WO2004025701A2

WO2004025701A2 - Load lock optimization method and system

Info

Publication number: WO2004025701A2
Application number: PCT/US2003/028213
Authority: WO
Inventors: Morgan D. Evans
Original assignee: Varian Semiconductor Equipment Associates, Inc.
Priority date: 2002-09-10
Filing date: 2003-09-10
Publication date: 2004-03-25
Also published as: EP1540558A2; AU2003270447A1; JP2005538567A; TW200406049A; AU2003270447A8; WO2004025701A3; KR20050042807A

Abstract

A method and system of optimizing a state change can include isolating a load at a first state, such as a wafer cassette, a wafer photoresist, a prosthetic device, or other device to be implanted, in a load lock of a wafer implanter, and bringing the load from the first state to a lock set point state, such as by pumping down the load lock to vacuum. The load may then be exposed to an environment at a second state, such as in an implant chamber with conditions desirable for implanting, that may be disturbed by the exposing, and the second state may then re-established. The time to re-establish the second state may be optimized as a function of the time to reach the lock set point state. For the implanter, the time to first implant may be optimized as a function of pump down time.

Description

LOAD LOCK OPTIMIZATION METHOD AND SYSTEM

Field

[0001] The methods and systems relate to semiconductor wafer processing, and more particularly to optimizing throughput of wafer processing. Background

[0002] A large variety of materials can be used in the production of wafer cassettes, with particular materials being used to meet various goals including particulate control, static discharge reduction, weight, cost and dimensional stability. While a particular material choice may achieve its goal, it may have negative effects on other aspects of wafer processing. For example, plastic cassettes or photoresist on a wafer can exhibit many structural properties that make their use desirable, however they may have negative effects on the vacuum needed during wafer processing. High and repeatable vacuum may be required in ion implanters during wafer processing to assure correct dose repeatability and uniformity as well as to maintain throughput of the implanter or tool, i.e., the number of wafers that may be processed over a given time. In adopting new cassette or photoresist materials, such as polycarbonate, in order to improve particulate control and/or dimensional stability, end users of such equipment may not realize that the vacuum system may need to be re-optimized to maintain tool performance. [0003] In a typical semiconductor wafer processing system, a wafer cassette may be loaded into a load lock station in preparation for transferring the wafers to a processing, or implant, chamber. A load lock vacuum pump can be used to bring down the pressure in the load lock to a level consistent with the implant chamber vacuum. The load lock may then be opened to the implant chamber and the wafer transferred into the chamber. In some instances, opening the load lock may result in a pressure burst in the chamber, such that a vacuum recovery period may be required to return the pressure within the chamber to levels acceptable for implanting, i.e., levels where the desired process parameter, such as implant dose repeatability, uniformity, energy contamination and/or tool throughput, can be achieved. Thus, the time to first implant, i.e., the time from the start of load lock pump down until implanting may be started can include a load lock pump down time and a process chamber vacuum recovery time.

[0004] Typically, load lock pump down may continue until a set point pressure in the load lock is reached and the load lock is opened to the implant chamber. When a pressure burst occurs, the vacuum recovery period may begin. Vacuum recovery within the chamber may continue until the implant chamber reaches the implant chamber set point pressure. The implant chamber set point pressure may be chosen to provide the desired repeatability and uniformity of the wafers processed. Though the load lock set point pressure and the implant chamber set point pressure may be kept constant, the various materials used in wafer processing can have widely disparate effects on the vacuum recovery period required due to outgassing, adsorption and absorption properties of the materials. Thus, the time to first implant, and consequently the throughput of the wafer processing system, may vary with the materials used, even though the load lock and chamber set point pressures can have been maintained. Summary

[0005] According to the methods and systems described herein, a method of optimizing a state change may comprise isolating a load at a first state, bringing the load from the first state to a lock set point state, exposing the load to an environment at a second state, wherein the second state may be disturbed by the exposing, re-establishing the second state and optimizing the time to re-establish the second state as a function of the time to reach the lock set point state. The second state may be re-established when the environment reaches an operating set point state. [0006] In optimizing the state change, the method and system may determine if a predefined number of data points has been obtained and may increment the time for bringing the load from the first state to the lock set point state when the predefined number of data points has not been obtained, so as to return to obtain additional data points. When the predefined number of data points has been obtained, the method and system may determine the optimum of the state change. In one embodiment, the optimum of the state change may be a minimum of a combination of the time for bringing the load from the first state to the second state and the time for reaching an operating set point state. The method and system may include determining if the minimum is finalized, returning to incrementing, isolating, bringing, exposing and re-establishing when the minimum is not finalized and ending when the minimum is finalized. [0007] In one embodiment, the method may be used with a wafer implanter where a wafer cassette of wafers may be inserted and sealed into a load lock of the wafer implanter. The load lock may be pumped down from an ambient pressure to a vacuum pressure and may then be opened to the implant chamber of the wafer implanter. For the implanter, re- establishing may include returning to an implant ready state and optimizing may include incrementing the time for pumping down the load lock and repeating the method to obtain additional data points for determining a minimum of a combination of the time for pumping down and the time for returning to an implant ready state. [0008] A system for implementing the method may include a system having means for determining pump down times for a load lock of the implanter with a load to reach incremental set points, means for determining a time to first implant for each respective set point based on the pump down time determined for the respective set point and a recovery time for an implant chamber to return to an implant ready state after the load lock is opened to the implant chamber upon reaching the respective set point and means for minimizing the time to first implant as a function of the pump down time. A computer-readable medium may contain instructions for controlling a processing system to implement the system. Brief Description Of The Drawings [0009] The following figures depict certain illustrative embodiments of the systems and methods in which like reference numerals refer to like elements. These depicted embodiments are to be understood as illustrative and not as limiting in any way. [0010] Fig. 1 may depict a flow chart for a method of optimizing state change process; [0011] Fig. 2 may depict a schematic representation of an implanter that may implement the method of Fig. 1; and [0012] Fig. 3 may depict a plot of results of the method of Fig. 1. Detailed Description Of The Preferred Embodiment

[0013] Referring now to Fig. 1, there can be shown a flow chart for a method 100, as shown in Fig. 1, that may optimize a process wherein a load in a first state is required to be brought to a second state in order to operate on the load. An exemplary process may include implanting a wafer, where the wafer may originally be at ambient pressure, the first state, and may need to be brought to a high vacuum, the second state, for implanting, as previously described. The implementation of the method 100 may be subsequently described herein in terms of the exemplary implanting process. However, it can be understood that the method 100 may find application in optimizing various processes having the generalized features that may be exemplified by the implanting process, including, but not limited to, other semiconductor wafer processing processes and other ion implantation processes, such as may be used for implanting prosthetics and other devices that may require a high level of surface hardness. [0014] For the exemplary implanting process, the method 100 may optimize the time to first implant of an ion implanter and thereby improve throughput of the implanter. Fig. 2 may show a schematic representation of portions of an ion implanter 10. The method 100 may begin by inserting, at 102, a load 12 into an isolation lock that may be used to isolate the load 12 while the load 12 may be brought from a first state to a second state at 104. For the exemplary implanter 10, the load 12 may be a wafer, or a cassette of wafers, 12 that may be inserted into a load lock 14 of the implanter 10 through an insertion opening 16. The load lock 14 may be at a first pressure during the insertion, referred to herein as ambient pressure. The wafer or wafer cassette 12 may be isolated, or sealed within load lock 14, by closing opening 16 and pump down of the load lock 14 can begin at 104 and can continue until the set point for the load lock 14 may be reached, as determined at 106. [0015] For the implanter 10 in this first iteration of method 100, the set point for the load lock 14 may be the set point pressure for the load lock, as previously described. This set point pressure may be a predetermined set point pressure for the implanter 10 and, as may be standard for such implanters, pressure measurements may be taken within the load lock 14 to determine, at 106, when the set point pressure may be reached. In general, a lock set point may correspond with the second state and a sensor in the lock may monitor the state change in the lock. When the lock set point may be reached, as determined at 106, the time required to reach the lock set point may be recorded at 108 and may be referred to as "set point time". When the lock set point may be reached, the isolation lock, or load lock 14, can be opened, at 110, to expose the load to the second state environment, i.e., opening the load lock 14 to the implant chamber 18 for the implanter example of Fig. 2, as by opening isolation valve 20 and slot 22. [0016] As discussed previously, opening the load lock 14 to the implant chamber 18 may cause a pressure burst at the implant chamber 18. Method 100 may account for such abrupt changes in the second state environment by including a state recovery period, at 112, during which the second state may be re-established, as determined at 114. The second state may be re-established when the environment may attain a predetermined operation set point. For the implanter 10, the pressure burst may raise the pressure within implant chamber 18 and re-establishing the second state at 112 may require reducing the pressure within implant chamber 18 until a chamber set point pressure can be attained. The chamber set point pressure may be a pressure at which implants may provide satisfactory repeatability and uniformity, as discussed previously. The time at which the second state is re-established may be recorded at 116 as the time to first operating condition, i.e., the time taken to reach a condition within the chamber 18 when an implant may begin. The isolation lock, or load lock 14, may then be closed, or isolated from the second environment, or implant chamber 18, at 118, and allowed to return to the first state, or ambient pressure for the implanter 10. [0017] The time to first operating condition may be plotted as a function of the set point time. Fig. 3 may show such a plot, with the results of an exemplary first iteration of method 100 for the implanter 10, as recorded at 108 and 116, plotted and labeled "Al". As may be known, optimization, as at 120, may require obtaining a sufficient number of data points. In the general case, monitoring the state change within the isolation lock may not provide the optimum results, as noted previously for the implanter 10 example. Thus, for method 100, the set point time may be incrementally changed, as at 122, and, using the set point time as the lock set point, method 100 may return to 102 to obtain new data points, which may be recorded at 108 and 116. Method 100 may increment the set point time and obtain additional data points until sufficient data points, as determined at 124, may be provided to perform the optimization and obtain the optimum point, as at 126. Thus, using the method 100 for the implanter 10, the time to first implant may be optimized as a function of the pump down time, or, conversely, the time to first implant may be minimized by optimizing the load lock pump down time. [0018] It can be understood that numerous optimization methods may be well-known in the art and may include manual curve fitting, curve fitting algorithms and other numerical analysis methods. The optimization method chosen may define the number of data points needed for the optimization. As an example, optimization may begin with the first set of data points, i.e., the predefined number of data points may be one, and a new optimum set point time can be determined after each incrementation. The optimization method chosen may also determine the sequence of incrementing the set point time and the number of data points required. The plot for the illustrated example of Fig. 3 may be based on the use of an E5-767 implanter with a turbomolecular pump (as provided by Varian Semiconductor Equipment Associates of Gloucester, Massachusetts) and a polycarbonate cassette of wafers. To illustrate the method, six data points, A1-A6, may be plotted and the optimization performed at 124 may include manually fitting a curve 30 to the data points A 1 -A6. It can be seen from curve 30 that a load lock pump down time of approximately sixty seconds may provide the shortest time to first implant of approximately 250 seconds. As may be understood by those of skill in the art, the curve 30 may depend on the specific equipment and materials used, as well as the optimization method chosen. [0019] When compared to using the standard load lock set point pressure, as indicated by Al, the time to first implant may be decreased by more than a factor of two. It can also be noted that increasing load lock pump down time may also increase uniformity and dose repeatability, as the outgassing of the cassette material, the wafers, and/or the photoresist on the wafers, may be lessened with increased load lock pump down. Thus, curve 30 may indicate that, for load lock pump down times greater than sixty seconds, further increases in uniformity and dose repeatability may be attained at the expense of increasing time to first implant.

[0020] While the methods and systems have been disclosed in connection with the preferred embodiments shown and described in detail, various modifications and improvements thereon will become readily apparent to those skilled in the art. As an example, the choice of optimization method may include the use of a processing system, such as implant controller 24, programmed to perform the optimization, i.e., programmed to control the incrementation of the set point and the determination of the number of data points to obtain. The controller 24 may also include timing means 24a and memory 24b to mark and record the set point time and the time to first operating condition or time to first implant, as described at 108 and 116 of method 100. [0021] The method 100 may be separate from or may be part of a production process. For example, the method 100 may be implemented on test specimens of the load, prior to actual production processing of loads. In one embodiment, the processing system can maintain optimization data for various test specimens and the production process can be set to operate at the optimized set point for the test specimen matching the production load. In one embodiment, method 100 can determine, at 128, if the optimization can be finalized for the test specimens. When method 100 determines that the optimization may be finalized, method 100 can exit with the optimized set point for the test specimens. [0022] Alternatively, method 100 may be implemented at the start of production processing, with the first loads serving to establish the optimization for the remainder of the loads to be processed. Implementation of method 100 may also be continued throughout production processing, with the optimization being updated with each load, or set of loads, processed. In one embodiment, optimization may not be finalized and 128 can return to 122 to increment the lock set point time and return to 102 for production processing of another load. The increment can be such that the lock set point may correspond with the optimum determined at 126.

[0023] As noted previously, the ion implanter 10 may illustrate but one example of the implementation of method 100. It can be understood that many different processes that may incorporate a state change may benefit from the use of method 100. Additional examples of state changes can include gaseous state changes, e.g., from an air environment to a single gas environment, liquid to gas or gas to liquid state changes, temperature changes, as well as pressure changes, as shown in the ion implanter example, and other state changes, or combinations thereof as may be known. For the ion implanter 10, pump 26 may perform a rough evacuation, or pump down of the load lock 14. A turbomolecular pump 28 may perform the final pump down to the set point pressure. Thus, the arrangement of the components shown in the figures may be merely for illustrative purposes and can be varied to suit the particular implementation of interest. Accordingly, items may be combined, expanded, or otherwise reconfigured without departing from the scope of the disclosed methods.

[0024] The implementation of the methods and the control of the systems as described herein in relation to the processing system, or controller 24, may not be limited to particular hardware or software configuration, and may find applicability in many processing environments used for the control of production processes. The methods can be implemented in hardware or software, or a combination of hardware and software. The methods can be implemented in one or more computer programs executing on one or more programmable computers that include a processor, such as controller 24, a storage medium readable by the processor, such as memory 24b, one or more input devices, and one or more output devices. In some embodiments, such as that of the ion implanter 10, a processing system or controller 24 may be part of the system where method 100 may be used. In other embodiments, the methods may be implemented on a computer in a network. User control for the systems and methods may be provided through known user interfaces. [0025] The computer program, or programs, may be preferably implemented using one or more high level procedural or object-oriented programming languages to communicate with a computer system; however, the programs can be implemented in assembly or machine language, if desired. The language can be compiled or interpreted. [0026] The computer programs can be preferably stored on a storage medium or device (e.g., CD-ROM, hard disk, or magnetic disk) readable by a general or special purpose programmable computer for configuring and operating the computer when the storage medium or device may be read by the computer to perform the procedures described herein. The method and system can also be considered to be implemented as a computer-readable storage medium, configured with a computer program, where the storage medium so configured may cause a computer to operate in a specific and predefined manner.

[0027] The aforementioned changes may also be merely illustrative and not exhaustive, and other changes can be implemented. Accordingly, many additional changes in the details and arrangement of parts, herein described and illustrated, can be made by those skilled in the art. It will thus be understood that the following claims may not to be limited to the embodiments disclosed herein. The claims can include practices otherwise than specifically described and are to be interpreted as broadly as allowed under the law.

Claims

What is claimed is:

1. A method of optimizing a state change, comprising: isolating a load at a first state; bringing the load from the first state to a lock set point state; exposing the load to an environment at a second state, the second state being disturbed by the exposing; re-establishing the second state; optimizing a time to re-establish the second state as a function of a time for bringing the load from the first state to the lock set point state.

2. The method of claim 1, wherein re-establishing the second state comprises determining when the environment reaches an operating set point state.

3. The method of claim 1, wherein optimizing comprises: determining if a predefined number of data points of time to re-establish the second state and time to bring the load from the first state to the lock set point state has been obtained; incrementing the time to bring the load from the first state to the lock set point state when the predefined number of data points has not been obtained; returning to isolating, bringing, exposing and re-establishing to obtain additional data points after incrementing; and determining an optimum of the state change when the predefined number of data points has been obtained.

4. The method of claim 3, wherein determining the optimum of the state change comprises: determining if the optimum of the state change is finalized; returning to incrementing, isolating, bringing, exposing and re-establishing when the optimum of the state change is not finalized; and exiting when the optimum of the state change is finalized.

5. The method of claim 1 wherein isolating comprises: inserting the load into a load lock of an ion implanter; and sealing the load in the load lock.

6. The method of claim 5, wherein bringing comprises pumping down the load lock from an ambient pressure to a vacuum pressure set point.

7. The method of claim 6, wherein: exposing comprises opening the load lock to an implant chamber of the ion implanter; and re-establishing comprises returning to an implant ready state.

8. The method of claim 7, wherein optimizing comprises: determining if a predefined number of data points of time to return to an implant ready state and time to pump down the load lock has been obtained; incrementing the time to pump down the load lock when the predefined number of data points has not been obtained; returning to isolating, bringing, exposing and re-establishing to obtain additional data points after incrementing; and determining a minimum of a combination of the time to pump down and the time to return to an implant ready state when the predefined number of data points has been obtained.

9. The method of claim 8, wherein determining a minimum comprises: determining if the minimum is finalized; returning to incrementing, isolating, bringing, exposing and re-establishing when the minimum is not finalized; and exiting when the minimum is finalized.

10. A method of optimizing throughput of an ion implanter, comprising: determining pump down times for a load lock with a load to be implanted to reach incremented set points; determining a time to first implant the load for each respective set point of the incremented set points based on the pump down time determined for the respective set point and a recovery time for an implant chamber to return to an implant ready state after the load lock is opened to the implant chamber upon reaching the respective set point; and determining a minimum for the time to first implant the load as a function of the pump down time.

11. The method of claim 10, comprising incrementing the set point until a predetermined number of incremented set points has been reached.

12. The method of claim 11, wherein the incrementing comprises incrementing the set point to correspond with the minimum.

13. The method of claim 10, comprising incrementing the set point to correspond with the minimum.

14. A system for optimizing throughput of a ion implanter, comprising: means for determining pump down times for a load lock with a load to be implanted to reach incremented set points; means for determining a time to first implant the load for each respective set point of the incremented set points based on the pump down time determined for the respective set point and a recovery time for an implant chamber to return to an implant ready state after the load lock is opened to the implant chamber upon reaching the respective set point; and means for determining a minimum for the time to first implant the load as a function of the pump down time.

15. The system of claim 14, comprising means for incrementing the set point until a predetermined number of incremented set points has been reached.

16. The system of claim 15, wherein the means for incrementing comprises means for incrementing the set point to correspond with the minimum.

17. The system of claim 14, comprising means for incrementing the set point to correspond with the minimum.

18. A system for optimizing throughput of an ion implanter, comprising: a load lock for isolating a load to be implanted; a pump to pump down the load lock to a set point; an implant chamber to accept the load for implanting when the load lock is at the set point; and a controller determining a time to first implant the load for respective incremented set points based on a pump down time determined for each of the respective incremented set points and a recovery time for the implant chamber to return to an implant ready state after the load lock is opened to the implant chamber upon reaching the respective set point and determining a minimum for the time to first implant the load as a function of the pump down time.

19. A computer-readable medium containing instructions for controlling a processing system to optimize throughput of a ion implanter by: controlling the processing system to determine pump down times for a load lock with a load to be implanted to reach incremented set points; controlling the processing system to determine a time to first implant the load for each respective set point of the incremented set points based on the pump down time determined for the respective set point and a recovery time for an implant chamber to return to an implant ready state after the load lock is opened to the implant chamber upon reaching the respective set point; and controlling the processing system to determine a minimum for the time to first implant the load as a function of the pump down time.

20. The computer-readable medium of claim 19, comprising controlling the processing system to increment the set point until a predetermined number of incremented set points has been reached.

21. The computer-readable medium of claim 20, wherein the controlling the processing system to increment comprises controlling the processing system to increment the set point to correspond with the minimum.

22. The computer-readable medium of claim 19, comprising controlling the processing system to increment the set point to correspond with the minimum.