary to have a given thickness. The thickness of the support membrane 10 is set to be not less than 1 µm and not more than 15 µm in instances in which the support membrane 10 comprises an organic membrane, because of its less absorption to the soft X-rays per unit thickness, and is set to be not less than 0.5 µm and not more than 5 µm in instances in which it comprises an inorganic membrane mainly composed of light elements of the periodic table. As materials to constitute the transmissive membrane 10 having an electrical resistivity of 10 Ω·cm or less, the membrane is constituted of one material selected from elementary simple substance such as beryllium, boron, silicon and aluminium conductive ceramics such as phosphorus-doped silicon carbide (SiC:P), and organic compounds such as poly(p-phenylene), polythiophene and polyaniline in which one material selected from the group consisting of arsenic pentafluoride, sulfuric acid anhydride and a halogen such as iodine has been added as a dopant. Provided that the dopant concentration is adjusted to such a level that may not hinder the object of the present invention. On the X-ray mask support (or maskblank) described above in detail, an X-ray absorption pattern can be formed according to the following procedures, to prepare an X-ray mask. Any X-ray absorbers having been used in conventional X-ray mask structures, such as thin films (having a thickness, for example, approximately of from 0.5 to 1 µm) of materials generally having a high density, as exemplified by gold, platinum, tungsten, tantalum, copper, nickel, and a material containing any of these, can be used in the present invention as the X-ray absorber formed on the above support membrane, without any particular limitations. Such an X-ray absorber is formed as follows: providing a plating electrode layer on the above support membrane, patterning thereon a single-layer or multi-layer resist by electron-beam drawing, followed by electroplating, for example, of gold to form a gold pattern serving as the X-ray absorber. Alternatively, a film of W or Ta is formed on the support membrane and a single-layer or multi-layer resist is formed thereon by electron-beam drawing, followed by plasma etching of the W or Ta layer, and thus the X-ray absorber can be formed. Particularly when an inorganic membrane is formed on a silicon substrate, the X-ray absorber may be formed before the back etching of a silicon wafer. Fig. 4 schematically illustrates a manner of carrying out X-ray proximity exposure by using a mask comprising a membrane 20 on which an absorber 22 has been formed. The mask is fixed to a mask stage 41 by a frame 21 by a chuck or like means. On the other hand, a silicon wafer 43 coated with an X-ray resist is fixed on a wafer chuck by a vacuum attraction or the like means. At this time a gap g called a proximity gap is provided between the mask membrane and the surface of the wafer resist so that the mask can be prevented from coming into contact with the wafer to prevent break of the mask. The gap g is made usually to range between 5 µm and 100 µm. X-ray exposure is carried out using the exposure apparatus thus constituted. First, an alignment mark on the mask and an alignment mark on the wafer are registered with alignment light 24 of visible or infrared light to make alignment so that the relationship between the mask and wafer may be brought into a predetermined positional relationship. After the alignment has been completed, X-rays 23 for exposure are irradiated for a necessary while through a shutter separately provided. Usually the exposure irradiation zone on the X-ray mask is smaller than the wafer, and hence it is common to continue to expose the whole wafer surface repeatingly the operation of alignment/X-ray irradiation/stage movement, sequentially. In order to prevent the accumulation of electrostatic charges on the support membrane 10 and the surface of the wafer with X-ray sensitive member thereon 43, the following alternative measures are taken: 1) the support membrane 10 and wafer with X-ray sensitive member thereon 43 are both grounded; 2) the support membrane 10 is electrically connected to the wafer with X-ray sensitive member thereon 43; or 3) a voltage is applied to each of the support membrane 10 and wafer with X-ray sensitive member thereon 43 so as to cancel the potential difference between the support membrane 10 and wafer with X-ray sensitive member thereon 43. As in the above, the support membrane can be effectively protected from being electrostatically charged by the X-ray exposure, by using in the X-ray mask support membrane the material having a given electrical resistivity and also by carrying out a suitable treatment for elimination of electric charges at least at the time of the X-ray exposure, so that deformation of the support membrane can be prevented. Deformation, which is caused by electrostatic attraction accompanying electrostatic charging is thus eliminated. It is thus possible to transfer a pattern with a high precision. The present invention will be described below in greater detail by giving Examples. Example 1 As a first example, firstly a support membrane. comprising silicon carbide (SiC) having a carbon content larger than the stoichiometric composition (silicon : carbon = 1 : 1) was prepared according to a method of firing an organic silicon compound, polysilastyrene. First, polysilastyrene (trade name: S-400; a product of Nippon Soda Co., Ltd.) was dissolved in toluene to prepare a 10 wt/V solution. This solution was coated using a spinner on a carbon substrate whose surface has been mirror-polished, to form a polysilastyrene film with a thickness of 30 µm. Next, this was put in a firing furnace together with the substrate to carry out firing in a dry nitrogen atmosphere at 200°C for 1 hour, and the temperature was raised up to 1,250°C at a rate of 100°C/min and maintained at that temperature for 25 hours. Thereafter the heating of the furnace was stopped to effect natural cooling to obtain a support membrane with a thickness of 2.8 µm, comprising silicon carbide. Electrical resistivity of the support membrane at this time was measured to find to be 0.1 Ω·cm. The resulting support membrane was stretchingly held on a circular support frame, and a gold pattern was further formed on the support membrane, thus preparing an X-ray mask. To effect elimination of electric charges, the support membrane was electrically connected to a wafer with X-ray sensitive member thereon with a conductor wire. Upon exposure of this X-ray mask no deformation was observed at all in the support membrane notwithstanding the irradiation of X-rays, and a high precision transfer of the pattern was performed. Example 2 As a second example, an SiC thin membrane was formed using a CVD (chemical vapor deposition) apparatus, on a silicon (Si) substrate of 2 mm in thickness, whose surface has been polished to 0.01 µm in root mean square roughness. First, using silane gas (SiH 4 ) and methane gas (CH 4 ) as starting material gases, an SiC layer was formed under the conditions of a flow rate ratio of SiH 4 /CH 4 = 0.2, a pressure of 4 Torr and a high frequency power of 90 W. The flow rate was controlled with a mass flow controller. Substrate temperature was maintained up to 600°C during the reactive formation of the layer. After 2 hours, the SiC layer with a thickness of 2.2 µm was formed. Thereafter the substrate was cooled, followed by back etching of the silicon substrate at central part its back side to a size of 40 mm⊘ to obtain an X-ray mask support having the SiC support membrane. The SiC support membrane thus obtained was amorphous, contained a lot of carbon, and had a specific resistance of 0.1 Ω·cm. A gold pattern was further formed on the support membrane to prepare an X-ray mask, and the support membrane and wafer with X-ray sensitive member thereon were electrically connected to each other to effect elimination of electric charges. Upon exposure of this X-ray mask, no enlargement of the ground potential of the support membrane was caused, and no deformation in the support membrane was observed. Example 3 As a third example, polyphenylene sulfide, as used for a heat-resistant structural material was used as the material for the membrane. As illustrated in Fig. 2, a thin layer 20 of polyphenylene sulfide with a thickness of 4 µm was stretchingly held on a circular frame 21, and adhesion-fixed with that state, followed by hot annealing at 200°C for 1 hour. Next, the above frame fitted with the support membrane was disposed standing in a vacuum chamber 30 as illustrated in Fig. 3, and its inside was evacuated to 10 -6 Torr. Thereafter, arsenic tetrafluoride AsF 5 34 was fed up to 1 Torr. A doping treatment was carried out at room temperature by leaving it overnight in the chamber. After the doping treatment, the inside of the vacuum chamber was restored to a normal pressure and thereafter the product was taken out to obtain a maskblank. The amount of the dopant was analyzed to find to be 5 wt.% based on the membrane material polyethylene sulfide. Electrical resistivity was also measured to obtain a specific resistance of 0.8 Ω·cm. An absorber was formed on the above maskblank to prepare an X-ray mask, and a means of applying a voltage on each of the support membrane and wafer with X-ray sensitive member thereon was provided to effect elimination of electric charges. Upon irradiation of X-rays (wavelength: 10 Å), no local electrostatic charging was caused, and also no deformation was observed in the support membrane.

Claims: An X-ray proximity exposure process comprising the following steps: arranging an X-ray mask (20-22) adjacent in close proximity to and spaced from a wafer (42) having an X-ray sensitive member (43) provided thereon, said X-ray mask (20-22) being electrically conductive and including a support frame (21), an X-ray transmissive support membrane (20) held thereon, and an X-ray absorber (22) provided in a patterned form on the surface of said support membrane (20); and exposing said X-ray sensitive member (43) on said wafer (42) to X-rays (23) directed through said X-ray mask (20-22); which process is characterised by: cancelling potential difference between said X-ray mask (20-22) and said wafer (42) by employing one of the following measures: (a) grounding both said support membrane (20) and said wafer (42); or (b) electrically connecting said support membrane (20) to said wafer (42); or (c) applying a common voltage to said support membrane (20) and said wafer (42); in each case said support membrane (20) having an electrical resistivity of 10 Ω.cm or less. A process as claimed in claim 1 wherein said support membrane (20) comprises either conductive ceramics material or conductive organic compound material. A process as claimed in claim 1 wherein said support membrane (20) is of organic material and is of thickness 1 µm to 15 µm. A process as claimed in claim 1 wherein said support membrane (20) is of inorganic material and is of thickness 0.5 µm to 5 µm. A process as claimed in any of the preceding claims wherein said X-ray mask (20-22) and wafer (42) are arranged with a proximity gap spacing of 5 µm to 100 µm. A process as claimed in claim 1 wherein said support membrane (20) is of an organic material and is stretched over said support frame (21). A process as claimed in any preceding claim wherein said X-ray mask (20-22) and said wafer (42) are arranged in parallel. A process as claimed in any preceding claim which includes aligning said X-ray mask (20-22) and said wafer (42) prior to exposure using either visible or infra-red light (24). A process as claimed in any preceding claim wherein said cancelling is performed at least during said step of exposing. A method of manufacturing an IC or ULSI semiconductor device having a patterned structure defined by X-ray lithography, wherein the patterning of said structure by X-ray lithography includes performing an X-ray proximity exposure by a process as claimed in any preceding claim.

Inventors: FUKUDA YASUAKI, FUKUDA, YASUAKI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1988
Main classification: G03F1
IPC codes: G03F7, G03F1