METHOD OF APPLYING A RESIST

Abstract: In a photoresist process which comprises a process for spin-coating a substrate (7) with a resist (10), a process for transferring a mask pattern (4, 5) onto the coated resist film followed by exposure (8), and a developing process for form­ ing a pattern on the substrate after the pattern (4, 5) has been exposed. When the developed pattern of the resist pulsates with the increase or decrease of parameters in the process of applying resist (10), the value of the parameter is set to a value that corresponds to an extreme value of the pulsation.

Description: The present invention relates to a method of applying a photoresist, and particularly to a method of applying a photoresist that is adapted for a process for producing highly integrated circuit elements where little variation is allowed in the size of the developed pattern. A method of applying a photoresist has been disclosed in Japanese Patent Laid-Open No. 155925/1984. According to this method, the photosensitive reaction is effected prior to applying the photoresist. However, nothing has been referred to in regard to optimizing the film thickness of the photoresist. Further, Japanese Patent Laid-Open Nos. 155930/1984, 155927/1984 and 155921/1984 disclose methods of applying a photoresist in relation to forming fine patterns. However, none of them teach to optimize the film thickness of the photoresist. On the other hand, JP-A-57 78 140 discloses a photoresist applying method according to the precharacterizing part of claim 1 and describes the importance to optimise the thickness of the photoresist coated on a substrate in order to obtain a good pattern. The object of the present invention is to provide a method of applying a resist, which is capable of increasing the production yields of highly integrated circuit elements while decreasing the variance in the developing size of elements formed on the silicon wafers. Said object is achieved, according to the present invention, by a photoresist applying method comprising a process for spin-coating a substrate with a photoresist, a process for transferring a mask pattern onto the coated photoresist film followed by exposure, and a developing process for forming a pattern on the substrate after the photoresist has been exposed, wherein the spin-coating process comprises the steps of: (a) determining a plurality of extreme values of a curve in which the width of the developed pattern of said photoresist pulsates sinusoidally in relation to the increase or decrease of a photoresist thickness , and (b) controlling said spin-coating by setting a parameter variable in response to the thickness of the photo-resist to a value that corresponds with one of said extreme values of said pulsation to minimize variations in the width of the developed pattern as caused by variations in said parameter. Preferably said parameter is the number of spinner revolutions applied to said substrate in forming the photoresist thereon. If a photoresist film is applied too thickly, little light enters into the interior of the photoresist film and the size changes after developing. This is called the bulk effect. With a contracted projection exposure apparatus which uses light of a single wavelength, however, the reflectance varies on the surface of the resist due to interference of light in the photoresist film. Therefore, interference in which uses are develops the size of the developed pattern pulsates sinusoidally as the film thickness increases. In such a case, variance in the size of the developed pattern can be minimized by setting the thickness of the photoresist film to a value that corresponds to an extreme value (a portion where the gradient of a curve that represents the dependency the developed size of the upon the thickness of photoresist film becomes zero) of pulsation. Fig. 1 is a diagram which shows the dependency of the developing size of the photoresist upon the thickness of the photoresist film according to an embodiment of the present invention; Fig. 2 is a section view of the photoresist; Fig. 3 is a diagram which illustrates the projection of mask pattern onto the photoresist; Fig. 4 is a diagram of distribution to explain the variance in the developing size; and Fig. 5 is a diagram which shows the dependency of the developing size of the photoresist upon the number of revolutions of the spinner. An embodiment of the invention will be described below in conjunction with Fig. 1. It was found that as the thickness of the photoresist film is increased by about 10 nm each time, the resist size (width ℓr of resist line 6 of Fig. 2, which should desirably be equal to the line width ℓm of a mask pattern which consists of a line portion 4 and a space portion 5) after developing pulsates as represented by a curve 3 in Fig. 1. A process for forming the resist line 6 on a silicon substrate 7 will be described below in conjunction with Fig. 3. First, the silicon substate 7 is spin-coated with a photoresist 10 to a thickness of about 1 µm. A projected image of mask pattern consisting of space portion and line portion is then irradiated with a source of ultraviolet rays. In this case, the intensity of light on the photoresist 10 is weak at a position which corresponds to the line portion and is strong at a position which corresponds to the space portion, as represented by a curve 9. If developing is performed after irradiation with ultraviolet rays, the photoresist melts at a portion which corresponds to the strongly irradiated space portion 5. The photoresist at a portion corresponding to the weakly irradiated line portion 4 does not melt but remains to form the resist line 6 shown in Fig. 2. This is a positive type of photoresist. In the case of a negative-type photoresist, on the other hand, a resist line is formed at a portion that corresponds to the space portion. When a silicon wafer is spin-coated with the photoresist to a thickness of about 1 µm, in general, the film thickness does not become uniform but varies by about 20 nm (200 angstroms). Therefore, variance develops in the width of resist line that is formed after developing. As is evident from Fig. 1, the range of variance of the line width greatly changes depending upon the set point (central value in the variance of resist film thickness) of the thickness of photoresist film. For instance, if the thickness of the photoresist film is denoted by t₃, the film thickness varies over a range ▵t; consequently, the developed pattern varies over a range ▵ℓ₃. However, if the thickness of the photoresist film is set to t₂ which corresponds to a minimum value of the curve 3, the developed pattern varies over a range ▵ℓ₂ despite the thickness of the photoresist film varing over the same range ▵t, which is a reduction into about one-fifth from the range ▵ℓ₃. Even if the thickness of the photoresist film is set to t₁ which corresponds to a maximum value 1 of the curve 3, the developing size varies over a range ▵ℓ₁, which is a reduction into about one-fifth from the range ▵ℓ₃. Here, the range ▵ℓ₃ generally has a value of 0.3 µm. This value becomes intolerable as the resist size approaches 1 µm. Here, it is presumed that the allowable value of variance in the developing size is ±▵ℓ with respect to the design size ℓd in the photoresist process for producing semiconductor elements. If now the setpoint of thickness of the photoresist film is erroneously set to t₃ in Fig. 1, the size of the developed pattern greatly, and the number of semiconductor elements relative to the developed pattern is distributed as represented by a curve 11. Many elements (those denoted by 12, 13) that lie outside the allowable pattern ℓd ± ▵ℓ become defective. However, when the setpoint of the thickness of the photoresist film is set to t₁ or t₂ in Fig. 1, variance in the developed pattern decreases as represented by a curve 14, and the proportion of defective elements decreases to less than 10% as distributed in the portions denoted by 15 and 16. In the foregoing were mentioned the pulsation of the resist developed pattern relative to the thickness of the photoresist film and the accompanying variance. When the thickness of the photoresist film cannot be directly measured, however, the developed pattern of the photoresist is measured relative to the number of revolutions of the spinner (spin-coating apparatus) which applies the resist to a thickness of about 1 µm. The thickness of photoresist film pulsates as represented by a curve 17 in Fig. 5. As the number of revolutions of the spinner is changed as denoted by N₁, N₂ and N₃, the developed pattern varies little as denoted by ▵ℓ₁ and ▵ℓ₂ when the number of revolutions N₁, N₂ correspond to extreme values 17, 18 even though the thickness of the photoresist film varies over the same range ▵t as that of Fig. 1. When the number of revolutions N₃ does not correspond to the extreme value, however, the developed pattern varies over an increased range ▵ℓ₃. Thus, by suitably setting the number of revolutions of the spinner, variance in the developed pattern can be reduced. Fig. 4 is a diagram which illustrates the variance in the developing size.

Claims: A photoresist applying method comprising a process for spin-coating a substrate (7) with a photoresist (10), a process for transferring a mask pattern (4, 5) onto the coated photoresist film followed by exposure (8), and a developing process for forming a pattern on the substrate (7) after the photoresist (10) has been exposed, wherein the spin-coating process comprises the steps of: (a) determining a plurality of extreme values of a curve in which the width of the developed pattern of said photoresist (10) pulsates sinusoidally in relation to the increase or decrease of a photoresist thickness , and (b) controlling said spin-coating by setting a parameter variable in response to the thickness of the photo-resist (10) to a value that corresponds with one of said extreme values of said pulsation to minimize variations in the width of the developed pattern (4, 5) as caused by variations in said parameter. A photoresist applying method according to claim 1, wherein said parameter is the number of spinner revolutions applied to said substrate (7) in forming the photoresist (10) thereon.

Inventors: ITO, TETSUO, KADOTA, KAZUYA, KOBAYASHI, KAZUNARI, NAKAGOMI, YOSHIYUKI, TANUMA, MASAYA
Applicants: HITACHI, LTD.
Application year: 1986
Main classification: G03F7
IPC codes: H01L21, B05D7, B05D1, G03F7, B05D3