Abstract:
A method and apparatus for dispensing a resist solution used in a semiconductor device manufacturing process senses the presence of air bubbles in the solution during delivery through a line feeding a dispensing pump. Air bubbles in the line are sensed by an optical photocoupler that senses changes in the intensity of light refracted through the solution caused by air bubbles entrapped in the solution. The sensor produces an air bubble indicating signal that can be used to activate an alarm or to stop the dispensing process.

Description:
TECHNICAL FIELD 
     The present invention broadly relates to semiconductor device manufacturing equipment, and deals more particularly with a method and apparatus for dispensing a solution of photoresist used in the manufacturing process. 
     BACKGROUND OF THE INVENTION 
     In processes for producing semiconductor devices from a semiconductor wafer, a number of techniques have been developed in order to form circuit patterns on the wafer. One of these processes employs photolithography which defines the circuit features on a wafer according to a specified pattern or mask. Subsequent manufacturing steps that are used to form a device include chemical and physical film depositions, etching, ion implantation, diffusion, annealing or thermal oxidation. Many of these processes require that a pattern of photoresist first be formed on the wafer substrate. The process for patterning the photoresist is referred to a photolithography process, which implies first depositing a uniform layer of photoresist onto the substrate, next exposing the photoresist layer to optical illumination through the mask, and then developing the exposed photoresist layer. The resist may be positive or negative, depending upon if it is to be removed or remain after the development of the irradiated regions. The development step may be carried out using wet chemical etching, dry plasma etching or by conversion to a volatile compound through the exposure radiation itself. The exposure radiation may be in the form of visible, deep ultraviolet or x-ray photons, or electron or ion beams of particles. The exposure can be made by a parallel process such as contact or projection printing from a mask, or by serially scanning one or more beams. 
     Prior to the application of the resist, the wafer surface must be cleaned, dehydrated and primed in order to improve the adhesion between the resist and the substrate. Cleaning steps are required because of the inevitable contamination which occurs during storage and handling between processing steps. As the minimum size of the circuit features is reduced, particles with diameters down to 10 nm and less have to be detected. Depending upon the expected contamination type and level, even chemical or dry cleaning will be required. Following surface cleaning, a dehydration bake is performed to evaporate most of the absorbed water. Once the wafer surface has been cleaned, a predetermined amount of resist is dispensed onto the wafer surface, either as the wafer remains motionless or is slowly rotated. Then, using a technique known as spin coating, the wafer is rapidly accelerated to a speed ranging between 2000 and 6000 RPM&#39;s. The acceleration stage is crucial to obtaining good uniformity since the solvents within the photoresist begin evaporating from the photoresist immediately after dispensing. After about 30 seconds of spin time, less than one percent of the originally dispensed amount of resist remains on the wafer surface, with the remainder having flown off with the spinning process. The typical resist thickness range is from 0.05-1.5 microns, and may not vary more than plus or minus 5 nm across a flat wafer, and plus or minus 10 nm from wafer to wafer. At this stage, the resist has a tacky consistency since less than ⅓ of the solvent remains. 
     Dispensing systems typically include a reservoir of the resist solution coupled with a pump that pumps the solution from the reservoir to a nozzle where a discrete quantity is dispensed onto the wafer surface. The composition of the resist solution is such that air bubbles may become entrapped therein as the solution is pumped from the reservoir, through the pump to the dispensing nozzle. The introduction of air bubbles into the solution can result in an incorrect amount of the solution being dispensed, and can also result in the deposit of resist on the wafer containing these air bubbles. In either case, the dispensing of an incorrect amount of the resist or dispensing resist containing entrapped air bubbles can result in inconsistencies in the resist layer which in turn can cause defects in circuit features that cause malfunctions in the operation of a semiconductor device. Consequently, the presence of air bubbles in the solution increases the likelihood of scrap and therefore decreases process yield of good devices. 
     In an attempt to eliminate air bubbles from the resist solution prior to the dispensing process, prior dispensing systems have employed a trap tank coupled between the solution reservoir and the pump. This tank acts as a holding tank where the air bubbles are allowed to settle out and/or are drained off from the solution. 
     In addition to the problems discussed above, air bubbles can become introduced into the resist solution as a result of leaks in the piping system connecting the reservoir, tank trap and pump. Moreover, in the event that the reservoir runs empty, the pump sucks air into the feed lines for a period of time until solution is added to the reservoir. During this period, the pump effectively draws air into the lines which becomes mixed with the solution. In each of these events, in addition to reducing process yields, valuable processing time is lost because human intervention is necessary to correct the problem and in some cases, the entire processing operation must be suspended while operators flush the lines and refill them with a fresh resist solution. 
     It would therefore be desirable to provide a resist dispensing system that virtually eliminates the possibility of air bubbles becoming entrapped in the solution, and drawn through the dispensing pump. The present invention is directed toward solving this problem. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, apparatus is provided for dispensing a liquid resist solution used in a semiconductor device manufacturing process. The apparatus includes a reservoir of the resist solution, a trap tank for trapping air bubbles entrapped in the solution, a pump for drawing the resist solution from the tank to a dispensing nozzle, and a sensor for sensing the presence of air bubbles in the resist solution flowing from the tank trap to a dispensing nozzle and a sensor for sensing the presence of air bubbles in a resist solution flowing from the tank trap to the pump. A controller receives signals from the bubble sensor and controls the operation of the dispensing process. The sensor preferably comprises a light source for directing light through the resist solution, and a photosensitive detection element for detecting changes in the light passing through the resist solution and originating from the light source. 
     According to another aspect of the invention, apparatus is provided for use with a plurality of processing stations for processing semiconductor wafers which monitors the presence of air bubbles in resist solution drawn from a tank to a dispensing pump. The apparatus comprises a plurality of sensors for respectively detecting the presence of air bubbles in the resist solution drawn from the associated tank to the corresponding pump, wherein each of the sensors produces a signal indicative of the presence of air bubbles in the solution, and a circuit connected with each of the sensors for receiving the signals produced by the sensors and for generating an alarm signal then the circuit receives a signal from any one of the sensors. The circuit preferably includes a logic circuit having a plurality if inputs for receiving the sensor signals, and an output for delivering a trigger signal, and a switch actuated by the trigger signal for outputting the alarm signal. 
     According to still another aspect of the invention, a method is provided for dispensing liquid resist solution used in a process for manufacturing semiconductor wafer devices. The method includes the steps of flowing the solution from a reservoir into a tank; removing at least some of the air bubbles contained in the resist solution within the tank; flowing the solution from the tank through a line to a dispensing nozzle; sensing the presence of air bubbles contained in the solution flowing through the line; dispensing the solution through the nozzle; and, issuing an alarm when air bubbles are sensed. The sensing step is performed by passing light through the solution flowing through a line and sensing changes in the intensity of light passing through the solution. The changes in the light intensity being indicative of the presence of air bubbles in the solution. 
     Accordingly, it is a primary object of the present invention to provide a method and apparatus for dispensing resist solution onto a semiconductor wafer that increases process yield and decreases equipment down time. 
     Another object of the invention is to provide a method and apparatus as mentioned above that provides early detection of air bubbles entrapped within the solution before the solution enters a dispensing pump. 
     A still further object of the invention is to provide a method and apparatus as described above which employs an interlock circuit cooperating with process control equipment that interrupts the operation of the equipment when air bubbles in the resist solution are sensed, to thereby prevent the dispensing of resist solution containing air bubbles. 
     These, and further objects and advantages of the present invention will be made clear or will become apparent during the course of the following description of a preferred embodiment of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings which form an integral part of the specification and are to be read in conjunction therewith, and in which like references numerals are employed to designate identical components in the various views; 
     FIG. 1 is a combined block and schematic diagram of apparatus for dispensing resist solution at multiple processing stations in accordance with the preferred embodiment; and, 
     FIG. 2 is a detailed schematic diagram of the control circuit and related sensors employed in the apparatus shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, the present invention generally relates to apparatus for dispensing a liquid resist solution from a nozzle  46  onto the surface of a semiconductor wafer  50  which is usually spinning during the dispensing process. Dispensing apparatus of this general type is part of a larger piece of equipment used in successively carrying out multiple manufacturing processes on a wafer  50 , which typically includes a plurality of processing stations for simultaneously processing wafers  50 . Two of the dispensing systems or “coaters” are generally shown by the numerals  10  and  12  which are respectively associated with two separate processing stations whose operation is controlled by a master controller  16 . Coaters  10  and  12  are identical, consequently only the details of coater  10  will be described herein. 
     A quantity of the resist solution is contained in a source tank or bottle  20  which is pressurized by a line  23  so as to feed the solution through a line  22  into a holding tank  26 . The bottle  20  and tank  26  thus act as a source or reservoir of the solution. A sensor  24  is employed to sense the presence of solution flowing through line  22  and is operated to generate a signal when solution in the line is no longer sensed. This signal acts as an alarm to alert an operator, or automatic controller, that the bottle  20  is empty or that it has otherwise malfunctioned. A vent line  28  is provided in tank  26  to allow the escape of air. The solution in the holding tank  20  flows through line  30  into a bubble trap tank  32  which is provided with an overflow drain  34 . The trap tank  32  functions to trap micro air bubbles present in the resist solution which, if not removed from the solution, remain entrapped within the solution as it is dispensed onto the surface of the wafer  50 . These micro air bubbles can cause imperfections in the layer of resist applied to the surface of the wafer  50 . These imperfections later result in feature defects of the devices formed from the wafer  50 , in turn causing the devices to be scraped. 
     A dispensing pump  40  draws solution from the trap tank  32  into the line  36  and delivers the solution to dispensing nozzle  46  through a filter  42  that is provided with a drain  44 . The pump  40  is controlled by the master controller  16  which has a plurality of control outputs  18  for controlling other functions of the associated manufacturing station. 
     In spite of the use of the trap tank  32 , some micro air bubbles may nevertheless remain entrapped in the solution and can be drawn into the pump  40 . Moreover, any air leaks in the line  36  or related connections will result in air being drawn into the line and mixed with the solution to create additional micro air bubbles. Finally, on some occasions, the solution sensor  24  may malfunction in which case it is not possible to detect when the bottle  20  runs empty of resist solution. When the bottle  20  is empty, and tank  26  is likewise depleted, continuing operation of the pump  40  results in air being drawn through the lines into the pump which become mixed with the last remaining solution, thereby creating micro air bubbles. In accordance with the present invention, in order to obviate the problem discussed above, a bubble sensor  38  is provided for sensing the presence of micro air bubbles entrapped in the resist solution immediately before it enters the dispensing pump  40 . Sensor  38  is mounted in the line  36  so as to sense bubbles flowing through the line. Although shown as being mounted near the trap tank  32 , sensor  38  may also be mounted so a to sense micro air bubbles in the solution immediately before the solution enters the pump  40  through line  36 . Bubble sensor  38  functions to sense changes in the intensity of light refracted through solution flowing through line  36 . The presence of air bubbles in the solution alters the light refraction, thereby causing the intensity of the refracted light to change in proportion to the density of the micro air bubbles present. When the sensor  38  detects a change in the refracted light intensity indicating the presence of micro air bubbles, it generates a signal that is delivered on line  39  to an interlock control circuit  14 , shown as a printed circuit board in FIG.  1 . Line  39  is shown as an input “R 1 ” to the circuit  14 . Inputs R 2 , R 3 , and R 4  respectively represent the inputs of identical sensors  38 A,  38 B, and  38 C that are employed to sense the resist solution flowing through other dispensing lines forming part of the coating unit  10 . 
     Control circuit  14  processes the signals received on inputs R- 1 -R 4 , and when a signal is received on one of these inputs indicating that micro air bubbles have been sensed, an LED  56  is turned on to indicate an alarm condition, an alarm signal is issued on line  52  to the master controller  16 . The master controller  16  is responsive to the alarm signal on line  52  to issue a signal on line  54  that terminates operation of the pump  40 . In effect, the sensor  38  and control circuit  14  cooperate with the master controller  16  to form an air bubble interlock that precludes dispensing resist solution onto the wafer  50  whenever air bubbles are present in the resist solution drawn into the pump  40 . 
     It should be mentioned here that the air bubble interlock described above may be conveniently retrofitted to existing dispensing systems and, in the event of a malfunction in the interlock system, it does not interfere with or adversely affect the remaining components of the dispensing system. 
     Referring now to also to FIG. 2 each of the sensors  38  comprises a source of light, such as an LED  58  and a phototransistor  60 . These two latter mentioned components are mounted such that the LED  58  emits light that passes through a resist solution and is picked up by the phototransistor  60 . As mentioned earlier, the presence of micro air bubbles in the solution alters the refracted index of the solution, thereby changing the intensity of light that is received by the phototransistor  60 . When the change in the intensity of light exceeds a preset, threshold value, transistor  60  turns on, thereby issuing a signal on line  61  which forms one input of a multi-input NAND gate  62 . The remainder of the inputs to NAND gate  62  are defined by the output lines of the sensors  38 A- 39 C. Normally, in the absence of any micro air bubbles being sensed, inputs to the NAND gate  62  are high and its output on line  63  is low. Line  63  forms one input to NOR gate  66 , a second input to NOR gate  66  being formed by line  65  which is connected to the output of NAND gate  64  associated with the second coater  12  (FIG.  1 ). NAND gate  64  functions identical to NAND gate  62 , having its inputs connected to receive the output signals from the sensors  41  associated with the second coater  12 . 
     Normally, lines  63  and  65  are low, consequently the output of NOR gate  66  is low. This low output from gate  66  is delivered to the inputs of three NOR gates  68  which function as a buffer to square up the signal before it is delivered to the coil  70  of a relay. Coil  70  controls a set of relay contacts  72  that selectively couple an LED  74  with a power source, and also couple a power source with the output, alarm signal line  52 . When the presence of micro air bubbles is sensed by any of the sensors  38 ,  41  at least one of the lines  61  switches from high to low, consequently the output of the corresponding NAND gate  62  switches from low to high. This high signal is gated through NOP gates  66  and  68  and functions to couple a voltage source  73  with the relay coil  70 , thereby energizing the latter. With coil  70  energized, contacts  72  are switched, thereby turning on the LED  74  and coupling line  52  with a voltage source so as to issue the alarm signal in line  52  to the master controller  16 . 
     It may appreciated at this point that a novel method of dispensing liquid resist solution has been provided that comprises the steps of flowing the solution from a reservoir into a tank; removing at least some of the air bubbles contained in the solution within the tank; flowing the solution from the tank through a line to a dispensing nozzle; sensing the presence of air bubbles contained in the solution flowing through the line; dispensing the solution through a nozzle; and issuing a alarm signal when air bubbles are sensed before these air bubbles are dispensed as part of the solution. 
     From the foregoing, it is apparent that the method and apparatus described above not only provide for the reliable accomplishment of the objects of the invention, but it do so in a particularly economical and efficient manner. It is recognized, of course, that those skilled in the art may make various modifications or additions to the preferred embodiment chosen to illustrate the invention without departing from the spirit and scope of the present contribution to the art. Accordingly, it is to be understood that the protections sought and to be afforded hereby should be deemed to extend to the subject matter claimed and all equivalents thereof fairly within the scope of the invention.