Pattern forming process and process for preparing semiconductor device utilizing said pattern forming process

Abstract: A process for forming an etching pattern, which comprises selectively irradiating a light to a clean surface of a material to be worked by etching so as to form radicals from a photoradical forming substance in an atmosphere of the substance, forming a modified portion having etching resistance at a photo-irradiated portion of the surface, and then subjecting a unmodified portion of the surface of the material to be worked to an etching treatment, thereby forming an etching pattern corresponding to a pattern formed by the irradiation.

Description: This invention relates to a method of producing a semiconductor device, for example a thin film transistor (TFT) utilising a pattern forming process, and in particular is concerned with the patterning of an insulation layer thereof of silicon nitride, SiN. In recent years, with the enlargement of apeture of an exposure device to be utilised in pattern forming technique utilising photolithographic technique, a large area device which is seen in a flat panel such as an active matrix liquid crystal display, a direct current plasma display, etc. is going to be commercially produced. The pattern forming process utilising photolithography to be used for the formation of various patterns which these device have includes steps of coating the predetermined surface of a material to be worked with a photoresist, subjecting the photoresist to treatments including pattern exposure, development to form a resist pattern, and effecting an etching by using a solvent or utilising a physical or chemical reaction in gas phase while using the resist pattern as a mask. In the preparation steps of semiconductor devices such as those including FETs, the photolithographic process to a sample substrate according to a desired pattern is an important technique. As described above, in the photolithographic process, complicated and cumbersome processes such as resist coating, pattern exposure, developing, etching, resist pell-off, etc. have been generally employed. When these devices are prepared according to the process as described above, due the more complicated process itself, cost is increased. Further due to generation of dust, etc. yield is lowered, also the cost will be increased as a whole. In view of the state of the pattern formation technique utilising photolithography as described above, it has been attempted to develop pattern formation techniques which do not require photolithography. There has been proposed a method utilising a photoetching process (see "Examination Research Report Concerning New Electronic Materials XIII" 62-M-273, Society of Electronic Industry Promotion of Japan). Photoetching is a process which effects an etching at a photoirradiated portion by a site selective photoirradiation to a material to be worked in a vacuum capable of maintaining a clean environment. None of the usual steps such as the coating of resist pattern, pattern exposure, and developing are required. Thereby a higher yield can be expected, there is little ionic shock, and also a great cost decrease is possible since cost of providing photoresist is eliminated. Thus, this process is attracting attention as an ideal pattern forming process. As one example of the photoetching processes, there has been known a process in which, by selectively irradiating an excimer laser to a polycrystalline silicon substrate added with phopsphorus in an atmosphere where chlorine gas and methyl methacrylate are permitted to coexist, a polymerised film at the non-irradiated dose is formed, while the polymerised film is not formed at the portion irradiated with a large irradiation dose, and then the substrate is etched by chlorine radicals to form a pattern (SEMI Technol. Symp. 1985, P. F-3-1). Further, a photoetching technique which performs the formation of a pattern shortening the cumbersome process to a great extent in place of the photolithographic process by use of the resist as described above is also described in Sekine, Okano, Horiike: The Fifth Dry Process Symposium Lecture Pretext, p. 97 (1983). In this essay, there is reported a process in which a substrate deposited with a polysilicon (p-Si) film thereon is set within a reaction chamber where chlorine gas is introduced, UV-ray is selectively irradiated through a photomask to the Si substrate by setting the photomask having a light shielding pattern in parallel to the substrate surface, whereby an etching proceeds only at the region where UV-ray is irradiated to form a pattern on the p-Si film. By use of the phototreatment apparatus utilizing the process, the steps of resist coating, developing, resist peel-off, etc. can be obviated to simplify the steps, improve the yield and reduce the cost to a great extent. Further, etching without the generation of damages by ion irradiation which are a problem in the reactive ion etching of the prior art is rendered possible. Nevertheless, the photoetching process as described above still has various problems to be solved. For example, as compared with the electron impingement dissociation area of the molecules in the plasma in the dry etching of the prior art, the light absorbing sectional area of the molecules by photons is smaller by about 1 to 2 orders, and therefore the amount of the radicals which are reactive molecules or atoms having effective non-bonding arms necessary for etching is small. For this reason, it is required to increase the irradiation dose. However, by the photoirradiation apparatus presently available, a sufficiently satisfactory irradiation dose cannot be obtained, and photoirradiation for a long time will be required. For example, J. Vac. Sci. Technol., B3 (5), 1507 (1985) discloses the acceleration of etching by use of an excimer laser which is a large output laser. But the irradiation area in this method is about some ten mm square. For example, when the treatment area is about A4 size, the time required for the etching treatment will be one hour or more. For obtaining a sufficient irradiation dose in the photoetching, the realization of enlarging a laser aperture and a laser output must be awaited, and it is difficult under the present situation to apply photoetching to the fine working process for the sheet system high speed etching accompanied with the enlargement of the device. As a process for improving such problems in the photoetching, the process in which a high speed etching is performed by combining the photoetching with the plasma etching has been proposed. This process comprises selectively irradiating an excimer laser to the surface of a material to be worked by etching while decomposing a gas of a hydrocarbon compound of which a part or all of the hydrogens is substituted with halogen atoms to deposite a polymerized film thereon, decomposing and removing the polymerized film at the photoirradiated portion, and further etching the surface of the material to be worked consequently exposed by halogen atom radicals, thereby obtaining an etching pattern corresponding to the photoirradiated pattern (Japanese Laid-open Patent Application No. 62-219525). However, even in such process, the photoirradiated area is narrow as several ten mm square, and the etching speed is about some 100 nm/min (1000 Å/min), the improvement of one order or more as compared with the photoetching as described above is not desired, and it could be unsatisfactory to apply to the fine working process for sheet system high speed etching accompanied with the enlargement of the device. As described above, the main preparation steps of semiconductor devices are the steps of forming a film of metal, semiconductor or insulator on the substrate and fine working the film to a desired pattern. Presently, in the film forming step and the etching step of performing the fine working for the preparation of these semiconductor devices, the main step comprises, rather than the step using a solution, the so-called dry step using a plasma or excited gas in vacuum or reduced pressure gas. However, for carrying out a desired fine working, in the photolithographic process generally employed, as described above, the complicated processes such as resist coating, pattern exposure, development, etching, resist peel-off, etc. have been employed. However, since a solution is used in the resist coating, developing, resist peel-off steps, all the steps cannot be made dry step. Since in the semiconductor preparation steps as described above the steps of performing in vacuum, in solution and in air are mixed therein, the surface of the samples will be oxidized and also the preliminary steps for the subsequent steps are required (e.g. steps of vacuuming for making the sample under a reduced state, washing and drying after a solution treatment), whereby there is the problem that the number of step may be increased and the steps become complicated. Also, the movement amount of the samples between the respective preparation apparatus will be increased, whereby losses in both time and space are increased. The increase of step number, complicatedness of the steps, increase of movement amount of samples involve the problem of increasing the attachment of dust. Further, by use of a resist and peel-off thereof, the resist peeled off becomes a dust to be attached on the sample surface, whereby there is the problem that performances of the device is deteriorated and also the yield is lowered. An alternative process to photoetching is described in UK Patent Application GB-A-997642 in which a metal, alloy, or semiconductor, thin film is selectively irradiated in an atmosphere of a photolysible oxidising gaseous substance to form an etchant resistant pattern and etching is performed by e.g. gaseous hydrogen fluoride. Thin film materials, that can be patterned in this manner, are chosen from (a) groups IV and transition metal elements of the Periodic Table, e.g. Sn, Ge, Si, Fe, Pb; (b) binary alloy films of group IV elements of the Periodic Table, e.g. Sn-Ge, Pb-Sn, Ge-Si, Pb-Ge; binary alloy films of transition metals, e.g. Ni-Fe, Ti-Fe; and (c) Intermetallic and semiconductor compound films of groups II-V, III-V, II-VI, e.g. GaAs, CdS, CdSe, ZnP, InTe, GaP, GaSb, InSe, InAs; (d) Elemental semiconductors Si, Ge, Examples of photolysible substances are given as: nitrogen dioxide, chlorine dioxide and mixtures of nitrous oxide and oxygen. The present invention as defined by the appended claims has been accomplished in view of the problems in the various etching techniques utilising the fine working process as described above, and it is intended to provide a process capable of forming a patterned portion having resistance to an etching by utilising the photoirradiation without utilising the formation step of the resist pattern in the photo-lithography, and forming an etching pattern at a high yield and at a low cost. In addition it is intended to realise a process for preparing an FET semiconductor device which can prevent oxidation of the surface, reduce the preparation steps and the movement amount of the device, improve the device performances and improve the yield. In the accompanying drawings: Fig. 1 and Fig. 2 are schematic illustrations of the apparatus which can be used in the process of the present invention; Fig. 3 is an upper view showing one example of preparation apparatus; Fig. 4 through Fig. 8 are schematic sectional views respectively illustrating the cleaning chamber 602, the sputtering film forming chamber 603, the plasma film forming chamber 604, the etching chamber 605 and the latent image chamber 606 in Fig. 3; Figs. 9A to 9G are step illustrations showing the preparation steps of an FET; Fig. 10 is a graph showing the dependency of drain voltage V D on the drain current I D in FET prepared in working Example 1; Figs. 11A to 11G are respectively illustrations for stepwise explaining the preparation steps during preparation of another FET by the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention enables an etching formation with a high yield, because site selective irradiation treatment of light in an atmosphere of a photoradical forming substance can easily accomplish a high yield for the formation of a pattern-shaped portion having an etching resistance. Also, according to the process of the present invention, it is possible to form an etching pattern at a low cost by reducing the cost required for the materials and eliminating steps necessary for the formation of a resist pattern as used in photolithography. Further, in the process of the present invention, a photoirradiation is not utilised for etching as in photoetching, but for the formation of a portion having an etching resistance by the modification of a predetermined portion on the surface of the material to be worked. When the etching of a silicon nitride film is practised by utilising the resist pattern formed by the modification of silicon nitride according to the process of the present invention, the resist pattern can be improved. Also, by dividing the formation step of etching resist utilising photoirradiation and etching treatment step by radicals, and independently performing each of them, it becomes possible to shorten the time required for formation of etching pattern to a great extent. The process of the present invention is further described below in detail by referring to the drawings. Fig. 1 and Fig. 2 illustrate the outline of respective apparatus each of which can be used for both selective irradiation and photolytic patterning. In Fig. 1 and Fig. 2 the sample to be worked 401 has a thin film 401b of the aforesaid silicon nitride insulator formed on a substrate 401a applied by a thin film forming technique. First, as shown in the drawings, the sample to be worked 401 is set at a predetermined position within a vacuum tank 402. The thin film 401b, on account of the characteristics in the film forming step thereof, when contacted with air, frequently has a naturally oxidised film, water and organic adherents formed on its surface. Accordingly, it may be necessary to carry out a cleaning treatment to remove native oxide and adherents from the surface of the thin film 401b. For the cleaning treatment, there may be employed a method in which an inert gas or hydrogen gas is introduced through a gas introducing pipe 404 into the vacuum tank 402 which is evacuated by an evacuation device 403, and a plasma is generated by actuation of plasma generators 405 and 405', and the naturally oxidised film, water, organic adherents, etc. on the surface of thin film 401b are removed by the radicals formed. Further, after the film formation of the thin film 401b, by setting the thin film within the vacuum tank 402 without exposure to air, the thin film 401b having a clean surface can be set within the vacuum tank 402 without performing the cleaning treatment. Also, after the film formation of the thin film 401b, by performing the surface modification by irradiating hydrogen ions, fluorine ions, etc. to its surface, a clean surface or a passivated surface (which can be easily cleaned and which makes it difficult to form a naturally oxidised film (native oxide) on the surface of the thin film 401b and difficult to attach water and other adherents to the surface) can be obtained. In the process of the present invention, for the effective site selective surface modification of the surface to be etched such as the thin film 401b surface, etc. with a good precision, it is extremely important that the surface to be etched is clean and substantially free of native oxide. Next, a gas of a photoradical forming substance (a photolysible oxidising gas) is introduced through a gas introducing pipe 404 into the vacuum tank 402, and light is irradiated site-selectively to the portion of the thin film 401b surface corresponding to the desired etching pattern. As the site-selective photoirradiation method, for example, the projection exposure system shown in Fig. 1 or the laser direct drawing system shown in Fig. 2 can be utilized. In the projection exposure system, as shown in Fig. 1 the exposure pattern possessed by an exposure mask 408 is projected to form an image by light from a light source 406 passed through an optical system including lenses 407 and 408, to effect a pattern exposure to the thin film 401b surface. On the other hand, in the direct drawing system, as shown in Fig. 2, a continuous laser beam oscillated from a laser beam source 410 passes through an optical modulator 411, a collimator lens 412 and a rotatory multi-planar mirror 413, and exposure is effected by a spot image formation on the surface of the thin film 401b within the vacuum tank 402 by a f lens 414 from a window 415. The site selective photoirradiation in the laser direct drawing system can be performed by scanning the image forming position of the laser beam modulated by the optical modulator 411 in one direction of a thin film 401b surface by rotation of the rotatory multi-planar mirror 413, at the same time moving the image forming position of the laser beam by delivering the material to. be worked 401 in the direction vertical to the scanning direction of the image forming position by use of a delivery system 416 of the material to be worked, and controlling the scanning by the rotatory multi-planar mirror, the delivery of the material to be worked, and the timings of irradiation and non-irradiation of the laser. The image source for photoirradiation to be used is chosen depending on the kind of the photoradical forming substance introduced into the vacuum tank 402. As the photoradical forming substance, a gas or gaseous compound, capable of forming oxygen radicals, is used. It may be selected from one or more of oxygen, nitrogen dioxide, ozone, nitrous oxide or carbon dioxide, for example. As the light source, one capable of generating light having a wavelength no longer than the UV-ray absorption edge wavelength of the photoradical forming substance can be utilised without any limitation. For example, when oxygen gas is used as the photoradical forming substance, since the UV-ray absorption edge wavelength in its radical generation is about 242 nm, a light source generating light having a shorter wavelength than that can be utilised, for example, rare gas excimer laser, rare gas halide excimer laser, etc. can be utilized. On the other hand, when nitrogen dioxide gas is used as the photoradical forming substance, since the UV-ray absorption edge wavelength in its radical regeneration is near 400 nm, for example, in addition to excimer lasers such as rare gas excimer laser, rare gas halide excimer laser, etc., continuous oscillation laser such as argon ion laser, krypton ion laser, He-Cd laser, etc. can be utilized, whereby there are included the advantages such that choice of light source can be expanded and also that handling is easy. Through the action of the radicals generated thus by photoexcitation, the photoirradiated portion of the thin film 401b is modified. The modified portion thus formed is not etched by halogen atom radicals such as fluorine radicals, chlorine radicals, etc., while the unmodified portion where no photoirradiation is effected remain in a state in which it can be etched by these etchant species. Next, when unnecessary remaining gas is discharged from the vacuum tank 402 by the evacuation device 403 to set a predetermined vacuum degree within the vacuum tank 402, a compound gas for etching is introduced through a gas introducing pipe 417, and microwave radiation is excited within a plasma pipe 419 with the microwave oscillated from a magnetron 418, and introduced through a transporting pipe 420 into the vacuum tank 402. As the compound for etching, substances capable of generating halogen atom radicals such as a mixture of Cl 2 and NF 3 can be preferably utilised. Fig. 3 is a schematic upper view showing the constitution of an example of a preferable apparatus for applying the process described above, which is used for forming a pattern on a sample 608. In Fig. 3, 601 is a loadlock chamber for introducing and taking out the sample 608, and 602 a cleaning chamber for cleaning the surface of the sample 608 by use of plasma. 603 is a sputtering film forming chamber for forming a film of a metal by sputtering. 604 is a plasma film forming chamber for forming a silicon nitride insulating film and semiconductor film by plasma. 605 is an etching chamber for effecting the chemical dry etching of the film. 609 is a conveyer for conveying the sample 608 by way of a conveying mechanism not shown, and is provided within a conveying chamber 610. The loadlock chamber 601, the cleaning chamber 602, the sputtering film forming chamber 603, the plasma film forming chamber 604, the etching chamber 605, the latent image chamber 606 and the conveying chamber 610 are all constituted vacuum-sealedly and equipped respectively with vacuum evacuation devices (not shown) for vacuum evacuation internally of the chambers. The loadlock chamber 601, the cleaning chamber 602, the sputtering film forming chamber 603, the plasma film forming chamber 604, the etching chamber 605 and the latent image chamber 606 are respectively arranged around the conveying chamber 610, and communicated to the conveying chamber 610 respectively through the gate valves 607b to 607g. The loadlock chamber 601 is also provided with, in addition to the gate valve 607b communicated to the conveying chamber 610, the gate valve 607a for taking in and out the sample 608 between the chamber and outside. Fig. 4 through Fig. 7 are respectively schematic sectional views for illustrating the constitutions of the cleaning chamber 602, the sputtering film forming chamber 603, the plasma film forming chamber 604, the etching chamber 605 and the latent chamber 606 in Fig. 3. Fig. 4 shows the cleaning chamber 602. In the same Figure, 621 is the gas introducing inlet provided above the cleaning chamber 602 for introducing cleaning gas into the cleaning chamber 602, 622 a sample holding stand for holding the sample 608 placed in the cleaning chamber 602, 623 a counter-electrode opposed to the sample holding stand 622, 624 the insulator for insulating the sample holding stand 622 from the vacuum vessel constituting the cleaning chamber 602, 625 a high frequency power source of 13.56 MHz, 200 W, which is connected to the sample holding stand 622. 626 is the matching box for taking matching between the sample holding side and the high frequency power source side. Fig. 5 shows the sputtering film forming chamber 603. In the same Figure, 631 is the gas introducing inlet for introducing the sputtering gas into the sputtering film forming chamber 603, 632 is the sample holding stand for holding the sample 608 placed in the sputtering film forming chamber 603, 633 is the counter-electrode provided as opposed to the sample holding stand 632 and applied with high frequency power, 634 the insulator for insulating the counter-electrode 633 from the vacuum vessel constituting the sputtering film forming chamber 603, 635 the high frequency power source of 13.56 MHz and 500 W, 636 the matching box for taking matching between the sample holding stand 632 and the high frequency power source 635 side, 637 the condenser for insulating direct currently the counter-electrode 634, and 638 the sputtering metal. Fig. 6 shows the plasma film forming chamber 604. In the same Figure, 641 is the gas introducing inlet for introducing the deposition gas into the plasma film forming chamber 604, 642 the sample holding stand for holding the sample 608 placed in the plasma film forming chamber 604, 643 the counter-electrode provided as opposed to the sample holding stand 642 and applied with high frequency power, 644 the insulator for insulating the counter-electrode 643 from the vacuum vessel constituting the plasma film forming chamber 604, 645 the high frequency power source of 13.56 MHz and 350 W, and 646 the matching box for taking matching between the sample holding stand 642 and the high frequency power source 645. Fig. 7 shows the etching chamber 605. In the same Figure, 652 is the sample holding stand for holding the sample 608 placed in the etching chamber 605, 653 the microwave plasma gas excitation device for generating the excited gas to be fed into the etching chamber 605, 651 the gas introducing inlet for feeding the etching gas to the microwave plasma gas excitation device 653, 654 the transporting pipe for transporting the excited gas generated in the microwave plasma gas excitation apparatus 653 to the etching chamber 605 and is provided at the position opposed to the sample holding stand 652. Fig. 8 shows the latent image chamber 606. In the same Figure, 661 is the gas introducing inlet for introducing the latent image gas into the latent image chamber 606, 662 the sample holding stand for holding the sample 608 set within the latent image chamber 606, 663 the KrF excimer laser which is the light source, 664 the illuminating optical system for illuminating the mask 665 as described below by the generated light at the light source 663, 665 the mask (or reticle) patterned by Cr on the quartz plate, 666 the projection optical system for forming image of the mask pattern on the surface of the sample 608, 667 the window for introducing the light coming out from the projection optical system 666 into the latent image chamber 606. Of the respective devices as described above, the light source 663, the illumination optical system 664, the mask 665, the projection optical system 666 are provided above the latent chamber 606. By making the apparatus constitution as described above, the respective steps for preparing a field effect transistor can be carried out consistently while maintaining a state shielded from air and reduced in a pressure without use of a resist. By doing so, a field effect transistor improved in performance can be prepared without oxidation of the surface of the member to be treated. Also, preliminary steps such as the step of vacuum evacuation from an atmospheric pressure between the respective steps and the steps of washing and drying after the solution treatment can be made unnecessary, whereby the productivity can be improved. Further, the number of steps and the movement amount between the steps can be reduced to reduce the chances of attachment of dust, and also the generation of dust due to resist peel-off does not occur, whereby the yield can be improved. The apparatus described by referring to Fig. 3 to Fig. 8 is not limited by the constitution as described above, but can be modified. Particularly, although the conveying chamber 610 is surrounded by the respective chambers, it may have a linear arrangement in which the respective chambers are arranged in series and provided with the conveying chamber. It is also possible to make the conveying chamber movable, thereby making it connectable and releasable for each chamber. The present invention is described in more detail by referring to Background Examples and Working Examples, the latter being examples of preferred embodiments. Background Example 1 A material to be worked having an A1 film with a thickness of 150 nm (1500 Å) formed on a silicon wafer as the substrate according to the sputtering method was set at the setting position of the material to be worked in the apparatus shown in Fig. 1. Next, after vacuum evacuation internally of the vacuum tank 402 by an evacuation device 403 to 10 -4 Pa (10 -6 Torr) or lower, argon gas was permitted to flow in at 20 SCCM, and the plasma generators 405 and 405', were actuated to generate a plasma, and the material to be worked was exposed in the atmosphere of the plasma generated for 20 minutes, and the Al film surface was cleaned by removing the Al oxide film and the adherents, etc. therefrom. Then, the vacuum tank 402 was internally vacuum evacuated to 10 -4 Pa (10 -6 Torr) or lower. Next, NO 2 gas was permitted through the gas introducing pipe 404 into the vacuum tank at 200 SCCM, and by use of an ArF excimer laser (wavelength 193 nm) as the light source 406, a pattern exposure was effected through the exposure mask 408 for formation of a desired exposure pattern at a laser oscillation frequency of 50 Hz for 5 minutes. Then, chlorine gas for etching was permitted to flow in through the gas introducing pipe 417 at 500 SCCM, excited by the magnetron 418 to form chlorine radicals, which were introduced through the transporting pipe 420 into the vacuum tank 402, and etching treatment was carried out under such state for 10 minutes. After completion of etching, the material to be worked was taken out from the apparatus, and its surface was observed. As the result, etching was not seen at the photoirradiated portion, and the silicon wafer which is the substrate exposed at the non-irradiated portion and was also etched. Background Example 2 After a polysilicon film (film thickness 400 nm (4000 Å)) was formed on a silicon wafer as the substrate according to the CVD method, subsequently in the same vacuum tank, hydrogen ions generated from hydrogen gas according to the ECR plasma method at a vacuum degree of 10 -1 Pa (10 -3 Torr) or less were irradiated to the above polysilicon film to effect its surface modification. The material to be worked thus obtained was set at the setting position of the material to be worked in the apparatus shown in Fig. 2 Next, after the vacuum tank 402 was internally vacuum evacuated to 10 -4 Pa (10 -6 Torr) or less, NO 2 gas was permitted to flow in through the gas introducing pipe 404 into the vacuum tank 402 at 100 SCCM, and by use of an argon ion laser as the laser beam source 410, continuous irradiation was effected under the conditions of an output of 1 W, a spot diameter of 50 µm, a parallel scanning speed of 20 cm/sec., and a delivery speed of the material to be worked of 2 mm/sec. to form a stripe-shaped irradiated pattern on the polysilicon film surface of the material to be worked. Then, NF 3 gas for etching was permitted to flow in through the gas introducing pipe 417 at 800 SCCM, excited by the magnetron 418 to form fluorine radicals, which were introduced through the transporting pipe 420 into the vacuum tank 402 to effect etching treatment in such state for 3 minutes. After completion of etching, the material to be worked was taken out from the apparatus, the etching depth at the etched position was measured by Taristep manufactured by Hobson-Taylor Company. As the result, it is confirmed that the photoirradiated portion was not etched, but the non-irradiated portion was etched to have a depth of 250 nm (2500 Å). As the result of its surface observation, a pattern of continuous unevenness corresponding to the photoirradiated portion and the unirradiated portion arranged in a stripe was observed. Working Example 1 At the setting position of the material to be worked in the vacuum tank 402 in the apparatus shown in Fig. 5, gases of SiH 4 20 SCCM, NH 3 500 SCCM and H 2 180 SCCM were permitted to flow through the gas introducing pipe 404 into the vacuum tank 402 and mixed, and by actuation of the plasma generators 405 and 405', an amorphous silicon nitride film (a-SiN:H film ) was formed with a thickness of 500 nm (5000 Å) on the substrate heated to 300 °C to form a material to be worked. The material to be worked was subjected subsequently to formation of etching pattern as described below in the state set in the vacuum tank 402, namely without contact with air by taking out of the apparatus. Next, after the vacuum tank was internally vacuum evacuated to 10 -4 nm (10 -6 Torr) or less, NO 2 gas was permitted to flow in through the gas introducing pipe 404 into the vacuum tank 402, and by use of an argon ion laser as the laser beam source 410, continuous irradiation was effected under the condition of an output of 1 W, a spot diameter of 50 µm, a parallel scanning speed of 20 cm/sec. and a delivery speed of the material to be worked of 2 mm/sec. to form a stripe-shaped irradiated pattern on the polysilicon film surface of the material to be worked. Then, Cl 2 gas 200 SCCM and NF 3 gas 100 SCCM for etching were permitted to flow in through the gas introducing pipe 417, these were excited by the magnetron 418 to form chlorine and fluorine radicals, which were introduced through the transporting pipe 420 into the vacuum tank 402 to effect etching treatment in such state for 10 minutes. After completion of etching, the material to be worked is taken out from the apparatus, the etching depth of the etched portion was measured by Taristep manufactured by Hobson-Taylor Company. As the result, it was confirmed that the photoirradiated portion was not etched, but the non-irradiated portion was etched with a depth of 300 nm (3000 Å) As the result of its surface observation, a pattern of continuous unevenness corresponding to the photoirradiated portion and the unirradiated portion arranged in stripe was observed. Further, when the dielectric constant and dielectric breakdown voltage of the a-SiN:H film (the portion remained without etching) subjected to the surface modification in the procedure of this Example were measured, the dielectric constant was found to be 7.0 unchanged as before the modification, while the dielectric breakdown voltage was improved by about 1.5-fold as 6 MV/cm as compared with that before the modification. Working Example 2 By utilizing the process of the present invention, TFT was prepared according to the procedure as described below. The outline of the steps is shown in Figs. 9A to 9G. First, on a glass substrate 421, an amorphous silicon film 422 (film thickness 500 nm (5000 Å)) was formed by plasma decomposition of SiH 4 by use of the plasma CVD method at a substrate temperature of 250 °C (Fig. 9A). Next, in the same vacuum region, the gas mixture of SiH 4 and PH 3 was plasma decomposed according to the plasma CVD method to form a film of an amorhous silicon n + -layer 423 (film thickness 100 nm (1000 Å)) at a substrate temperature of 250 °C (Fig. 9B). Further, the substrate at the stage having formed the layer 423 was set in the apparatus shown in Fig. 2 and after cleaning of the surface in the same manner as in Background Example 2, the layer 423 was subsequently etched to a desired pattern in the same apparatus (Fig. 9C ) . On the substrate surface including the layer 423 formed in a pattern, an a-SiN:H film 424 (film thickness 400 nm (4000 Å)) was formed in the same manner as in Working Example 1 (Fig. 9D) and further subsequently the surface modification and the etching treatment were effected in the same manner as in Working Example 1 (Fig. 9E). Finally, as described in Background Example 1, an A1 film 425 (film thickness 300 nm (3000 Å)) was formed according to the sputtering method (Fig. 9F), set in the apparatus in Fig. 1 subjected to cleaning of the surface, photoirradiation and to etching treatment similarly as in Background Example 1 and then taken out from the apparatus to obtain an FET (Fig. 9G ) . The site selective exposure for the respective etching steps as described above controlled the pattern of the exposure mask and the scanning conditions of laser beam, etc. so that the channel width of the FET prepared may be 25 µm and the channel length 10 µm. Dependency of the drain current I D of the obtained TFT on the drain voltage V D was examined. The results are shown in Fig. 10. As is apparent from the results in Fig. 10, the TFT obtained in this Example was found to have good characteristics with little trap site under gate channel. Working Example 3 Referring to the apparatus described in Figs. 3 to 8, the preparation procedures of this Example in preparing an IG-FET are described. Fig. 11A to Fig. 11G are each schematic sectional views showing the structure of the grown film in each preparation step. The quartz plate 671 shown as the sample 608 in Figs. 3 to 8 was placed through the gate valve 607a into the loadlock chamber 601, and the loadlock chamber 601 was internally vacuum evacuated to 10 -4 Pa (10 -6 Torr) or lower by means of a vacuum evacuation device (not shown). In the following, the sample 608 is called hereinafter as the quartz plate 671. The quartz plate 671 may be also called as such, also including the case when a deposited film was provided on the quartz plate 671. The conveying chamber 610 was evacuated constantly by means of a vacuum evacuation device (not shown) to be maintained at a pressure of 10 -4 Pa (10 -6 Torr) or lower. The gate valve 607b was opened, the quartz substrate 671 which is a sample was received by the conveyor 609, placed in the conveying chamber 601 and the gate valve 607b was closed. As the next step, the cleaning chamber 602 was previously vacuum evacuated internally to 10 -5 Pa (10 -7 Torr) or lower by means of a vacuum evacuation device (not shown), the gate valve 607c was opened and the quartz substrate 608 was mounted by the conveyor 609 on the sample holding stand 622 in the cleaning chamber 602, followed by closing the gate valve 607c. The cleaning chamber 602 was vacuum evacuated internally to 10 -5 Pa (10 -7 Torr)or lower by a vacuum evacuation device (not shown). Through the gas introducing inlet 621, a gas for cleaning of the surface of the quartz substrate 671, Ar 50 SCCM in this case, was introduced into the cleaning chamber 601, and the operation of the vacuum evacuation device (not shown) was controlled to a pressure of 11 Pa (0.08 Torr) in the cleaning chamber 602. Next, a high frequency of 13.56 MHz and 100 W was applied to the sample holding stand 622 while controlling the matching box 626 to generate a plasma in the space between the sample holding stand 622 and the counter-electrode 623. The sample holding stand 622 was insulated direct-currently by the condenser (not shown) within the matching box 626, and therefore the sample holding stand 622 generates a negative direct current bias voltage of about -60 V from the difference in mobility between electrons and ions, by the voltage Ar ions were accelerated to be impinged to the surface of the quartz substrate 671 and remove the contamination on the surface by sputtering, thus giving a clean surface. The treatment time was about 600 sec. Feeding of the gas was stopped after completion of the treatment, and the cleaning chamber 602 was vacuum evacuated to an internal pressure of 10 -5 Pa (10 -7 Torr) or lower. The gate valve 607c was opened, the quartz substrate 671 was taken out by the conveyor 609, the gate valve 607c was closed and the cleaning chamber 602 was again vacuum evacuated so as to maintain an internal pressure of 10 -5 Pa (10 -7 Torr) or lower. Next, the plasma film forming chamber 604 was vacuum evacuated internally to 10 -5 Pa (10 -7 Torr) or lower by a vacuum evacuation device (not shown). Subsequently, the gate valve 607e was opened, the quartz substrate 671 was introduced into the plasma film forming chamber 604 by the conveyor 609 and placed on the sample holding stand 642 previously heated to 250 °C by a heater (not shown), followed by closing of the gate valve 607e. Then, the plasma film forming chamber 604 was internally vacuum evacuated to 10 -5 Pa (10 -7 Torr)or lower by a vacuum evacuation device (not shown), and the quartz substrate 671 was heated to a temperature of 250 °C. Next, SiH 4 was introduced at 100 SCCM and H 2 at 740 SCCM through the gas introducing inlet 641 into the plasma forming chamber 604 to control the operation of the vacuum evacuation device (not shown) so that the pressure of the plasma film forming chamber 604 may become 67 Pa (0.5 Torr). A high frequency of 13.56 MHz and 100 W was applied to the counter-electrode 643 while controlling the matching box 646 to generate a plasma in the space between the sample holding stand 642 and the counter-electrode 643 to effect plasma decomposition of the introduced gas, thereby depositing an amorphous silicon (a-Si) film 672 thereon to 500 nm (5000 Å) (see Fig. llA). After film formation, the gas introduced was stopped and vacuum evacuation effected to 10 -5 Pa (10 -7 Torr) or lower internally of the plasma film forming chamber 604, and thereafter SiH 4 was introduced at 20 SCCM, H 2 at 80 SCCM, PH 3 at 400 SCCM through the gas introducing inlet 641 into the plasma film forming chamber 604 to control the operation of the vacuum evacuation device (not shown) so that the pressure of the plasma film forming chamber 604 may become 67 Pa (0.5 Torr). Next, a high frequency of 13.56 MHz and 300 W was applied to the counter-electrode 643 while controlling the matching box 646 to generate a plasma in the space between the sample holding stand 642 and the counter-electrode 643 and effect plasma decomposition of the introduced gas, thereby depositing an n + -amorphous silicon (n + -Si) film 673 to 1000 nm (1000 Å) (see Fig. 11B). After film formation, the introduced gas was stopped, the plasma film forming chamber 604 vacuum was evacuated internally to 10 -5 Pa (10 -7 Torr)or lower, the quartz substrate 671 was taken out by the conveyor 609 by opening the gate valve 607e, the gate valve 607e was closed, and the plasma film forming chamber 604 was vacuum evacuated so as to maintain an internal pressure of 10 -5 Pa (10 -7 Torr) or lower. Next, the gate valve 607g of the latent image chamber 606 previously evacuated to 10 -5 Pa (10 -7 Torr) by a vacuum evacuation device (not shown) was opened, the quartz substrate 671 was introduced by the conveyor 609 into the latent image chamber 606 and mounted on the sample holding stand 662, and the gate valve 607g was closed. Subsequently, the latent image chamber 606 was internally vacuum evacuated by a vacuum evacuation device (not shown) to 10 -5 Pa (10 -7 Torr)or lower. NO 2 gas was introduced through the gas introducing inlet 662 into the latent image chamber 606 to control the vacuum evacuation device so that the internal pressure may become 133 Pa (1 Torr). Subsequently, a laser beam with a wavelength of 248 nm oscillated by a KrF excimer laser which is the light source 663 was irradiated uniformly by the illumination optical system 664 to the mask 665 on which a pattern for formation of a channel (channel width: 25 µm, channel length: 10 µm) had been formed, and the pattern image of the mask 665 formed through the window 667 onto the surface of the n + -Si film 673 on the quartz substrate by the projection optical system 666. As the material of the window 667, quartz was employed for permitting the laser beam with the wavelength of 248 nm so as to be not absorbed but transmitted. On the surface of the n + -Si film 673 where the mask image was formed, NO 2 and Si caused the photochemical reaction only at the portion where the light was irradiated, to form an SiO x layer with a thickness of about 2 nm (20 Å) (not shown) on the n + -poly-Si surface by exposure for 10 minutes. At the portion where light was not irradiated, such reaction was not proceeded, and consequently a negative pattern of the mask is formed on the surface of the n + -Si film 673. In other words, the n + -Si film 673 was modified to SiO x and a latent image was formed. As the light source 663, a KrF excimer laser was employed here, the lamp light sources such as xenon lamp, low pressure mercury lamp, high pressure mercury lamp, etc. and UV-ray lasers such as ArF excimer laser, XeCl excimer laser, Ar laser, etc. have also same effects. After formation of the latent image as described, the gas introduced was stopped, the latent image chamber 606 internally was vacuum evacuated 10 -5 Pa (10 -7 Torr) or lower, the gate valve 607g was opened, the quartz substrate 671 was taken out by the conveyor 609 and the gate valve 607g was closed. Then, the latent image chamber 606 was vacuum evacuated again so as to maintain the internal pressure at 10 -5 Pa (10 -7 Torr) or lower. Next, the gate valve 607f of the etching chamber 605 previously evacuated by a vacuum evacuation device (not shown) to 10 -5 Pa (10 -7 Torr) was opened, the quartz substrate 671 was introduced by the conveyor 609 into the etching chamber 605 and mounted on the sample holding stand 652, followed by closing of the gate valve 607f. Then, the etching chamber 605 was vacuum evacuated internally to 10 -5 Pa to (10 -7 Torr) or lower by a vacuum evacuation device (not shown). Subsequently, through the gas introducing inlet 651, a gas for etching the n + -Si film 673 applied with the latent image, Cl 2 at 500 SCCM in this case, into the microwave plasma gas excitation device 653 to control the operation of a vacuum evacuation device (not shown) so that the pressure in the etching chamber 605 may become 33 Pa (0.25 Torr). Subsequently, the microwave of 2.45 GHz and 700 W generated by a microwave generating device (not shown) was supplied to the microwave plasma gas excitation device 653, and the excited molecules Cl 2 * and Cl* excited by plasma formation of the etching gas were supplied into the etching chamber 605 through the transporting pipe 654 which is made of quartz with a whole length of 20 cm and an inner diameter of 40 mm. With an SiO 2 layer having a latent image pattern on the surface of the excited molecule n + -Si film 673 reaching the quartz substrate 671 as the mask, a chlorine compound which is a volatile substance was generated through the reaction with the n + -Si film 673, whereby the n + -poly-Si film was etched to form a channel (see Fig. 11C). Then, the gas introduced was stopped, and the etching chamber was internally vacuum evacuated to 10 -5 Pa (10 -7 Torr) or lower, the gate valve 607f was opened, the quartz substrate 671 was taken out by the conveyor 609, the gate valve 607f was closed and the etching chamber 605 again internally vacuum evacuated so as to maintain a pressure of 10 -5 Pa (10 -7 Torr) or lower. Next, the gate valve 607e of the plasma film forming chamber 604 with the internal pressure already made to 10 -5 Pa (10 -7 Torr) or lower was opened, the quartz substrate 671 was introduced into the chamber by the conveyor 609 and mounted on the sample holding stand 642 previously heated to 350 °C by a heater (not shown) and then the gate valve 607e was closed. Subsequently, the plasma film forming chamber 604 was internally vacuum evacuated to 10 -5 Pa (10 -7 Torr) or lower by a vacuum evacuation device (not shown) and heated until the temperature of the quartz substrate 671 may become 350 °C. Through the gas introducing inlet 641, SiH 4 at 13 SCCM, H 2 at 120 SCCM, and NH 3 at 360 SCCM were introduced into the plasma film forming chamber 604, and the operation of a vacuum evacuation device (not shown) was controlled so that the pressure of the plasma film forming chamber may become 67 Pa (0.5 Torr). A high frequency of 13.56 MHz and 80 W was applied to the counter-electrode 643 while controlling the matching box 646 to generate a plasma in the space between the sample holding stand 642 and the counter-electrode 643 and effect plasma decomposition of the introduced gas, thereby depositing an amorphous silicon nitride (a-SiN) film 674 to 400 nm (4000 Å) (see Fig. 11D). After the film formation, the introduced gas was stopped, the plasma film forming chamber 604 was internally evacuated to 10 -5 Pa 10 -7 Torr) or lower, than the gate valve 607e was opened, the quartz substrate 671 was taken out by the conveyor 609, the gate valve 607e was closed, and the plasma film forming chamber 604 was again vacuum evacuated so as to maintain the internal pressure of 10 -5 (10 -7 Torr) or lower. Next, the gate valve 607g was opened, the quartz substrate 671 introduced into the latent image chamber 606 by the conveyor 609 and mounted on the sample holding stand 662, the gate valve 607g was closed to prepare a latent image of SiO x film on the a-SiN film 674, which is the same as the latent image film for channel formation formed on the n + -Si film 673 as described above, on the same position. Next, the a-SiN film 674 was etched with the latent image as the mask to form a gate insulating film, and as the etching gas, Cl 2 at 900 SCCM and NF 3 at 100 SCCM were fed through the gas introducing inlet 651 to effect etching of the a-SiN film 674 similarly as the n + -Si film 673 (see Fig. 11E). Then, the above introduced gas was stopped, the etching chamber 605 was internally vacuum evacuated to 10 -5 Pa (10 -7 Torr) or lower, the gate valve 607f was opened, the quartz substrate 671 was taken out by the conveyor 609, the gate valve 607f was closed, and the etching chamber 605 was again vacuum evacuated so as to maintain the inner pressure at 10 -5 Pa (10 -7 Torr) or lower. Next, the gate valve 607d of the sputtering film forming chamber 603 previously vacuum evacuated internally to 10 -5 Pa (10 -7 Torr) or lower by a vacuum evacuation device (not shown) was opened, the quartz substrate 671 was introduced into the sputtering film forming chamber 603 by the conveyor 609 and mounted on the sample holding stand 632, and the gate valve 607d was closed. Subsequently, the sputtering film forming chamber 603 was internally vacuum evacuated to 10 -5 Pa (10 -7 Torr) or less by a vacuum evacuation device (not shown). Through the gas introducing inlet 631, Ar at 50 SCCM was introduced into the sputtering film forming chamber 603, and the operation of a vacuum evacuation chamber (not shown) was controlled so that the pressure in the sputtering film forming chamber 603 may become 7 Pa (0.05 Torr). A high frequency of 13.56 MHz and 500 W was applied on the counter-electrode 633 while controlling the matching box 636 to generate a plasma in the space between the sample holding stand 632 and the counter-electrode 633 and to effect plasma decomposition of the introduced gas, and the aluminium mounted as the sputtering metal 638 on the counter-electrode 633 was sputtered to deposit an Al thin film 675 to 400 nm (4000 Å) on the quartz substrate 671 (see Fig. 11F). After film formation of the Al thin film 675, the introduced gas was stopped, the sputtering film forming chamber 603 was internally vacuum evacuated to 10 -5 Pa to (10 -7 Torr) or lower, the gate valve 607d was opened, the quartz substrate 671 was taken out by the conveyor 609 the gate valve 607d was closed, and the sputtering film forming chamber 603 was again vacuum evacuated so as to maintain the internal pressure of 10 -5 Pa (10 -7 Torr) or lower. Next, the gate valve 607g was opened, the quartz substrate 671 was introduced by the conveyor 609 into the latent image chamber 606 and mounted on the sample holding stand 662, the gate valve 607g was closed, and a latent image layer for electrode was prepared on the surface of the Al thin film 675 according to the same procedures as in the twice latent image forming steps as described above. However, the composition of the latent image formed here is AlOx. Next, the gate valve 607f was opened, the quartz substrate 671 was mounted on the sample holding stand 672, and the Al thin film was etched with the latent image layer as the mask according to the same procedures as the etching step as described above to form source, drain and gate electrodes (see Fig. 11G). As the etching gas, Cl 2 was fed at 1000 SCCM through the gas introducing inlet 651 to effect etching similarly as the n + -Si film 673 and the a-SiN film 674 (see Fig. 11E). Then the introduced gas was stopped, the etching chamber 605 was internally vacuum evacuated to 10 -5 Pa (10 -7 Torr) or lower, the gate valve 607f was opened, the quartz substrate 671 was taken out by the conveyor 609, the gate valve 607f was closed, the gate valve 607b was opened, the quartz substrate 671 was placed in the loadlock chamber 601, the gate valve 607b was closed, the internal pressure of the loadlock chamber 601 was returned internally to atmospheric pressure, and the quartz substrate having FET formed thereon was taken out.

Claims: A method of producing a semiconductor device having an insulator layer of silicon nitride SiN patterned by etching, wherein the preparation and patterning of said insulator layer of silicon nitride is performed by sequential steps of: (i) providing in vacuum a substrate having formed thereon a thin film insulator layer of silicon nitride SiN the surface of which, in accordance with its formation in said vacuum exclusive of air or following cleaning in said vacuum by radicals of an inert gas or hydrogen gas ambient plasma, is free of naturally oxidised film, water and organic adherents; (ii) exposing the as formed or the as cleaned surface of said thin film insulator layer of silicon nitride to a reduced pressure atmosphere of a photolysible oxidising gaseous substance and irradiating said surface selectively, according to a predetermined pattern, with light of wavelength not longer than the UV absorption edge of said gaseous substance, thereby generating oxygen radicals and oxidising the irradiated surface portion of said surface to define an oxide mask pattern; and, without exposure to air; (iii) dry etching the non-irradiated surface portion of said surface left exposed by said oxide mask pattern, by exposure to halogen atomic radicals, while leaving the irradiated surface portion of said surface of silicon nitride SiN unetched. A method according to claim 1 wherein said photolysible oxidising gaseous substance is at least one of oxygen, ozone, nitrous oxide, carbon dioxide and nitrogen dioxide. A method according to either of claims 1 or 2 wherein said step (iii) of dry etching is performed by exposure to halogen atomic radicals generated from a mixture of Cl 2 and NF 3 . A method according to any preceding claim wherein sequential steps (ii) and (iii) are conducted in different treatment spaces. A method according to claim 4 wherein the substrate is transferred between the different treatment spaces via a conveying chamber adjacent to each treatment space. A method of producing a thin film transistor, which method is a method of producing a semiconductor device according to claim 1 and is performed by: providing a substrate of quartz (671) and forming thereon by plasma decomposition of SiH 4 in a vacuum treatment space a thin film of amorphous silicon (672); forming on said thin film of amorphous silicon, a thin film of amorphous silicon n + -type (673) by plasma decomposition of SiH 4 and PH 3 in the same vacuum treatment space; without contact with air producing a mask pattern by irradiating the amorphous silicon n + -type thin film surface (673), which surface is free of naturally oxidised thin film, water and organic adherents as formed, selectively according to a predetermined pattern, in a reduced pressure atmosphere of NO 2 by a argon ion laser or KrF excimer laser, or other laser or lamp source having a wavelength not longer than the UV absorption edge of said NO 2 , thereby forming a SiO x layer on the irradiated portions of said amorphous silicon n + type thin film surface; without contact with air dry etching the n + -type thin film of amorphous silicon using chlorine atomic radicals generated with Cl 2 gas, thereby leaving the irradiated portions of the n + -type thin film of amorphous silicon unetched; forming on the substrate surface including the patterned n + -type thin film of amorphous silicon, an α-SiN:H thin film insulator layer (674) by a plasma generated in a gaseous ambient of SiH 4 , NH 3 , and H 2 ; then without contact with air; exposing the as formed α-SiN:H thin film insulator layer surface to a reduced pressure ambient of NO 2 gas and selectively irradiating the surface thereof with an argon ion laser to form a SiO x pattern; drying etching the non-irradiated portion of said surface of the α-SiN:H thin film using chlorine and fluorine radicals generated by plasma in a Cl 2 and NF 3 gas ambient thereby leaving the irradiated portions of said α-SiN:H thin film unetched; then without contact with air forming a sputtered thin film of Al (675) on the surface of the substrate including the patterned α-SiN:H thin film insulator layer; without contact with air producing an AlO x pattern by irradiating the surface of the sputtered thin film of Al, which surface is free of naturally oxidised thin film, water, and organic adherents, selectively according to a predetermined pattern, using an ArF excimer laser in an NO 2 gas ambient; and dry etching the non-irradiated portion of said surface of the sputtered thin film of Al to define source and drain contacts, gate electrode and wiring of said thin film transistor, by use of chlorine atomic radicals generated in chlorine gas, while leaving the irradiated portion of said surface of the sputtered thin film of Al unetched.

Inventors: KAWATE SHINICHI, KOMATSU TOSHIYUKI, SATO YASUE, YAGI TAKAYUKI, KAWATE, SHINICHI, KOMATSU, TOSHIYUKI, SATO, YASUE, YAGI, TAKAYUKI, KAWATE, SHINICHI, CANON KABUSHIKI KAISHA, KOMATSU, TOSHIYUKI, CANON KABUSHIKI KAISHA, SATO, YASUE, CANON KABUSHIKI KAISHA, YAGI, TAKAYUKI, CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: G03F7
IPC codes: G03F7, H01L21