Abstract:
A method of lowering critical dimensions. A film layer and a photoresist layer are sequentially formed over a substrate. The photoresist layer is exposed and developed to form a plurality of first openings. A first baking of the photoresist layer is carried out, permitting the photoresist layer to flow. A second baking is next carried out so that the width of the first openings is reduced linearly with time until a desired dimension is reached. Using the photoresist layer as a mask, the film layer is etched to form a plurality of second openings.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a photolithographic method. More particularly, the present invention relates to a method of reducing a critical dimension of a patterned photoresist layer during the manufacturing of integrated circuits. 
     2. Description of Related Art 
     Due to the rapid development of integrated circuit fabricating techniques, dimensions of individual semiconductor devices have become smaller so that more devices are integrated into a single silicon chip. However, miniaturization of a semiconductor device depends very much on controlling critical dimensions in photolithography. 
     Raising the line width resolution in a photolithographic operation beyond 0.18 μm in the current state of technology is rather difficult unless a light source having a shorter wavelength is used, along with a short wavelength photoresist. Reducing wavelength of light source, however, means that old machines have to be entirely replaced by new, costly machines. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of lowering the critical dimensions of a semiconductor device without needing to replace existing manufacturing equipment. 
     To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a method of reducing a critical dimension of a patterned photoresist layer, which is suitable for a substrate having a patterned photoresist layer formed thereon. In the method, the patterned photoresist layer is baked at a first temperature, wherein the first temperature is higher enough to make the patterned photoresist layer expand laterally. The patterned photoresist layer is then baked at a second temperature to make the patterned photoresist layer expand laterally at a fixed rate. 
     The first temperature is greater than a melting point or a glass transition temperature of the patterned photoresist layer. 
     If the patterned photoresist layer is made of a deep-UV material and the patterned photoresist layer is used as a deep ultraviolet photoresist, the patterned photoresist layer is baked at the first temperatures of about 145° C. for 90 seconds. 
     If the patterned photoresist layer is made of a deep-UV material and the patterned photoresist layer is used as a deep ultraviolet photoresist, the patterned photoresist layer is baked at the second temperature of about 161° C. for 70 seconds. 
     To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a method of reducing a critical dimension of a patterned photoresist layer, which is suitable for a substrate having a film layer and a patterned photoresist layer sequentially formed thereon. A plurality of first openings are formed in the patterned photoresist layer. In the method, the patterned photoresist layer is baked at a first temperature. The first temperature is higher enough to make the patterned photoresist layer expand laterally. Then, the patterned photoresist layer is baked at a second temperature to make the patterned photoresist layer expand laterally at a linear rate, so that widths of the first openings are reduced at the linear rate to a desired dimension. The film layer is etched to form a plurality of second openings while using the photoresist layer as a mask. 
     The first temperature is greater than a melting point or a glass transition temperature of the patterned photoresist layer. 
     If the patterned photoresist layer is made of a deep-UV material and the patterned photoresist layer is used as a deep ultraviolet photoresist, the patterned photoresist layer is baked at the first temperature of about 145° C. for 90 seconds. 
     If the patterned photoresist layer is made of a deep-UV material and the patterned photoresist layer is used as a deep ultraviolet photoresist, the patterned photoresist layer is baked at the second temperature of about 161° C. for 70 seconds. 
     According to the method of this invention, the second openings can be narrowed to a critical dimension of around 80nm without needing to purchase additional equipment. Moreover, temperature used in the second baking can be adjusted to control the reduction rate of the first opening in the photoresist layer. 
     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, 
     FIG. 1 is a graph showing the variation of the critical dimension of a photoresist opening versus temperature; 
     FIG. 2 is a graph showing the variation of the critical dimension of a photoresist opening versus baking time at different baking temperatures; 
     FIG. 3 is a flow chart showing the steps needed to lower the critical dimensions according to the preferred embodiment of this invention; 
     FIGS. 4A through 4C are schematic cross-sectional views showing the progression of steps in reducing critical dimensions according to the flow chart in FIG. 3; and 
     FIG. 5 is a graph showing the variation of critical dimension versus second baking time for an opening in a deep ultraviolet photoresist layer having different duty ratios. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
     A photolithographic process can be subdivided into several steps including a photoresist deposition, a soft bake, an exposure, a post-exposure bake, a photoresist development and a hard bake. However, the hard baking step can be skipped depending on actual requirement. 
     Major constituents of photoresist include resin, sensitizer and solvent. In general, the sensitizer polymerizes with the resin or with itself when exposed to light so that the light-exposed portion of a photoresist layer is polymerized to form a structurally stable compound that resists dissolution by a developer. 
     However, if the photoresist layer is heated to a temperature above a melting point or glass transition temperature (Tg), the photoresist material is able to expand laterally. Such horizontal expansion of photoresist material gradually reduces the width of openings in the photoresist layer. FIG. 1 is a graph showing the variation of the critical dimension of a photoresist opening versus temperature. As shown in FIG. 1, the variation of critical dimension is non-linear. When the temperature is low, the rate of reduction of critical dimension is rather slow. As temperature of the photoresist layer increases, fluidity of the photoresist layer also increases leading to a faster rate of reduction of the critical dimension. 
     FIG. 2 is a graph showing the variation of the critical dimension of a photoresist opening versus baking time at different baking temperatures. If the photoresist layer is heated to a temperature A (shown in FIG. 1) for a period, curve A results (shown in FIG.  2 ). On the other hand, if the photoresist layer is heated to a higher temperature C (shown in FIG. 1) for the same period, curve C results (shown in FIG.  2 ). However, both curves A and C in FIG. 2 are non-linear. In other words, when the photoresist is heated to a temperature A or C, reduction of critical dimension is not directly proportional to the heating time. 
     Nevertheless, it is possible to find a temperature somewhere between point A and point C such as point B where the rate of reduction of opening width is a constant. 
     The temperature point B is likely to be a point of inflexion along the curve in FIG.  1 . When the photoresist layer is heated to a temperature B, width of openings in a photoresist layer reduces linearly with time according to curve B in FIG.  2 . 
     Temperature B is selected to carry out a reduction of critical dimension. FIG. 3 is a flow chart showing the steps needed to lower the critical dimensions according to the preferred embodiment of this invention. FIGS. 4A through 4C are schematic cross-sectional views showing the progression of steps in reducing critical dimensions according to the flow chart in FIG. 3. A photoresist material that is sensitive to deep ultraviolet light is used throughout to illustrate the method of this invention. 
     With reference to FIG.  3  and FIG.  4 A - FIG. 4C, first, as shown in FIG. 4A, a film layer  410  and a patterned photoresist layer  420  is formed over a substrate  400 . There are openings  430  in the patterned photoresist layer  420 . Each opening  430  has a width a  10  and a distance of separation b between neighboring openings  430 . The photoresist layer  420  is formed by a series of steps including coating (step  300 ), soft-baking (step  310 ), light exposure (step  320 ), post-exposure baking (step  330 ), photoresist development (step  340 ) and hard-baking (step  350 ). 
     A first baking (step  360 ) of the photoresist layer  420  is carried out. This is followed by a second baking (step  370 ) to form a structure as shown in FIG.  4 B. At a high temperature, the photoresist layer  420  flows laterally so that a photoresist profile  425  as shown in FIG. 4B is obtained. Ultimately, the original openings  430  are transformed into openings  435 . To carry out the first baking (step  360 ) and the second baking (step  370 ) of the photoresist layer, two hot plates each pre-heated to a pre-determined temperature are all that is required. 
     In the aforementioned first baking (step  360 ) operation, the temperature must be higher than the melting point of the photoresist layer  420  or the glass transition temperature (Tg) so that the photoresist layer  420  is able to flow. In the second baking (step  370 ) operation, a temperature must be selected such that the width of the opening  430  reduces linearly with heating time. In other words, a temperature such as point B in FIG. 1 should be chosen. 
     Using the photoresist layer  425  as a mask, the film layer  410  is etched (step  380 ) to form a patterned film layer  415  with openings  440  as shown in FIG.  4 C. 
     When the photoresist layer  425  is made of, for example, a deep ultraviolet (deep-UV) material and the photoresist layer  425  is used as a deep ultraviolet photoresist. The deep ultraviolet photoresist of a Deep-UV lithography process is using, for example, 248- or 193-nm light as a light source. The first baking (step  360 ) operation can be carried out at about 145° C. for about 90 seconds. The second baking (step  370 ) can be carried out at about 161° C. for about 70 seconds so that width a of each opening  430  is reduced from about 254 nm to around 149 nm. When openings  440  are subsequently formed in the film layer  415 , the bottom portion of each opening  440  has a width of about 100 nm, only. In other words, there is a difference of roughly 154 nm between the width of opening  430  and the width of opening  440 . 
     FIG. 5 is a graph showing the variation of critical dimension versus second baking time for an opening in a deep ultraviolet photoresist layer having different initial duty ratio. The initial duty ratio (the ratio a:b in FIG. 4A) ranges from 1:0.8 to 1:8. After the openings  430  have been heated by the second baking step, an almost linear opening width versus baking time curve is obtained. Furthermore, by adjusting the temperatures used in the first (step  360 ) and the second baking (step  370 ) operations, respectively, a reduction rate of about 1 nm/sec for each opening  430  can be obtained. Hence, engineers are able to form an opening with the desired critical dimension with ease. For example, as shown in FIG. 5, critical dimension of the opening  430  can be reduced from 278 nm to 85 nm within 120 seconds when the duty ratio is 1:8. 
     In summary, the advantages of the method in this invention include: 
     1. The production of devices with a line width smaller than 0.1 μm is possible without sophisticated equipment. Therefore, superior products can be made without needing to purchase costly new machines. 
     2. By adjusting the temperatures in the first and the second baking operations, respectively, a suitable reduction rate for the openings can be obtained. Hence, engineers are more capable of designing openings with the desired critical dimension. 
     3. The method is also suitable for reducing openings in a patterned photoresist layer that has a range of line width to distance of separation ratios. 
     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.