Chemical blending system with titrator control

Claims

What is claimed is:

1. A chemical blending system for blending at least first and second chemical constituents to obtain blended chemical having a desired chemical concentration, including:

a first constituent inlet for receiving a first chemical constituent;

a second constituent inlet for receiving a second chemical constituent;

a blending tank fluidly coupled to the first and second constituent inlets, for receiving the chemical constituents and holding the blended chemical;

a recirculation line having an inlet and an outlet in the blending tank;

pump means having a suction end and a pressure end in the recirculation line, for recirculating blended chemical from the blending tank through the recirculation line to blend the chemical constituents;

at least a first regulating mechanism coupling the first constituent inlet to the recirculation line between the recirculation line inlet and the suction end of the pump means, for regulating the amount of at least the first chemical constituent supplied to the recirculation line;

a first concentration measuring instrument having first operating characteristics, for measuring concentration of the blended chemical in the blending tank;

a second concentration measuring instrument having second operating characteristics different than the first operating characteristics, for measuring concentration of the blended chemical in the blending tank;

memory for storing digital data representative of:

a blending control program; and

a concentration qualification range representative of chemical concentrations within a qualification range of the desired chemical concentration; and

a digital processor coupled to the memory, first and second concentration measuring instruments and to the first regulating mechanism, for executing the blending control program and controlling at least the first regulating mechanism as a function of the concentration measurements provided by the first and second concentration measuring instruments to blend the first and second chemical constituents to a concentration within the qualification range.

2. The chemical blending system of claim 1 wherein:

the system further includes a second regulating mechanism coupling a source of the second chemical constituent to the recirculation line between the recirculation line inlet and the suction end of the pump means, for regulating the amount of the second chemical constituent supplied to the recirculation line; and

the digital processor is coupled to the second regulating mechanism, and controls the first and second regulating mechanisms as a function of the concentration measurements provided by the first and second concentration measuring instruments to blend the first and second chemical constituents to a concentration within the qualification range.

3. The chemical blending system of claim 1 wherein:

the memory further includes memory for storing digital data representative of first constituent injection information characterizing the relationship between a controlled parameter of the first regulating mechanism and expected corresponding changes in the concentration of the blended chemical; and

the digital processor controls the first regulating mechanism as a function of the first constituent injection information and differences between the concentration measurements provided by the first and second concentration measuring instruments and the desired chemical concentration.

4. The chemical blending system of claim 3 wherein:

the system further includes a second regulating mechanism coupling a source of the second chemical constituent to the recirculation line between the recirculation line inlet and the suction end of the pump means, for regulating the amount of the second chemical constituent supplied to the recirculation line;

the memory further includes memory for storing digital data representative of second constituent injection information characterizing the relationship between a controlled parameter of the second regulating mechanism and expected corresponding changes in the concentration of the blended chemical; and

the digital processor is coupled to the second regulating mechanism, and controls the second regulating mechanism as a function of the second constituent injection information and differences between the concentration measurements provided by the first and second concentration measuring instruments and the desired chemical concentration to blend the first and second chemical constituents to a concentration within the qualification range.

5. The chemical blending system of claim 4 wherein the digital processor further includes:

first update means for updating the first constituent injection information as a function of the difference between expected changes in blended chemical concentrations caused by control of the first regulating mechanism, and actual changes in the blended chemical concentrations caused by control of the first regulating mechanism; and

second update means for updating the second constituent injection information as a function of the difference between expected changes in blended chemical concentrations caused by control of the second regulating mechanism, and actual changes in the blended chemical concentrations caused by control of the second regulating mechanism.

6. The chemical blending system of claim 3 wherein the digital processor further includes means for updating the first constituent injection information as a function of the difference between expected changes in blended chemical concentrations caused by control of the first regulating mechanism, and actual changes in the blended chemical concentrations caused by control of the first regulating mechanism.

7. The chemical blending system of claim 1 wherein the first concentration measuring instrument includes measurement means for providing a relatively fast measurement response time with respect to the response time operating characteristics of the second concentration measuring instrument.

8. The chemical blending system of claim 7 wherein the first concentration measuring instrument includes measurement means for providing a substantially instantaneous measurement response time.

9. The chemical blending system of claim 7 wherein the first concentration measuring instrument includes a conductivity-type measuring instrument.

10. The chemical blending system of claim 1 wherein the second concentration measuring instrument includes measurement means for providing a relatively high degree of measurement accuracy with respect to the degree of measurement accuracy operating characteristics of the first concentration measuring instrument.

11. The chemical blending system of claim 10 wherein the second concentration measuring instrument includes a titration analyzer measuring instrument.

12. A chemical blending system for blending at least first and second chemical constituents to produce blended chemical having a desired chemical concentration within a qualification range, including:

a first constituent inlet for receiving the first chemical constituent;

a second constituent inlet for receiving the second chemical constituent;

blending apparatus fluidly coupled to the first and second constituent inlets, for receiving and blending the first and second chemical constituents to produce the blended chemical;

a first regulating mechanism for regulating the amount of at least one of the chemical constituents received by the blending apparatus;

a first concentration measuring instrument having first operating characteristics, for measuring concentration of the blended chemical in the blending apparatus;

a second concentration measuring instrument having second operating characteristics different than the first operating characteristics, for measuring concentration of the blended chemical in the blending apparatus; and

a control system coupled to the first and second concentration measuring instruments for controlling the first regulating mechanism, including:

first control means for actuating the first regulating mechanism to control chemical blending as a function of concentration measurements from the first concentration measuring instrument when the measured concentration is less than a first blend concentration set point which is less than the qualification range;

second control means for actuating the first regulating mechanism to control chemical blending as a function of concentration measurements from the second concentration measuring instrument when the measured concentration is greater than the first blend concentration setpoint and less than a lower qualification range setpoint of the qualification range; and

third control means for qualifying the blended chemical by causing the first regulating mechanism to discontinue the receipt of the chemical constituents when the concentration measurements from the second concentration measuring instrument are within the qualification range.

13. The chemical blending system of claim 12 and further including fourth control means for actuating the first regulating mechanism as a function of concentration measurements from the first concentration measuring instrument when the measured concentration is less than a second blend concentration setpoint which is less than the first blend concentration setpoint.

14. The chemical blending system of claim 12 wherein:

the control system further includes stored first constituent injection information characterizing a relationship between a controlled parameter of the first regulating mechanism and expected corresponding changes in the concentration of the blended chemical;

the first control means includes means for actuating the first regulating mechanism as a function of the first constituent injection information and the difference between the concentration measurements from the first measuring instrument and the desired concentration, to add a finite quantity of the chemical constituent to the blending apparatus in an attempt to increase the blended chemical concentration to a concentration within the qualification range; and

the second control means includes means for actuating the first regulating mechanism as a function of the first constituent injection information, and the difference between the concentration measurements from the second measuring instrument and the desired concentration, to add a finite quantity of the chemical constituent to the blending apparatus in an attempt to increase the blended chemical concentration to a concentration within the qualification range.

15. The chemical blending system of claim 14 wherein the control system further includes first update means for updating the first constituent injection information as a function of the difference between expected changes in blended chemical concentration caused by control of the first regulating mechanism, and actual changes in the blended chemical concentration caused by control of the first regulating mechanism.

16. The chemical blending system of claim 14 and further including fourth control means for actuating the first regulating mechanism to continuously add the chemical constituent to the blending apparatus when the measured concentration is less than a second blend concentration setpoint which is less than the first blend concentration setpoint, in an attempt to increase the blended chemical concentration to a concentration which is greater than or equal to the second blend concentration setpoint.

17. The chemical blending system of claim 12 and further including:

a second regulating mechanism for regulating the amount of a second chemical constituent received by the blending apparatus; and

second regulating mechanism control means for actuating the second regulating mechanism to control chemical blending as a function of concentration measurements from at least one of the first and second measuring instruments when the measured concentration is greater than the qualification range.

18. The chemical blending system of claim 17 wherein the control system further includes stored second constituent injection information characterizing a relationship between a controlled parameter of the second regulating mechanism and expected corresponding changes in the concentration of the blended chemical, and the second regulating mechanism control means includes means for actuating the second regulating means as a function of second constituent injection information and the difference between the concentration measurements from the measuring instrument and the desired concentration, to add a finite quantity of the second chemical constituent to the blending apparatus in an attempt to decrease the blended chemical concentration to a concentration within the qualification range.

19. The chemical blending system of claim 18 and further including second update means for updating the second constituent injection information as a function of the difference between expected changes in blended chemical concentration caused by control of the second regulating mechanism, and measured changes in the blended chemical concentration caused by control of the second regulating mechanism.

20. The chemical blending system of claim 12 wherein the first concentration measuring instrument includes measurement means for providing a relatively fast measurement response time with respect to the response time operating characteristics of the second concentration measuring instrument.

21. The chemical blending system of claim 20 wherein the first concentration measuring instrument includes measurement means for providing a substantially instantaneous measurement response time.

22. The chemical blending system of claim 12 wherein the first concentration measuring instrument includes a conductivity-type measuring instrument.

23. The chemical blending system of claim 12 wherein the second concentration measuring instrument includes measurement means for providing a relatively high degree of measurement accuracy with respect to the degree of measurement accuracy operating characteristics of the first concentration measuring instrument.

24. The chemical blending system of claim 12 wherein the second concentration measuring instrument includes a titration analyzer measuring instrument.

25. The chemical blending system of claim 12 wherein:

the blending apparatus includes:

a blending tank fluidly coupled to the first and second constituent inlets, for receiving the chemical constituents and holding the blended chemical;

a recirculation line having an inlet and an outlet in the blending tank; and

pump means having a suction end and a pressure end in the recirculation line, for recirculating blended chemical from the blending tank through the recirculation line to blend the chemical constituents; and

the first regulating mechanism couples the first constituent inlet to the recirculation line between the recirculation line inlet and the suction end of the pump means.

26. The chemical blending system of claim 25 wherein the first regulating mechanism includes a valve.

27. The chemical blending system of claim 12 wherein the first control means, second control means and third control means include digital control means and associated memory.

28. A chemical blending system for blending concentrated chemical with a diluent to obtain blended chemical having a desired chemical concentration within a qualification range, including:

a concentrated chemical inlet for receiving concentrated chemical;

a diluent inlet for receiving diluent;

a blending tank fluidly coupled to the concentrated chemical inlet and the diluent inlet, for receiving and blending the concentrated chemical and diluent to produce the blended chemical;

a concentrate flow regulating mechanism responsive to concentrate control signals, for controlling the flow of concentrated chemical from the concentrated chemical inlet into the blending tank;

a first concentration measuring instrument having first operating characteristics, for measuring concentration of the blended chemical in the blending tank and providing first instrument readings as a function of the measurements;

a second concentration measuring instrument having second operating characteristics different than the first operating characteristics, for measuring concentration of the blended chemical in the blending tank and providing second instrument readings as a function of the measurements;

memory for storing data representative of:

concentrate injection information representative of the relationship between the concentrate control signals and blended chemical concentration changes induced by actuation of the concentrate flow regulating mechanism;

a fine blend setpoint concentration which is less than the qualification range; and

qualification range setpoint concentrations representative of chemical concentrations within a qualification range of the desired chemical concentration; and

a digital processor coupled to the memory, first and second concentration measuring instruments and the concentrate flow regulating mechanism, and including means for:

(a) monitoring first instrument readings of blended chemical concentration measurements provided by the first concentration measuring instrument;

(b) generating concentrate control signals as a function of the concentrate injection information and the first instrument readings, to actuate the concentrate flow regulating mechanism and add a finite quantity of the chemical concentrate in an attempt to increase the blended chemical concentration to the desired concentration if the monitored first instrument reading is less than or equal to the fine blend setpoint concentration;

(c) monitoring second instrument readings of blended chemical concentration measurements provided by the second concentration measuring instrument if the monitored first instrument reading is greater than the fine blend setpoint concentration;

(d) generating concentrate control signals as a function of the concentrate injection information and the second instrument readings, to actuate the concentrate flow regulating mechanism and add a finite quantity of the chemical concentrate in an attempt to increase the blended chemical concentration to the desired concentration if the monitored second instrument reading is greater than the fine blend setpoint concentration and less than the qualification range setpoint concentrations;

(e) repeating functions (a)-(d) until the second instrument reading is within the qualification range setpoint concentrations.

29. The chemical blending system of claim 28 wherein:

the memory further includes data representative of a coarse blend setpoint concentration which is less than the fine blend setpoint concentration; and

the digital processor further includes means for:

(f) generating concentrate control signals to actuate the concentrate flow regulating mechanism and continuously add chemical concentrate to increase the blended chemical concentration if the monitored first instrument reading is less than the coarse blend setpoint concentration; and

(g) repeating functions (a)-(d) and (f), but not (e), until the second instrument reading is within the qualification range setpoint concentrations.

30. The chemical blending system of claim 29 wherein:

the system further includes a recirculation line having an inlet and an outlet in the blending tank;

the system further includes pump means having a suction end and a pressure end in the recirculation line, for recirculating blended chemical from the blending tank through the recirculation line to blend the chemical constituents; and

the concentrate flow regulating mechanism couples the concentrated chemical inlet to the recirculation line between the recirculation line inlet and the suction end of the pump means.

31. The chemical blending system of claim 29 wherein the digital processor further includes concentrate injection information update means for updating the concentrate injection information as a function of the difference between expected changes in blended chemical concentration caused by functions (b) and (d), and actual changes in blended chemical concentration caused by functions (b) and (d).

32. The chemical blending system of claim 29 wherein:

the system further includes a diluent flow regulating mechanism responsive diluent control signals, for controlling the flow of diluent into the blending tank;

the memory further includes data representative of diluent injection information representative of the relationship between the diluent control signals and blended chemical concentration changes induced by actuation of the diluent flow regulating mechanism; and

the digital processor further includes means for:

(h) generating diluent control signals as a function of the diluent injection information and at least one of the first and second instrument readings, to actuate the diluent flow regulating mechanism and add a finite quantity of the diluent in an attempt to decrease the blended chemical concentration to the desired concentration if the monitored instrument reading is greater than the qualification range setpoint concentrations; and

(i) repeating functions (a)-(d), (f) and (h), but not (e) or (g), until the second instrument reading is within the qualification range setpoint concentrations.

33. The chemical blending system of claim 32 wherein the digital processor further includes:

concentrate injection information update means for updating the concentrate injection information as a function of the difference between expected changes in blended chemical concentration caused by functions (b) and (d), and actual changes in blended chemical concentration caused by functions (b) and (d); and

diluent injection information update means for updating the concentrate injection information as a function of the difference between expected changes in blended chemical concentration caused by function (h), and actual changes in blended chemical concentration caused by function (h).

34. The chemical blending system of claim 33 wherein:

the system further includes a recirculation line having an inlet and an outlet in the blending tank;

the system further includes pump means having a suction end and a pressure end in the recirculation line, for recirculating blended chemical from the blending tank through the recirculation line to blend the chemical constituents; and

the concentrate flow regulating mechanism couples the concentrated chemical inlet to the recirculation line between the recirculation line inlet and the suction end of the pump means.

35. The chemical blending system of claim 28 wherein the first concentration measuring instrument includes measurement means for providing a relatively fast measurement response time with respect to the response time operating characteristics of the second concentration measuring instrument.

36. The chemical blending system of claim 35 wherein the first concentration measuring instrument includes measurement means for providing a substantially instantaneous measurement response time.

37. The chemical blending system of claim 28 wherein the first concentration measuring instrument includes a conductivity-type measuring instrument.

38. The chemical blending system of claim 28 wherein the second concentration measuring instrument includes measurement means for providing a relatively high degree of measurement accuracy with respect to the degree of measurement accuracy operating characteristics of the first concentration measuring instrument.

39. The chemical blending system of claim 28 wherein the second concentration measuring instrument includes a titration analyzer measuring instrument.

40. The chemical blending system of claim 28 wherein:

the concentrate flow regulating mechanism includes a concentrate flow regulating valve;

the memory for storing concentrate injection includes a concentrate injection constant representative of chemical concentration changes as a function of the length of time that the valve is actuated; and

the digital processor means for performing function (b) includes means for:

(b.sub.1) determining the difference in concentration between the concentration reading provided by the first concentration measuring instrument and the desired concentration;

(b.sub.2) accessing the memory and determining a concentrate flow regulating valve actuation time as a function of the concentration difference determined by function b.sub.1 and the concentration injection constant; and

(b.sub.3) actuating the concentrate flow regulating valve for the actuation time determined by function b.sub.2.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is a chemical blending system. In particular, the present invention is a computer-controlled system for blending batches of concentrated chemicals from two or more chemical components for subsequent use in semiconductor fabrication facilities.

2. Description of the Related Art

Chemical generation or blending systems are used in a variety of industrial applications to blend two or more components or constituents to a desired concentration. In semiconductor fabrication facilities, for example, concentrated chemicals (which are usually provided by commercial chemical suppliers in solution with water) are mixed or diluted with DI (deionized) water before being sprayed on or otherwise applied to semiconductor wafers. Table 1 below lists a number of chemicals used in semiconductor fabrication facilities, and the concentration (in weight %) in which these chemicals are typically provided by suppliers.

TABLE 1 ______________________________________ Percentage Concentrate Chemical Symbol in Water ______________________________________ Hydrofluoric Acid HF 49% Acetic Acid HAC 99.7% Nitric Acid HNO.sub.3 71% Phosphoric Acid H.sub.3 PO.sub.4 80% Potassium Hydroxide KOH 30% Tetramethyl Ammonium TMAH 25% Hydroxide Hydrochloric Acid HCl 37% HF and Ammonium BOEs -- Fluoride Mixtures Ammonium Hydroxide NH.sub.4 OH 28-30% Sulfuric Acid H.sub.2 SO.sub.4 93-98% ______________________________________

When used in semiconductor fabrication facilities, the concentrated chemicals described above are commonly diluted with DI water (i.e., a diluent) to desired concentrations or assays. Concentrations in these applications are typically described in terms of weight % (weight percent) of concentrated or pure chemical in water. Hydrofluoric Acid (HF), for example, is often diluted with high purity water to concentrations ranging from about 0.5%-5% HF by weight when used for etching and cleaning processes. Tetramethyl Ammonium Hydroxide (TMAH) is often diluted to about 2.38 weight % for use as a positive photoresist developer. Non-aqueous blended chemicals, and blended chemicals with three or more components, can also be generated.

Chemical blending systems blend the chemicals to a desired concentration which is sometimes known as the nominal or qualification concentration. A high degree of accuracy is also required. The range or window of acceptable concentrations surrounding the qualification concentration is known as the qualification range, and can be defined as a weight % error with respect to the qualification concentration, or by upper and lower qualification range concentrations.

A known chemical blending system which is commercially available from FSI International of Chaska, Minn., the assignee of the present invention, is disclosed generally in commonly assigned application Ser. No. 08/355,671, filed Dec. 14, 1994 and entitled "Apparatus For Blending Chemical And Diluent Liquids". This chemical blending system includes a mix tank for the blended chemical, a recirculation line having an inlet and outlet in the mix tank, and a pump in the recirculation line. A source of a first constituent of the blended chemical, such as DI water which is used as a diluent, is coupled to the mix tank through an inlet and supply line. A source of a second constituent of the blended chemical, such as the concentrated chemical to be diluted, is coupled to the recirculation line through an inlet, source line and adding valve. The adding valve is located in the recirculation line on the suction side of the pump (i.e., between the pump and the inlet of the recirculation line) , and is controlled by a microprocessor-based control system. When the pump is operating and the adding valve is open, concentrated chemical is drawn into the recirculation line. Recirculation of the blended chemical through the recirculation line causes the blended chemical and added concentrate to be thoroughly mixed.

Concentration of the blended chemical is monitored by conductivity-type sensors in the recirculation line between the pump and inlet. The sensors are coupled to the control system through analyzers that convert the conductivity readings provided by the sensors to concentration values used by the control system.

The control system initiates a chemical blending cycle by filling the mix tank with a desired quantity of DI water and activating the pump to recirculate the blended chemical within the tank and recirculation line. The concentrate adding valve is then opened to provide a continuous flow of concentrated chemical into the recirculation line. During this continuous injection phase of the blending cycle the concentration of the blended chemical is monitored continuously and compared to a coarse blend setpoint. The coarse blend setpoint can be empirically determined, and represents a concentration which is sufficiently less than the qualification concentration that the continuous addition of concentrated chemical will approach, but not exceed or overshoot, the qualification concentration if the addition of concentrated chemical is stopped when the measured concentration has increased to the coarse blend setpoint. Once the control system determines that the measured blended chemical concentration has reached the coarse blend setpoint, it closes the concentrate adding valve.

The control system then periodically opens and closes the concentrate adding valve during a periodic injection phase. Relatively small quantities of the concentrated chemical are added during the time periods that the valve is open, and the added concentrated chemical is mixed with the blended chemical while the valve is closed. The concentration of the blended chemical is continuously measured and compared to the qualification concentration during this periodic injection phase. To ensure that the concentration measurements are made in homogeneous and thoroughly blended chemical, the duty cycle of the period during which the concentrate adding valve is open is relatively short compared to the duty cycle of the time period during which the valve is closed. Furthermore, to minimize the chances that the concentration will exceed the qualification range, the duty cycle of the time period during which the valve is open is relatively short so as to increase the concentration in relatively small increments. In one embodiment, for example, the open valve duty cycle is about six seconds while the closed valve duty cycle is about twenty-four seconds. When the measured concentration reaches the qualification concentration, the control system qualifies the blended chemical batch and ceases further concentrated chemical addition. The blended chemical can then be pumped to its point of use.

The conductivity-type sensors used in the chemical blending system described above are capable of providing continuous and almost instantaneous measurements of the blended chemical concentration. The accuracy of the measurements provided by the conductivity-type sensors is also good. Nonetheless, blended chemical concentration variations within the range of accuracy that can be provided through the use of conductivity-type sensors can result in semiconductor fabrication process variations. These process variations can detrimentally affect the physical and electrical characteristics of the semiconductor wafers being processed. The problems associated with these process variations will become even more critical as the circuit geometries on the wafers become smaller and the circuit patterns more complex. Chemical blending systems capable of blending chemicals to higher concentration accuracy levels or tolerances are therefore needed to keep pace with other advances in semiconductor fabrication processes.

The use of titration analyzers to measure the concentration of blended chemicals produced by chemical blending systems is also known. Titration analyzers are commercially available from a number of suppliers including Applikon Dependable Instruments of the Netherlands, through its North American distributer Applikon Analyzers, Inc. of Kingwood Tex. When actuated, analyzers of this type draw a sample of the blended chemical. The sample is then titrated with reagents and its pH or pH inflect