Patent Publication Number: US-6656280-B2

Title: Resist recycling apparatus and method for recycling the same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a divisional application of application Ser. No. 09/616,379, filed Jul. 13, 2000 now U.S. Pat. No. 6,503,568 which is hereby incorporated by reference in its entirety for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a resist recycling apparatus and a method for recycling the same, and more particularly, this invention relates to a process for recycling resist wastes generated by a process for coating a resist on a substrate. 
     This application is a counterpart of Japanese application Serial Number 278589/1999, filed Sep. 30, 1999, the subject matter of which is incorporated herein by reference. 
     2. Description of the Related Art 
     A photolithography technique is generally used in the manufacturing of a semiconductor device. 
     Spin coating is generally used to form resist thin films on a semiconductor substrate. In a typical spin coating, liquid resist is dropped on to a silicon wafer by a nozzle. Spinning of the wafer spreads the resist over the wafer. After spinning, only a thin film of the resist is left on the surface of the wafer. 
     In excess of 95% of the resist is wasted in spin coating. Therefore, a lot of resist recycling techniques have been proposed, such as Japanese laid open number HEI 8-203804 and so on. 
     These prior arts cannot control the viscosity of the resist sufficiently to reuse recycled resist in the manufacturing of a semiconductor device. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a resist recycling apparatus and a method for recycling a resist. 
     According to one aspect of the present invention, for achieving the above object, there is provided a resist recycling apparatus, comprising a viscosity control tank being supplied with wasted resist liquid, a solvent tank supplying a solvent to the viscosity control tank, a viscometer measuring s viscosity of the resist in the viscosity control tank, a control portion calculating the resin density of the resist according to the viscosity measured by the viscometer and the temperature of the resist, determining an amount of solvent to supply to the viscosity control tank according to the difference between the calculated resin density and a predetermined resin density, and a filter removing dust from the resist. 
     According to another aspect of the present invention, for achieving the above object, there is provided a method for recycling a resist comprising supplying waste resist to a viscosity control tank, measuring the viscosity of the resist in the viscosity control tank, calculating the resin density according to the measured viscosity and the temperature of the resist in the viscosity control tank, determining the amount of solvent to add to the resist according to the difference between the calculated resin density and the predetermined resin density, supplying a solvent to the viscosity control tank, and removing dust from the resist. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter that is regarded as the invention, the invention, along with the objects, features, and advantages thereof, will be better understood from the following description taken in connection with the accompanying drawings, in which: 
     FIG. 1 is a schematic diagram showing a resist recycling apparatus according to a first preferred embodiment of this invention. 
     FIGS. 2A and 2B show a cup structure of a spin coater in this invention. 
     FIG. 3 shows the relationship between the resin density and the Viscosity Current Value. 
     FIG. 4 shows the relationship between the viscosity and the Viscosity Current Value. 
     FIG. 5 shows relationship s between the resin density and the Viscosity Current Value when the temperatures of the resist is changed. 
     FIG. 6 shows relationships between the coefficients A, B and the temperature of the resist. 
     FIG. 7 shows a resist recycling flow in this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic diagram showing a resist recycling apparatus according to a first preferred embodiment of this invention. 
     This resist recycling apparatus has a viscosity control tank  101 , a solvent tank  102 , a waste liquid tank  103 , a resist supply port  104  for supplying used resist, a gas supply port  105  for supplying gaseous nitrogen, a flow control valve  106  for controlling a flow rate of the solvent from the solvent tank to the viscosity control tank  101 , an agitator  107  for agitating the viscosity control tank  101 , a gaseous nitrogen supply pipe  108 , a gas exhaust pipe  109 , a pump  110 , a plurality of valves  111 ˜ 115 , a filter  116  for filtering a resist, a filter  117  for filtering gaseous nitrogen, a measure  118  for measuring the weight of the viscosity control tank  101 , an ultrasonic viscometer  119  for measuring the viscosity and the temperature of the resist in the viscosity control tank  101 , and a control portion  120  for controlling the flow control valve or etc. 
     The viscosity control tank  101  is a tank for controlling the viscosity of resist. The used resist, which is collected in a resist coating apparatus, is supplied into the viscosity control tank  101  through the valve  111  from the resist supply port  104 . Solvent thinner is supplied into the viscosity control tank  101  through the control valve  106  from the solvent tank  102 . 
     The gaseous nitrogen is supplied into the viscosity control tank  101  through the valve  112  and the filter  117 . This gaseous nitrogen prevents the collected resist from coming into contact with the air. This gaseous nitrogen is also used to move the resist by the pressure of the gaseous nitrogen after the viscosity control. 
     The agitator  107  to make the temperature and the viscosity of the resist uniform agitates the resist in the viscosity control tank  101 . 
     The viscosity and the temperature of the resist are measured by the ultrasonic viscometer  119 . The data measured by the viscometer is sent to the control portion  120 . 
     The ultrasonic viscometer  119  in this embodiment has a vibrating pole that vibrates in a constant cycle and constant amplitude. The value of an electric current to vibrate the vibrating pole shows the viscosity. The viscosity is calculated based on this value of an electric current. The value of an electric current is sent to the control portion  120  as a measured data. This value of an electric current is called Viscosity Current Value hereinafter. 
     The solvent tank  102  holds thinner which is the solvent of the resist in this embodiment. 
     The wasted liquid tank  103  holds waste resist. The resist held in this tank  103  is delivered to the viscosity tank  101  by the pump  103   a.    
     There are two ways to supply waste resist in to the viscosity control tank  101 . One way is to supply through the resist supply port  104 , and another way is to supply from the waste liquid tank  103 . 
     After the control of the viscosity of the resist, the resist is supplied as recycled resist through the filter  116 . 
     In this embodiment, waste resist is collected separately from the edge rinse liquid or the back rinse liquid. 
     FIGS. 2A and 2B show a cup structure for collecting waste resist. A spin chuck  202  adsorbs a semiconductor wafer  201 . A cup inside  203  has two layers and it moves up and down. During a resist coating, a cup is set downward as shown in FIG.  2 A. 
     A resist liquid is deposited on the wafer. During the resist liquid deposition, the wafer is spinning, and the resist liquid is scattered out about the same height of the semiconductor wafer surface. 
     The scattered resist gets into the upper layer of the inside of the cup and is collected through the upper layer. The collected resist is held in the waste liquid tank  103  or supplied directly to the resist supply port  104 . 
     During a back rinse or an edge rinse, the cup is set upward as shown in FIG.  2 B. When back rinse or edge rinse is done in this state, the rinse liquid is scattered out. 
     The scattered rinse liquid gets into the lower layer of the inside of the cup and is collected through the lower layer. The collected rinse is recycled after processes of filtering etc. 
     The method for resist recycling in this invention is described below. 
     Most important thing in the resist coating process is the uniformity of the resist thickness. The thickness of the resist is mainly determined by the viscosity of the resist. A resin density (ratio of resin included in resist liquid) determines the viscosity of resist liquid. 
     A method for measuring viscosity of resist and resin density is described below. 
     FIG. 3 shows the relationship between the resin density and the Viscosity Current Value, in this embodiment, the value of an electric current, which shows the viscosity of resist as described above, is used. 
     The detailed experiments of the inventor of this invention confirm the relationship of the Viscosity Current Value X and the resin density Y. This relationship is expressed by using a logarithmic function (natural logarithm) as shown below. 
     
       
           Y=ALnX+B  (A,B: coefficients)   (1)  
       
     
     This relationship makes it possible to calculate the ratio of resin included in resist liquid. 
     The viscosity itself pointed out by the ultrasonic viscometer is not used in the above expression. The Viscosity Current Value is used in the above expression. 
     FIG. 4 shows the relationship between the viscosity and the Viscosity Current Value. 
     The relationship of the Viscosity Current Value X and the viscosity is expressed by using a tertiary function 
     If the Expression (1) is expressed using the viscosity, the Expression (1) becomes very complex, so the calculation of the resin density becomes very difficult. However, by using the Viscosity Current Value, it becomes very simple, and the calculation can be performed more precisely. 
     FIG. 5 shows relationships between the resin density and the Viscosity Current Valve when the temperatures of the resist are changed. 
     The temperature of the resist determines the coefficients A, B of the expression 1. 
     FIG. 6 shows relationships between the coefficients A, B and the temperature of the resist. 
     More detailed experiments revealed that relationships between the coefficients A, B and the temperature are linear. 
     The coefficients A, B are expressed as below. 
     
       
           A=CT+D  (C,D: coefficients, T: temperature)   (2)  
       
     
     
       
           B=ET+F  (E,F: coefficients, T: temperature)   (3)  
       
     
     These expressions are applied to the expression (1). When the Viscosity Current Value X is measured at the temperature T, the resin density Y is expressed as below. 
     
       
           Y= ( CT+D ) LnX+ET+F    (4)  
       
     
     In this embodiment, the resist used is FF134A made by Sumitomo Chemical Industry. The expression (4) of this resist is described below. 
     
       
           Y= (0.0164· T+ 0.5764) LnX+ (−0.0987· T− 3.3168)   (5)  
       
     
     The expression (4) is applied to various resists by changing coefficients C,D,E,F. 
     FIG. 7 shows a resist recycling flow in this invention. The resist recycle flow of the invention will be described in conjunction with FIG.  1  and FIG.  7 . 
     Step 1 
     Predetermined resin density Y 1  is inputted to the control portion  120 . The used resist, which is collected in a resist coating apparatus, is supplied into the viscosity control tank  101  from the resist supply port  104 . The used resist may be supplied from the waste resist tank  103  by a pump. 
     Step 2 
     The agitator  107  agitates the resist in the viscosity control tank  101 . After the temperature and the viscosity of the resist are made uniform, the ultrasonic viscometer  119  measures the viscosity and the temperature of the resist 
     Step 3 
     The measured data are sent to a control portion  120 , and the control portion  120  calculates the resin density Y 2  of the used resist by using the expression (5). 
     Step 4 
     The control portion  120  calculates the amount of thinner that should be added to the used resist. The amount of thinner is calculated according to the difference between a predetermined resin density Y 1  and the measured resin density Y 2 . 
     For example, if predetermined resin density is 25%, then when measured resin density of 1 kg of the used resist is 30%, 200 g of thinner should be added. The relationship described above is expressed as below. 
     
       
           I=L ( Y 1/ Y 2−1)   (6)  
       
     
     In the expression (6), predetermined resin density is denoted Y 1 , measured resin density of the used resist is denoted Y 2 , the amount of the used resist is denoted L (g), and the amount of solvent that should be added is denoted I (g). 
     Step 5 
     The control portion  120  outputs the signal to operate the control valve  106 . The control valve operates to add the thinner in the viscosity control tank  101 . The amount of thinner added at this time, is 90% of the calculated amount. The agitator  107  agitates the resist in the viscosity control tank  101   
     Step 6 
     The ultrasonic viscometer  119  measures the viscosity and the temperature of the resist, and the control portion  120  calculates the amount of thinner that should be added to the used resist again. 
     Step 7 
     Operations of STEP 5 and STEP 6 are repeated, until the measured resin density become within the predetermined range. When the measured resin density becomes within the predetermined range, controlling of the viscosity ends. 
     Step 8 
     After the control of the viscosity of the resist, the resist is supplied as a recycled resist through the filter  116 . 
     This resist recycle apparatus has two kinds of nozzles illustrated in FIG. 1 as  121 , that add the solvent thinner. One nozzle is a nozzle for a large amount, and another nozzle is a nozzle for a small amount. The amount of thinner determines which nozzle to use. 
     When a used resist is collected as waste liquid, a solvent is generally evaporated with passage of time. 
     For example, the resist is used on condition that the resin density is 25%. When this resist is collected after use, the resin density becomes larger than 25%. Solvents are added until the resin density becomes a predetermined value in this invention. 
     The addition of the calculated amount of solvent is divided into several times in this invention. The amount of added solvent gets smaller and smaller with every addition, and the resin density is measured after every addition. This makes the control of resin density more precise. 
     The method for resist recycling of this invention makes the control of resin density of resist more precise, so waste resist liquid is recycled as a resist in good condition. 
     A percentage is used in the description of the embodiment, but a ratio is used in coefficients of expressions and figures, and their meaning is the same. For example, if the resin density in a figure indicates 0.24, it means 24%. 
     It is very easy to change a percentage to a ratio. It is also easy to change a ratio to percentage. Both ways are applied in this invention.