Method of forming a thin film pattern with a trapezoidal cross section

Abstract: The present invention provides a method of forming a thin film pattern with a trapezoidal cross section. In this method, a resist pattern (102a) with an inverted-trapezoidal cross section is formed on a thin film (2). Using the resist pattern (102a) with the inverted-trapezoidal cross section as a mask, the thin film (2) is dry-etched. A resist pattern (2a) is left with the resist pattern (102a) used as a mask. The resist pattern (2a) has a trapezoidal cross section.

Description: The present invention relates generally to a method of forming a thin film pattern according to the preamble of claim 1, and to a method of performing a photoetching process selectively on such thin films as a polysilicon film, an insulating film and a metallic film. A method for forming and filling holes in a layer on a substrate having the features of the preamble of claim 1 is known from EP-A-0 057 738. According to this known method a thin film pattern, having a trapezoidal cross section is formed under a resist pattern having an inverted trapezoidal cross section. After provision of holes in this manner, those holes are filled in order to provide a flat surface particularly suited for generated multilayer conductor structures. Extended Abstracts/Electrochemical Society, Vol. 87-2, 1987, pages 676-677 and Extended Abstracts Vol. 90-1, 6-11, May 1990, pages 217-218 respectively, disclose particular methods for forming tape part film profiles having a controlled taper. In accordance with recent development in miniaturization of semiconductor devices, a dry etching technique is widely used in the process of etching thin films. The dry etching technique is generally classified into isotropic etching such as CDE (Chemical Dry Etching) and anisotropic etching auch as RIE (Reative Ion Etching). In particular, RIE is widely used since it allows a photoresist pattern to be reproduced with fidelity on a thin film to be etched. In the anisotropic etching, a side face of a thin film pattern is formed substantially vertical to a substrate. However, when a new thin film is formed over the thin film pattern having the substantially vertical side face, the "step coverage" of the new thin film is deteriorated. In other words, the new thin film has a difference between the film thickness t1 at its flat area and film thickness t2 at its side area. The relationship between t1 and t2 is: t1 < t2 Consequently, in the anisotropic etching, that part of the thin film corresponds to Δ t (= t2 - t1). The above problem will now be explained with reference to the accompanying drawings. Figs. 1A to 1D are cross-sectional views illustrating the steps of a process for fabricating a conventional semiconductor device. As shown in Fig. 1A, a first polysilicon layer 2, which is undoped or has a low impurity concentration, is formed on a base layer 1 (a silicon substrate or a silicon oxide film). A photoresist is coated over the first polysilicon layer 2. The photoresist is selectively etched by means of photoetching, thus obtaining a photoresist pattern 3A, 3B. Referring to Fig. 1B, the polysilicon layer 2 is selectively etched by means of RIE (one of anisotropic etching techniques), using the photoresist pattern 3A, 3B as a mask. Thus, polysilicon patterns 2A and 2B are obtained. In the step of Fig. 1C, a second polysilicon layer 4 having a high impurity concentration is formed over the base layer 1 and polysilicon patterns 2A and 2B. In Fig. 1C, a photoresist is coated over the second polysilicon layer 4, and the photoresist is selectively etched by means of photoetching, thereby obtaining a photoresist pattern 5. Using the pattern 5 as a mask, the second polysilicon layer 4 is selectively etched by means of RIE and a polysilicon pattern 4, 4A is obtained. In the step of Fig. 1D, the second polysilicon 4 is left on the side walls of the polysilicon patterns 2A and 2B. In the resultant condition, impurities move from the second polysilicon layer 4 with high impurity concentration to the polysilicon patterns 2A and 2B with low impurity concentration. Consequently, the impurity concentration of the polysilicon patterns 2A and 2B varies. For example, where the polysilicon patterns 2A and 2B constitute resistors of an integrated circuit, the resistance values of the resistors vary and the characeristic of the integrated circuit changes. In order to solve the above problem, it is thought that the second polysilicon layer 4 on the side walls of the polysilicon patterns 2A and 2B are entirely removed. In this case, however, the base layer 1 is over-etched and is damaged by RIE. As a result, the reliability of the semiconductor device is deteriorated. The present invention has been made in consideration of the above circumstances, and an object of the invention is to provide a method of forming a thin film pattern, wherein no portion of a new thin film is left on the side wall of an old thin film pattern, formed by anisotropic etching avoiding over-etching of a base layer. This object is solved by a method of forming a thin film pattern of the initially defined type having the characterizing features of claim 1. Preferred embodiments are listed in the dependent claims. By this method, there is obtained a thin film pattern with a trapezoidal cross section. Since the thin film pattern has a trapezoidal cross section, it is possible that a new thin film formed on the thin film pattern has a uniform thickness. Thus, even if a new thin film is formed by anisotropic etching, no portion of the new thin film is left on the side wall of the thin film pattern. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Figs. 1A to 1D are cross-sectional views illustrating the steps of a process for fabricating a conventional semiconductor device; Figs. 2A to 2D are cross-sectional views illustrating the steps of a process for fabricating a semiconductor device according to a first embodiment of the invention; Figs. 3A to 3F are cross-sectional views illustrating the steps of a process for fabricating a semiconductor device according to a second embodiment; Figs. 4A to 4F are cross-sectional views illustrating the steps of a process for fabricating a semiconductor device according to a third embodiment; and Figs. 5A and 5B are cross-sectional views illustrating the steps of a process for fabricating a semiconductor device according to a fourth embodiment. Embodiments of the present invention will now be described with reference to the accompanying drawings. [First Embodiment] Figs. 2A to 2F are cross-sectional views illustrating the steps of a process for fabricating a semiconductor device according to a first embodiment. Referring to Fig. 2A, a first polysilicon layer 2, which is undoped or has a low impurity concentration, is formed on a base layer 1 (e.g. silicon oxide film or a silicon substrate). A positive type photoresist is coated on the polysilicon layer 2. The photoresist is selectively etched by means of photoetching. In the step of photoetching, an image reverse method is employed. The image reverse method will be described in detail in connection with the second and third embodiments. The angle 1 defined by the upper surface of the first polysilicon layer 2 and each side wall of the resist patterns 3A and 3B is acute and less than 90°. Each of the resist patterns 3A and 3B has an inverted-trapezoidal cross section. In the subsequent step of Fig. 2B, using the resist patterns 3A and 3B as a mask, the first polysilicon layer 2 is selectively etched by means of dry etching (e.g., RIE). The angle 2 defined by the upper surface of the base layer 1 and each side wall of the silicon patterns 2A and 2B is obtuse and more than 90°. Each of polysilicon patterns 2A and 2B has a trapezoidal cross section. The patterns 2A and 2B constitute, for example, resistors of an integrated circuit. After the photoresist pattern 3A and 3B are removed, a second polysilicon layer 4 having a high impurity concentration is formed over the base layer 1 and polysilicon patterns 2A and 2B. In this case, the thickness of the second polysilicon layer 4 is uniform. Specifically, the thickness t1 of the second polysilicon layer 4 on the base layer 1 and the thickness t2 of the second polysilicon layer 4 on the side walls of the polysilicon patterns 2A and 2B have the relationship: t1 = t2 This is because the cross section of each of polysilicon patterns 2A and 2B is trapezoidal and the side wall thereof is inclined. Then, a photoresist is coated on the second polysilicon layer 4, and the photoresist is selectively etched by means of conventional photoetching, thereby obtaining a photoresist pattern 5. In the step of Fig. 2D, using the photoresist pattern 5 as a mask, the second polysilicon layer 4 is selectively etched by means of anisotropic etching (e.g. RIE). Thus, the second polysilicon layer 4 is completely removed, without being left on the side walls of the polysilicon patterns 2A and 2B, and without over-etching. A polysilicon pattern 4A is obtained at the region masked by the photoresist pattern 5. The polysilicon pattern 4A constitutes, for example, a wiring layer in an integrated circuit or a gate electrode of a transistor. According to the above method of manufacturing the semiconductor device, a thin film pattern having a trapezoidal cross section can be formed, and a new thin film having a uniform thickness can be formed on this thin film pattern. The above method has the following principal advantages: 1) For example, when a second thin film (conductive film) is formed over a first thin film, the second thin film on the side wall of the first thin film can be removed only by anisotropic etching the second thin film. 2) If a wiring thin film layer with a trapezoidal cross section is formed, the step coverage of an insulating layer covering the wiring layer can be improved. 3) When a new wiring layer is formed over the wiring thin film layer, with an insulating layer interposed therebetween, the step coverage of the new wiring layer can also be improved. Thus, the stepwise severing of the new wiring layer can be prevented. [Second Embodiment] A description will now be given of a second embodiment of the invention wherein a first method of controlling the angle of the side wall of a thin film pattern with a trapezoidal cross section is employed. Figs. 3A to 3F are cross-sectional views illustrating the steps of a process for fabricating a semiconductor device according to the second embodiment. Referring to Fig. 3A, a first polysilicon layer 2 is formed on a substrate 1. A positive type photoresist 102 with a thickness of about 1.0 µm to 2.0 µm is coated on the polysilicon layer 2. Then, a prebaking process is carried out. In the positive type photoresist 102, the base polymer is, for example, phenol novolak resin, and diazo naphtho-quinone. The photoresist 102 contains, for example, azo dye. Using a stepper (not shown), the positive type photoresist 102 is selectively irradiated with ultraviolet 104, with a reticle employed as a mask, as shown in Fig. 3B. The condition for the selective exposure is that, for example, a g-ray (wavelength: 436 nm; g-ray is generally called "ultraviolet") is used and the quantity of ultraviolet is 300 mJ/cm 2 . The photoresist 102 is subjected to selective exposure, and a selectively photo-sensitized region 102a is obtained. Alkali-soluble indene carboxylic acid is produced in the photo-sensitized region 102a. Subsequently, as shown in Fig. 3C, the substrate 1 coated with photoresist 102 is put in heat-treatment furnace 105. In the furnace 105, the substrate 1 is heated in an ammonia gas atmosphere 106. The heat treatment is performed at a temperature of 95 to 120°C for 75 minutes. At the photo-sensitized region, the indene carboxylic acid is crosslinked and alkali-insoluble indene is produced. This step is called an image reverse heat treatment step. In the step of Fig. 3D, using an aligner (not shown), the entire surface of the photoresist 102 is irradiated with ultraviolet 107, thus effecting total surface exposure. For example, for the total surface exposure, ultraviolet (wavelength: 200 to 450 nm) is employed in a quantity of 0.36 mJ/cm 2 . Non-sensitized region 102b of the photoresist 102 is newly sensitized by the total surface exposure, and alkali-soluble indene carboxylic acid is produced. In the step illustrated in Fig. 3E, the substrate 1 is immersed in an alkali water solution, and the photoresist 102 is developed. Consequently, the region 102b where indene carboxylic acid is produced is removed and the region 102a where indene is produced remains. In this way, regarding the positive type photoresist 102, the region 102a sensitized in the selective exposure step (first exposure) described with reference to Fig. 2B is left, and the image is reversed, compared to the normal positive type resist. This type of photoetching is called an image reverse method. As is shown in Fig. 3E, regarding the photoresist pattern 102a obtained by the image reverse method, an acute angle 1 is defined by the side wall of the pattern 102a and the upper surface of the polysilicon layer 2. In addition, the photoresist pattern 102a has an inverted-trapezoidal cross section. In the following step illustrated in Fig. 3F, using the photoresist pattern 102a as a mask, anisotropic etching (e.g. RIE) is employed to etch the polysilicon layer 2. Thus, like in the first embodiment, a polysilicon pattern 2a having a trapezoidal cross section is obtained. According to this second embodiment, an azo dye is contained in the photoresist 102. Since the photoresist 102 contains the azo dye, the ultraviolet transmissivity decreases, and the quantity of ultraviolet reaching the deepest part of the photoresist 102 decreases. Where the quantity of light reaching the deepest part decreases, the sensitized part 102 is tapered. By changing the quantity of dye in the photoresist 102, the degree of tapering of the sensitized part 102a varies. In other words, the angle 1 of the inclined side wall of the photoresist pattern 102a varies. For example, if a large quantity of dye is contained in the resist 102 and the image reverse method is carried out, the angle 1 is shifted from 90° towards 0°. If the quantity of dye is small, the angle 1 approaches 90°. If the inclination angle 1 of the side wall of the resist pattern 102a varies, the angle 2 of the side wall of the polysilicon pattern 2a obtained by the method of the present invention also varies. Taking advantage of this phenomenon, the angle 82 of the side wall of the polysilicon pattern 2a can be controlled. For example, if the angle 1 approaches 90°, the angle 2 also approaches 90°. If the angle 1 approaches 0°, the angle 2 also approaches 180°. More specifically, the inclination angle 2 of the side wall of the polysilicon pattern 2a can be varied by altering the quantity of dye contained in the photoresist 102. Accordingly, since the shape of the cross section of the polysilicon pattern 2a can be freely controlled, the thin film pattern can have a wide margin. The method of controlling the cross sectional shape of the resist pattern is not limited to the changing of dye quantity in the photoresist 102; the quantity of ultraviolet 104 may be altered in the selective exposure process illustrated in Fig. 3B. For example, if the quantity of ultraviolet 104 is decreased, the quantity of ultraviolet reaching the deepest part of the photoresist 102 decreases. Utilizing this fact, the inclination angle 1 is controlled, as stated above; thus, the inclination angle 2 of the side wall of the thin film pattern can be controlled. Other methods of altering the angle 1 of the side wall of the resist pattern are to change the temperature and/or time for the heat treatment described with reference to Fig. 3C, or to change the exposure amount in the total surface exposure step described with reference to Fig. 3D. As a result, the cross sectional shape of the thin film pattern can be changed. [Third Embodiment] A third embodiment of the invention, wherein an image reverse resist is employed, will now be described. Figs. 4A to 4F are cross-sectional views illustrating the steps of a process for fabricating a semiconductor device according to the third embodiment. Referring to Fig. 4A, a polysilicon layer 2 is formed on a substrate 1. An image reverse resist 202 is coated on the polysilicon layer 2. An example of the image reverse resist 202 is AZ5214E (HOECHST JAPAN Corp.). After the image reverse resist 202 is coated, prebaking is carried out. As is shown in Fig. 4B, the photoresist 202 is selectively exposed by a method similar to that illustrated in Fig. 3B, thus obtaining a photosensitized region 202a. The condition for selective exposure is the use of g-rays in a quantity of 500 mJ/cm 2 . In the step illustrated in Fig. 4C, the substrate 1 is placed on the hot plate 205, and the substrate 1 is heated to reverse an image. The heat treatment is performed under the condition that the temperature is 95 to 120°C and the time is 1 to 5 minutes. By the heat treatment, alkali insoluble indene is produced in the sensitized region 202a. In the third embodiment, the image reverse resist is used as photoresist; thus, the heat treatment may not be carried out, for example, in an ammonia atmosphere. Subsequently, as shown in Fig. 4D, the total surface of the photoresist 202 is exposed in the same method as described with reference to Fig. 3D, thus forming a new sensitized region 202b. The condition for the total surface exposure is the use of ultraviolet (200 to 450 nm) in a quantity of 0.36 mJ/cm 2 . In the step illustrated in Fig. 4E, the photoresist 202 is developed in the same method as described by referring to Fig. 3E, thereby obtaining a photoresist pattern 202a. In the step of Fig. 4F, using the photoresist pattern 202a as a mask, the polysilicon layer 2 is etched by anisotropic etching (e.g. RIE), in a manner similar to that illustrated in Fig. 3F, thus obtaining a polysilicon pattern 2a with a trapezoidal cross section. According to the above method of fabricating the semiconductor device, the image reverse resist 202 is used as photoresist, and therefore the time required for heat treatment illustrated, for example, in Fig 3C, can be reduced. In the third embodiment, too, it is possible to add a dye to the image reverse photoresist 202 and adjust the quantity of added dye, thereby controlling the inclination angle 1 of the side wall of the photoresist pattern 202a. By controlling the angle 1, the inclination angle 2 of the side wall of the thin film pattern 2a can be controlled. In the selective exposure step shown in Fig. 4B, if the exposure amount of ultraviolet 104 is controlled, the inclination angle 1 of the side wall of resist pattern 202a can be controlled and accordingly the angle 2 of the side wall of the thin film pattern can also be controlled. [Fourth Embodiment] A fourth embodiment of the invention, wherein a second method of controlling the inclination angle of the side wall of a thin film pattern with a trapezoidal cross section is employed, will now be described. Figs. 5A to 5B are cross-sectional views illustrating the steps of a process for fabricating a semiconductor device according to the fourth embodiment. Referring to Fig. 5A, a parallel plate type RIE apparatus 300 has a lower electrode 301, to which an AC source 303 is connected, and an upper electrode 302 serving as an ion generating source facing the lower electrode 301. A substrate 1 having a photoresist pattern 3a is placed on the lower electrode 301. In the fourth embodiment, the frequency of the RF generator 303 is about 200 KHz, and the thickness of the resist pattern 3a is about 1.5 µm. In this state, ions are emitted from the upper electrode 302 to etch a polysilicon layer 2 selectively, using the resist pattern 3a as mask. Fig. 5B shows the state in which the etching has been completed. In the fourth embodiment, when ions are radiated, the temperature of the upper electrode 302 is made to differ from that of the lower electrode 301, thus providing a temperature gradient therebetween. By virtue of the temperature gradient, the inclination angle 2 of the side wall of the polysilicon pattern 2a can be controlled. According to experimental data, where the inclination angle 1 was 180°-108° and the temperature of both the upper and lower electrodes 302 and 301 was set at 50°C, the inclination angle 2 was 113°. Where the inclination angle 1 was 180°-108° and the temperature of the upper electrode 302 was set at 60°C and that of the lower electrode 301 was set at 20°C, the inclination angle 2 was 118°. In this manner, when the temperature gradient of 40°C was provided between the upper electrode 302 and the lower electrode 301, the inclination angle 2 changed by about 5°. Where the inclination angle 1 was 180°-112° and the temperature of both the upper and lower electrodes 302 and 301 was set at 50°, the inclination angle 2 was 127°. In addition, where the inclination angle 1 was 180°-112° and the temperature of the upper electrode 302 was set at 60°C and that of the lower electrode 301 was set at 20°C, the inclination angle 2 was 130°. As has been described above, by providing a temperature gradient between the upper electrode 302 and the lower electrode 301, the inclination angle 2 of the side wall of the thin film pattern 2a can be controlled. In the fourth embodiment, it is desirable that the temperature gradient be provided such that the temperature of the lower electrode 301 be lower than that of the upper electrode 302. By doing this, the acceleration of ions can be enhanced and the inclination angle 2 can be further changed. In the first to fourth embodiments, the thin film pattern was formed of an undoped or low-impurity-concentration polysilicon; however, any type of thin film capable of being subjected to anisotropic etching may be employed (e.g. a high-impurity-concentration polysilicon, a silicon oxide film, a metallic film of aluminum, etc.). Other thin film formed over the thin film pattern is not limited to the high-impurity concentration polysilicon; for example, any type of thin film capable of being subjected to anisotropic etching may be employed (e.g. an undoped polysilicon, a silicon oxide film, a metallic film of aluminum, etc.). According to this invention, the thin film can be etched by means of dry etching as much as possible, without lowering the reliability of the semiconductor device. Accordingly, semiconductor devices having fine element structure can be fabricated with a high yield, and this contributes to the development in miniaturization of element structure. At present, a minimum rule of a resist pattern obtained by an image reverse method is about 0.8 µm. It is thought, however, that the minimum rule decreases to less than 0.8 µm, for example, a half micron or a quarter micron. Needless to say, the present invention is applicable, even if the minimum rule of a resist pattern having an inverted-trapezoidal cross section becomes a half micron or a quarter micron.

Claims: A method of forming a thin film pattern comprising the steps of a) forming a thin film (2) on a major surface of a substrate (1); b) forming a resist pattern (3A, 3B) having an inverted-trapezoidal cross section ( 1 ) on said thin film (2); c) dry-etching the thin film (2) selectively using said resist pattern (3A, 3B) as a mask thereby forming a thin film pattern (2A, 2B) having a trapezoidal cross section ( 2 ) ; d) removing said resist pattern (3A, 3B) ; characterized by further comprising the steps of e) forming another thin film (4) over said substrate (1) and said thin film pattern (2A, 2B) having a uniform thickness; f) forming another resist pattern (5) on said another thin film (4); and g) anisotropic etching said another thin film (4) selectively using said another resist pattern (5) as a mask thereby completely removing said another thin film (4) without leaving it on side walls of said thin film pattern (2a, 2B) and without over-etching of said substrate (1). The method according to claim 1, characterized in that the anisotropic etching in step g is R.I.E. etching. The method according to claim 1, characterized in that the dry etching in step C is RIE. The method according to claim 1, characterized in that the resist pattern (3A, 3B) having an inverted-trapezoidal cross section is obtained by the steps of forming a resist film (102) on the thin film (2); exposing the resist film (102) selectively, to ultraviolet light and forming a photo-sensitized region (102a) in the resist film (102), the photo-sensitized region being soluble in a developing liquid; subjecting the resist film (102) having the photo-sensitized region (102a) to heat treatment, thereby producing a substance in the photo-sensitized region (102a), which substance is insoluble in the developing liquid; exposing the total surface of the resist film (102) to ultraviolet light and forming another photo-sensitized region (102b), which is soluble in the developing liquid, in those regions of the resist film (102) which are other than the photo-sensitized region (102a); and removing said other photo-sensitized region (102b) by use of the developing liquid. The method according to claim 4, characterized in that the cross-sectional shape of the resist pattern (3A, 3B) is determined by controlling the quantity of exposure light in the step of exposing the resist film (102) selectively. The method according to claim 4, characterized in that the cross-sectional shape of the resist pattern (3A, 3B) is determined by adding a dye to the resist film (102) and controlling the quantity of the added dye. The method according to claim 6, characterized in that said dye is an azo dye. The method according to one of claims 4 to 7, characterized in that the resist film (102) formed on the thin film (2) is a novolak-based positive resist; and the step of subjecting the resist film (102) having the photo-sensitized region (102a) to heat treatment, thereby producing a substance in the photo-sensitized region (102a), which substance is insoluble in the developing liquid, is carried out in a ammonia atmosphere (106). The method according to one of claim 4 to 7, characterized in that the resist film is an image reverse resist (202). The method according to one of claims 1 to 3, characterized in that the cross-sectional shape of the thin film pattern (2A, 2B) is determined such that the dry etching is performed by RIE, the RIE is carried out by use of a parallel-plate reactor (300) capable of independently controlling the temperature of an upper electrode (302) and the temperature of a lower electrode (301), and a temperature difference , which is caused between the upper electrode (302) and lower electrode (301), is controlled.

Inventors: HARAGUCHI HIROSHI, KUMAMARU KUNIAKI, OTANI YASUTO, TSUJI HITOSHI, HARAGUCHI, HIROSHI, KUMAMARU, KUNIAKI, OTANI, YASUTO, TSUJI, HITOSHI, Haraguchi, Hiroshi, c/o Intell. Property Div., Kumamaru, Kuniaki, c/o Intell. Property Div., Otani, Yasuto, c/o Intell. Property Div., Tsuji, Hitoshi, c/o Intell. Property Div.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1991
Main classification: H01L21
IPC codes: G03F7, H01L21, H01L27