Abstract:
A method for performing immersion lithography on a semiconductor wafer is disclosed. The method includes positioning the semiconductor wafer beneath a lens and applying a fluid between a top surface of the semiconductor wafer and the lens. An additive can be provided to the top surface so that any droplet of the fluid that forms on the top surface of the semiconductor wafer will have a contact angle between about 40° and about 80°.

Description:
BACKGROUND  
       [0001]     The present invention relates to the fabrication of substrates such as semiconductor wafers, and more specifically, to fluid-based processes such as immersion lithography for patterning one or more layers of the semiconductor substrate.  
         [0002]     Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. Since then, integrated circuits have generally followed the two year/half-size rule (often called Moore&#39;s Law), which means that the number of devices on a chip doubles every two years. Today&#39;s fabrication plants are routinely producing devices having 0.13 micron and even 90 nm feature sizes.  
         [0003]     Due to the ever shrinking feature sizes, changes have been made throughout the semiconductor manufacturing process. For example, lithography is a mechanism by which a pattern on a mask is projected onto a substrate such as a semiconductor wafer. In areas such as semiconductor photolithography, it has become necessary to create images on the semiconductor wafer which incorporate minimum feature sizes under a resolution limit. Lithographic systems must use shorter light wavelengths to form the smaller features. One solution has been a process called immersion lithography. Immersion lithography uses a transparent fluid to fill the space between a projection lens of a scanning or step-and-repeat lithography system and the substrate (e.g., semiconductor wafer) surface.  
         [0004]     For further example, in a 193-nm wavelength exposure system, it is common to use water as the fluid between the projection lens and the substrate surface. This works well because the lens can be designed with numerical apertures higher than one, which allows the lithography system to produce smaller images and thereby shrink the feature sizes.  
         [0005]     There are a number of practical issues to implementing immersion lithography. For one, maintaining a consistent bubble-free fluid between the lens and the wafer surface is very difficult. There are basically three approaches to the problem. The first approach is to submerge the entire wafer and lens in a pool of water. The issue with this approach is that a complex system of servo motors and laser interferometers are required to accurately move the chuck, and submerging some or all of this system is difficult to achieve. The second approach is to limit the pool size to the top of the chuck. This technique would keep all of the chuck control mechanisms out of the water but would add considerable mass to the chuck that must rapidly accelerate. The third technique is to dispense the water between the lens and the wafer with a nozzle and rely on surface tension to maintain a “puddle”. However, bubbles can still form between the lens and the wafer surface due to the fact that water droplets can be created all over the wafer surface, and not just at the puddle. When an unexposed portion of the wafer that includes a water droplet receives the puddle, air can be trapped, thereby causing one or more bubbles.  
         [0006]     It is desired to provide a method for use with fabrication processes such as immersion lithography that reduces or otherwise eliminates any bubbles that may occur between on the wafer surface. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features may not be drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.  
         [0008]      FIG. 1  is a perspective view of a semiconductor wafer being processed in an immersion lithography system according to one embodiment of the present invention.  
         [0009]      FIG. 2  is a side cross-sectional view of the semiconductor wafer and immersion lithography system of  FIG. 1 .  
         [0010]      FIGS. 3 and 4  are close-up views of water on a top surface of the semiconductor wafer of  FIGS. 1 and 2 . 
     
    
     DETAILED DESCRIPTION  
       [0011]     It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the present invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are merely examples and are not intended to be limiting. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Moreover, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed interposing the first and second features, such that the first and second features may not be in direct contact.  
         [0012]     Referring to  FIG. 1 , a 193 nm immersion lithography system  10  is an example of a system and method that can benefit from different embodiments of the present invention. The immersion lithography system  10  includes a stage (or chuck)  12  and a plurality of stage control mechanisms  14 , which may use such conventional devices such as servos for controlling the movement of the stage  12 . The immersion lithography system  10  also includes one or more lenses  16  through which an image can be projected. In the present embodiment, the immersion lithography system  10  also includes a nozzle  18  for providing a fluid.  
         [0013]     As shown in  FIG. 1 , a wafer  20  can be placed on the stage  12  and both can be moved by the stage control mechanisms  14 . Also, the nozzle  18  emits water  22  to form a puddle  24  on a top surface of the wafer  20 . In the present embodiment, the puddle  24  does not cover the entire top surface of the wafer  20 .  
         [0014]     In the present embodiment, the immersion lithography system  10  is a puddle-type system. The nozzle  18  dispenses the water  22  between the lens  16  and the wafer  20 . Surface tension causes the water  22  to form the puddle  24 . In some embodiments, the stage  12  may be recessed for receiving the wafer  20 . A lip around the stage&#39;s edge allows the puddle  24  to extend off the edge of the wafer  20  during edge die exposure.  
         [0015]     Referring also to  FIG. 2 , the exposure system  10  may include many additional components, including patterning devices (e.g., masks), light producing mechanisms, additional lenses and optical elements, laser measurement systems, and so forth, collectively represented by the box  30 . These additional components can be dependent on various factors that are a choice of design.  
         [0016]     The puddle  24  does not cover the entire top surface of the wafer  20 , but instead covers a step area  32  that is associated with a step-and-repeat type exposure system. In one embodiment, the step area  32  may correspond to one die on the wafer. It is understood that in other embodiments, different multiples of die can be covered by a single step area  32 . Furthermore, in some embodiments, a reduced exposure area  34  may be exposed to a pattern at any one time, while the step area  32  is being scanned in a direction  36  to expose an entire reticle image. Once the step area  32  has been exposed, the stage  12  moves (relatively) so that a next step area  38  can be exposed.  
         [0017]     The stage  12  steps from location to location across the wafer  20 , scanning the reticle image for each step location. In order to achieve high throughput, the stage  12  must accelerate rapidly, move accurately to the next step area, settle, scan the image and then step to the next step area all in a short period of time.  
         [0018]     It has been noticed that on occasion, one or more water droplets  40  may appear on the top surface of the wafer  20 . The water droplets  40  may be a result of overspray from the nozzle  18 , may result from the scanning or stepping movement of the stage  12 , or may result for some other reason.  
         [0019]     In the example of  FIG. 2 , the water droplet  40  appears on the step area  38  of the wafer  20  that has not yet been exposed. When the stage  12  moves the wafer  20  so that the step area  38  is aligned to receive a new puddle, one or more air bubbles can form.  
         [0020]     Referring now to  FIGS. 3 and 4 , as the puddle  24  approaches the water droplet (designated  40   a  and  40   b  in  FIGS. 3 and 4 , respectively), a contact angle ( 50   a  and  50   b  in  FIGS. 3 and 4 , respectively) is very important as to whether or not air is trapped between the puddle and the water droplet. In  FIG. 3 , the contact angle  50   a  of the water droplet  40   a  is relatively high (e.g., about 85°). As a result, a significant amount of air  52   a  is trapped, resulting in the creation of bubbles. In  FIG. 4 , the contact angle  50   b  of the water droplet  40   b  is about 60°. As a result, practically no air  52   b  is trapped, resulting in no bubbles. Through experimentation, it has been determined that a preferred range of contact angle is between 40° and 80°, although other angles may also be suitable.  
         [0021]     The contact angle  50   a ,  50   b  can be controlled by the composition of a top layer  54  of the wafer  20 . The top layer  54  can be various layers, such as photoresist or top antireflective layer (top ARC). It is understood that in some embodiments, a photoresist layer can be used alone for forming patterned microelectronic structures, while in other embodiments, one or more antireflective layers may be used. Furthermore, a top ARC layer is often used to prevent lens contamination. Typically, a top ARC layer is transparent to deep ultra-violet (DUV) light used in photolithography processing and has an index matched with the underlying photoresist.  
         [0022]     The top layer  54  may include surfactants, polymers, or combinations thereof. If the top layer  54  is too hydrophobic, as is shown in  FIG. 3 , bubbles  52   a  can occur. If the top layer  54  is too hydrophilic, swelling may occur due to diffusion of the water into the hydrophilic layer (and vice versa). If swelling occurs, the results of the lithographic process will be deteriorated. Therefore, a balance between hydrophilic and hydrophobic is desired, either by treating the top layer  54 , by modifying the fluid (e.g., water)  22 , or both.  
         [0000]     Monomer Ratio  
         [0023]     To get a contact angle between hydrophilic and hydrophobic, the monomer ratio of a polymer photoresist or top ARC can be modified. The following polymers are known to be hydrophilic:  
                                                       poly(vinyl alcohol)   PVAL           poly(vinyl chloride)   PVC           polyamide   PA           poly(acrylic acid)   PAA           polyacrylonitrile   PAN           poly(ethylene oxide)   PEOX           poly(vinyl acetate)   PVAC           poly(vinyl butyral)   PVB           poly (p-hydroxystyrene)   PHS                      
 
         [0024]     as well as cellulose such as:  
                                                       cellulose acetate   CA           cellulose acetate butyrate   CAB           cellulose acetate propionate   CAP           cellulose nitrate   CN           cellulose propionate   CP           ethyl cellulose   EC                      
 
         [0025]     Furthermore, common commercial hydrophilic copolymers are copolymers made of polyethylene oxide (PEO) and crystallizable polyamide, polyurethane or polyester (PBT). These materials can be used to make a hydrophobic polymer more hydrophilic. The following polymers are known to be hydrophobic:  
                                                       silicone               polyethylene   PE           poly(phenylene oxide)   PPO           poly(phenylene sulfide)   PPS           polystyrene   PS                      
 
         [0026]     Further still, polymers with acid labile functional group known to be hydrophobic include:  
         [0027]     poly(4-t-butoxycarbonyloxystyrene) PBOCST  
         [0000]     Additives  
         [0028]     Additives can also be used in conjunction with, or independently of, adjusting the monomer ratio of the top layer  54 . The following end groups may be added to make a hydrophobic polymer more hydrophilic:  
                                                       hydroxyl   OH           amide   CONH           carboxy   COOH                      
 
 In addition, additives can be added to the water  22  to make the desired contact angle. 
 
 Other Treatments 
 
         [0029]     Furthermore, other treatments can be used either separately or in combination with one or more of the above-referenced treatments to achieve the desired contact angle. For example, a physical treatment such as exposing the top layer  54  to a plasma source can be used. Also, a chemical treatment of spraying the top layer  54  with an additive, such as one of the additives discussed above, can be used. Another example is to modify the polymer of the top ARC layer by using a fluorine polymer.  
         [0030]     The foregoing has outlined features of several embodiments according to aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions and alterations herein without departing from the spirit and scope of the present disclosure.