Abstract:
A multilayer photoresist process in photolithography, which is applicable on a substrate having a composite photoresist layer with a desired thickness formed thereon. The present invention provides a process, comprising the following steps. A photoresist layer is formed on a substrate, and subsequently exposed through a photomask, followed by the developing process to pattern the photoresist. Then, the patterned photoresist layer is stabilized. This sequence is repeated untill at least another one layer is deposited and patterned on the substrate. Each photoresist layer has almost the same pattern with the underlying patterned photoresist layer. Many thin photoresist layers are accumulated to form a composite photoresist layer with a desired thickness.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 89102173, filed Feb. 10, 2000. 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a photolithography technology. More particularly, the present invention related to a multilayer photoresist process in photolithography for increasing photolithography process window magnitude. 
     2. Description of Related Art 
     In the semiconductor photolithography, the exposure resolution and the depth of focus are two key parameters that determine photolithography quality. According to Rayleigh&#39;s theory, the exposure resolution and the depth of focus in a photolithography process can be expressed as follows: 
     
       
         Resolution= K   1   ×λ/NA   
       
     
     
       
         
           DOF=K 
           2 
           ×λ/NA 
           2 
         
       
     
     Where λ is the wavelength of incident light, NA is the numerical aperture of the lens, and K 1  and K 2  are factors controlled by both the optical system and the photoresist, These parameter values depend on the photoresist material, the photoresist thickness, and the exposure method. 
     Generally, the thinner the photoresist layer is, the lower K 1  value, the higher K 2  value, and the higher exposure margin are. The K 1  value is reduced corresponding to the exposure resolution is increased. The K 2  value is increased corresponding to the depth of focus is increased. Therefore, the thinner the photoresist layer is, the better the pattern quality is, that is, the larger the lithography process window is. However, the over thin photoresist layer can not usually resist the plasma etching, therefore, the photoresist layer must have a certain thickness to serve as an etching mask. 
     To resolve the above problems, the bilayer process is used in photolithography process in the prior art. The bilayer process, which is the top layer and the bottom layer with different material are coated on a wafer in sequence. The top photoresist layer is patterned, and subsequently the bottom layer is dry-etched. The top patterned photoresist layer is in combination with the bottom layer to form a thick composite photoresist layer. However, the bilayer method includes overlapping two layers and subsequent patterning the top photoresist layer and dry etching the bottom layer. Therefore, the top photoresist material should be different from the bottom photoresist material in etching selectivity. 
     SUMMARY OF THE INVENTION 
     Based on the foregoing, the present invention provides a multilayer photoresist process in photolithography to solve the above problems of concurrently taking the photoresist thickness and the pattern quality into consideration. Many thin photoresist layers are accumulated to form a composite photoresist with a desired thickness. 
     The present invention further provides a bilayer photoresist process in photolithography to solve the problems of the bilayer photoresist process of the prior art. 
     According to a preferred embodiment of the present invention, the present invention provides a process, comprising the following steps. A photoresist layer is formed on a substrate, and subsequently exposed through a photomask, followed by the developing process to pattern the photoresist. Then, the first patterned photoresist layer is stabilized to avoid the transformation of constituents due to the influence with the subsequent exposing and developing steps of the overlying photoresist layers. This sequence is repeated untill at least another one layer is deposited and patterned on the substrate. Each photoresist layer has almost the same pattern with the underlying patterned photoresist layer. The photoresist layers are accumulated to form a composite photoresist layer with a desired thickness. Photoresists are of two types: positive, which on exposure to light become more soluble in a developer solution, and negative, which become less soluble, and any one photoresist layer in the composite photoresist layer can be positive or negative. 
     According to the present invention, each photoresist layer has almost the same pattern as the underlying patterned photoresist layer. If both the photoresist layer and the underlying photoresist layer are the same type photoresists such as positive or negative. The same photomask containing the desired pattern is applied to the same type photoresist. If the type of the top photoesist layer is different from the type of the underlying photoresist layer, such that one is positive (negative), and the other is negative (positive). While the two photoresist layers are exposed through the photomasks with patterns, the pattern in one photomask is complementary to that in another photomask. 
     Accordingly, the present invention provides a method to form a composite photoresist with a desired thickness. The composite photoresist layer is composed of many thin photoresist layers. Every thin photoresist layer is thinner than the composite photoresist layer. The thinner the photoresist layer is, the larger the lithography process window is. Therefore, the large lithography process window is obtained by accumulating many thin layers to a desired thickness rather than by forming a single photoresist layer with the same desired thickness. 
     In addition, the magnitude of the lithography process window depends on the monolayer thickness. In the present invention, a composite photoresist is composed of many thin photoresist layers to increase the total photoresist thickness. Meanwhile, the magnitude of the lithography process window is kept large. 
     Furthermore, in the present invention, the photoresist layer is stabilized before the subsequent light-exposure, and photoresist development steps for the overlying photoresist layer. In the case of bilayer photoresist, both the two photoresist layers can use the same material. Therefore, comparing with bilayer process of the prior art, the photoresist material is easily found in the present invention. 
     Accordingly, many thin photoresist layers are combined in a composite photoresist with a desired thickness to increase the magnitude of the lithography process window. In addition, the magnitude of the lithography process window depends on the monolayer thickness. Therefore, the present invention provides a method to increase the total photoresist thickness, and keep the lithography process window large. Furthermore, In the case of bilayer photoresist process, the photoresist layer is stabilized before subsequent light-exposure, and photoresist development steps for the overlying photoresist layer. Therefore, both the two photoresist layers can use the same material, and therefore the photoresist material and light-exposure conditions are easily found. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings, 
     FIGS. 1A to  1 D are schematic, cross-sectional views showing the bilayer photoresist process with two positive photoresists according to the first preferred embodiment of this invention, wherein the second positive photoresist layer is conformally formed on the first positive photoresist layer and the substrate. 
     FIGS. 2A to  2 D are schematic, cross-sectional views showing the bilayer photoresist process with two negative photoresists according to the second preferred embodiment of this invention, wherein the second negative photoresist layer is conformally formed on the first negative photoresist layer and the substrate. 
     FIGS. 3A to  3 D are schematic, cross-sectional views showing the bilayer photoresist process with two negative photoresists according to the third preferred embodiment of this invention, wherein the planarized second negative photoresist layer is formed on the first negative photoresist layer. 
     FIGS. 4A to  4 D are schematic, cross-sectional views showing the bilayer photoresist process with one positive photoresist and one negative photoresist according to the fourth preferred embodiment of this invention, wherein the second photoresist layer is negative with a planarized surface formed on the first positive photoresist layer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     According to a preferred embodiment of the present invention, the bilayer photoresist process is put forward as an example to illustrate the change in the patterning steps for the different type of photoresist existing in the bilayer photoresist. The three examples is provided to illustrate the change in the patterning steps for the different bilayer photoresist system. 
     EXAMPLE 1 
     FIGS. 1A to  4 D are schematic, cross-sectional views showing the change in the patterning steps for the different bilayer photoresist system according to one preferred embodiment of this invention. 
     Referring to FIGS. 1A and 1B, a substrate  100  is provided with the first positive photoresist layer  110  formed thereon. The first positive photoresist layer  110  is subsequently exposed through a photomask  120  by a light source  130 . Subsequent to the development step, the first patterned positive photoresist layer  110   a  is resulted. Then, the first patterned positive photoresist layer  110   a  is stabilized by, for example, baking to prevent the influence of subsequent light exposure, photoresist development steps for the second positive photoresist layer  140 . 
     Referring to FIGS. 1C and 1D, the second thin positive photoresist  140  is conformally formed over the first positive photoresist  110   a  and the substrate  100 . The conformal forming step includes thinning down the second positive photoresist layer. The second positive photoresist layer  140  is subsequently exposed through a photomask  120  by a light source  130 . Subsequent to the development step, a thick patterned photoresist layer  180  is resulted, and the patterned photoresist  180  thickness is the sum of the first positive photoresist  110   a  thickness and the second positive photoresist  140   a  thickness. 
     In the present example, the second positive photoresist  140  conformally formed over the first positive photoresist  110   a  and the substrate  100  is a necessary condition to enlarge the magnitude of the lithography process window due to the thin exposed portion of the second positive photoresist  140  (as shown in FIG.  1 C). 
     EXAMPLE 2 
     FIGS. 2A to  2 D and FIGS. 3A to  3 D, are schematic, cross-sectional views showing the bilayer photoresist processes with two negative photoresists. The difference for the two processes is in forming the second negative photoresist layer. In the first case, the second negative photoresist layer is conformally formed, and in the second case, the second negative photoresist layer is planarized formed. 
     Referring to FIGS. 2A and 2B, a substrate  200  is provided with the first negative photoresist layer  210  formed thereon. The first negative photoresist layer  210  is subsequently exposed through a photomask  220  by a light source  230 . Subsequent to the development step, the first patterned negative photoresist layer  210   a  is resulted. Then, the first negative photoresist layer  210   a  is stabilized by, for example, baking to prevent the influence of subsequent light exposure, photoresist development steps for the second negative photoresist layer  240 . 
     Referring to FIGS. 2C and 2D, the second thin negative photoresist  240  is conformally formed over the first negative photoresist  210   a  and the substrate  200 . The second negative photoresist layer  240  is subsequently exposed through a photomask  220  by a light source  230 . Subsequent to the development step, a thick patterned photoresist layer  280  is resulted, and the patterned photoresist  280  thickness is the sum of the first positive photoresist  210   a  thickness and the second positive photoresist  240   a  thickness. 
     Referring to FIGS. 3A and 3B, a substrate  300  is provided with the first negative photoresist layer  310  formed thereon. The first negative photoresist layer  310  is subsequently exposed through a photomask  320  by a light source  330 . Subsequent to the development step, the patterned first negative photoresist layer  310   a  is resulted. Then, the first negative photoresist layer  310   a  is stabilized by, for example, baking to prevent the influence of subsequent light exposure, photoresist development steps for the second negative photoresist layer  340 . 
     Referring to FIGS. 3C and 3D, the planarized second negative photoresist layer  340  is formed over the first negative photoresist  310   a  and the substrate  300 . The second negative photoresist layer  340  is subsequently exposed through a photomask  320  by a light source  330 . Subsequent to the development step, a thick patterned photoresist layer  380  is resulted, and the patterned photoresist  380  thickness is the sum of the first positive photoresist  310   a  thickness and the second positive photoresist  340   a  thickness. 
     In the above two cases, only the portion of the second negative photoresist layer formed on the top of the patterned first negative photoresist is exposed to light. Therefore, whether the second negative photoresist layer is formed conformally or not, the lithography process window is large due to the same thin thickness for the exposed portion of the second negative photoresist in both cases. 
     EXAMPLE 3 
     FIGS. 4A to  4 D, are schematic, cross-sectional views showing the bilayer photoresist processes with one positive photoresist layer and one negative photoresist layer according to the preferred embodiment of the present invention. 
     Referring to FIGS. 4A and 4B, a substrate  400  is provided with the positive photoresist layer  410  formed thereon. The positive photoresist layer  410  is subsequently exposed through a photomask  420  by a light source  430 . Subsequent to the development step, the patterned positive photoresist layer  410   a  is resulted. Then, the positive photoresist layer  410   a  is stabilized by, for example, baking to prevent the influence of subsequent light exposure, photoresist development steps for the negative photoresist layer  440 . 
     Referring to FIGS. 4C and 4D, the planarized negative photoresist layer  440  is formed on the positive photoresist  410   a  and the substrate  400 . The negative photoresist layer  440  is subsequently exposed through a photomask  450  by a light source  460 . Subsequent to the development step, a thick patterned photoresist layer  480  is resulted. The pattern in the photomask  450  and the pattern in the photomask  420  are complementary to each other, and then the pattern in the positive photoresist layer is the same as the pattern in the negative photoresist layer. 
     In addition, analogy to the examples 1 to 3, if the upper photoresist layer is positive, and the lower photoresist is negative. The upper positive photoresist layer is conformally formed on the lower negative photoresist layer, followed by the developing step. The pattern in the photomask applied on the positive photoresist layer and the pattern in the photomask applied on the negative photoresist layer are complementary to each other, and then the pattern in the positive photoresist layer is the same as the pattern in the negative photoresist layer. 
     Accordingly, the present invention provides a method to form a composite photoresist layer with a desired thickness. The composite photoresist layer is composed of two thin photoresist layers. The lithography process window is larger for every thin photoresist layer in the composite photoresist layer than that for a single composite photoresist layer with a desired thickness. Generally speaking, the present invention provide a process to form a composite photoresist layer, and the lithography process window is larger for many thin photoresist layers than that for a single thick photoresist layer with a desired thickness. 
     In addition, the magnitude of the lithography process window depends on the monolayer thickness. According to the preferred embodiment of the present invention, a bilayer photoresist is composed of two thin photoresist layers to increase the total photoresist thickness. Meanwhile, the lithography process window is kept large. 
     According to the preferred embodiment of the present invention, in the case of bilayer photoresist, the first photoresist layer is stabilized before subsequent light-exposure, and photoresist development steps for the second photoresist layer. Therefore, both the two photoresist layers can use the same material, and the photoresist material and light-exposure conditions are easily found in the present invention comparing with that in the prior art. 
     After the second photoresist layer is formed on the substrate, more photoresist layers can be accumulated on the second photoresist layer to increase the total thickness of a composite photoresist to increase the plasma etch resistance for the subsequent etching process. If the type of the overlying second photoesist layer is different from the type of the first photoresist layer, and then the pattern in the first photomask is complementary to the pattern in the second photorsist layer. In addition, if the upper layer is a positive photoresist layer, the positive photoresist layer is conformally formed on the lower layer to thin down the thickness of the exposed portion of the positive photoresist to obtain the large lithography process window. 
     According to the preferred embodiment of the present invention, two thin photoresist layers are combined in a composite photoresist with a desired thickness to enlarge the magnitude of the lithography process window, and the magnitude of the lithography process window depends on the monolayer thickness. The present invention provides a process to increase the total photoresist thickness. Meanwhile, the magnitude of the lithography process window is kept large. Furthermore, the first forming photoresist layer is stabilized before subsequent light-exposure, and photoresist development steps for the second forming photoresist layer. Therefore, both the two photoresist layers can use the same material, and the photoresist material and light-exposure conditions are easily found. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.