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Vacuum and Instrumentation Training Products and Materials from MKS Instruments, Inc.

MKS Instruments, Inc. is a global leader in the supply of process measurement and control instrumentation and advanced vacuum components. We serve markets that depend on advancements in material science and material processing to improve their products, ranging from sophisticated semiconductors to industrial tool coatings. These processes often involve building up thin layers of materials through the interaction of gases and solids in tightly controlled environments.

We control these environments with products that manage the flow rates of entering and exiting process gases, thus controlling the mixture and pressure within a process chamber. We also offer products that analyze and monitor the composition of these gases and isolate them from the environment outside of the chamber.

Realizing that an understanding of vacuum and related process monitoring and control instrumentation is a key area for individuals who are preparing to become technical workers in the semiconductor industry, MKS has developed an integrated set of instructional hardware and literature for the teaching of vacuum and instrumentation practice. The centerpiece is the MKS Vacuum Training System (model VTS-1B), a tabletop vacuum system which replicates the key features and functions of a full-scale process tool. The following material provides an overview of the VTS-1B, an outline of the documented exercises that may be performed with the system, downloadable student worksheets that were developed in cooperation with Austin Community College, and pointers to further information and resources.

What are some of the vacuum and instrumentation concepts that are important in the semiconductor industry and how can they be taught?

 

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Figure 1 is a simplified schematic representation of a "typical" semiconductor process tool. A semiconductor process tool embodies a wide variety of features that are intertwined with vacuum theory and practice.
The basic MKS Vacuum Training System (Figure 2) incorporates the overall architecture and a variety of the features of a process tool into a student-friendly table-top classroom apparatus. The software that is provided allows a personal computer to be used for set up and for establishing recipes. The software also has graphical displays which enhance the student's ability to understand the various facets of vacuum and vacuum systems.
Add-on components are available that enhance the capabilities of the basic system. The high vacuum version with residual gas analysis capability is shown in Figure 3. Other options can include digital instrumentation and RF compatible chambers and fixturing.

Descriptions of Exercises

The instructor's guide for the VTS-1B system currently contains about thirty exercises. The following provides an outline of these exercises and how these relate to semiconductor process tools.

Vacuum system set up (starting with individual components):

The students are presented with a collection of individual components. They must assemble the components into a functional system. They learn:

  • Identification and physical characteristics of various components, assembly of components using commonly used fittings, basic functions and relationships of the various components, electronic control systems, and entering parameters and menus on the computer.
  • Instrument set up: scaling, zeroing, etc.
  • Initial troubleshooting (base pressure capability and rate-of-rise leak test).

Vacuum Basics

These exercises delve into the basics of vacuum practice.

  • Pumping system operation.
  • Pumpdown time constant: what it is and how the time constant can be used as a diagnostic tool. Also, why control of the time constant is important to maintaining a low contamination environment. This encompasses the use of low conductance bypass lines and explains the adverse effects of adiabatic cooling during pumpdown.
  • Leaks: identifying real and virtual leaks.
  • Locating leaks. Using simple techniques, leaks are intentionally introduced into the system and the students must locate and repair the leaks. Once the concepts of mass flow are understood, students determine actual leak rates as expressed in terms of standard condition volumetric flow.
  • Outgassing and permeation: effects of different materials, identifying these effects by observing the pressure vs time signatures.
  • Throughput (Q) and its relationship to pumping speed or line conductance and pressure. This lays a foundation for understanding pressure control and mass flow control systems.

Gauging

A bewildering variety of gauges are used to monitor and control the system pressure. By comparing gauges, students will understand the differences between indirect (as exemplified by Pirani and ionization gauges) and the precision direct vacuum gauges (as exemplified by the capacitance manometer) that are used to control processes. Students learn about critical issues such as transient response, gas sensitivity, accuracy and repeatability, etc.

  • Dynamic response characteristics: at various pressures and with differing pumpdown rates, a comparison is made between the behaviors of the convection Pirani gauge and the capacitance manometer. Students learn that the operating principles of a gauge have a direct influence on what the gauge is indicating and, under some circumstances, some gauges may be giving a totally wrong impression of what is actually taking place in the chamber.
  • Gauge calibration principles: a Pirani gauge is calibrated against a capacitance manometer of known accuracy using air and other gases.
  • Operation of an ion gauge. This exercise also demonstrates desorption effects and illustrates the bake-out process.

Closed-Loop Pressure Control

All semiconductor processes live or die by the repeatability and stability of their closed loop control systems. All tools have a variety of pressure and flow control systems. These exercises familiarize the students with pressure control systems in a variety of modes. The effects of various tuning parameters (proportional, integral, derivative) are seen in the way that the system responds while acquiring and maintaining specific set point pressures.

  • Manual pressure control: students control the system using manual control valves. This explains the basic function of a closed loop system and demonstrates the importance of the proper sizing of components, in terms of control range.
  • Auto pressure control - downstream. This configuration is commonly found in chemical vapor deposition (CVD) and ashing tools. Students enter various set points and tuning parameters and observe pressure stability and throttle valve operation.
  • Auto pressure control - upstream. This configuration is commonly found in physical vapor deposition (PVD) tools.

Mass Flow Controllers

Mass flow controllers (MFCs) are the devices by which process and purge gases are delivered to the tool. Mass flow is usually measured in units of standard cubic centimeters per minute (sccm). The mass flow is actually an indication of how many molecules per minute of gas are being delivered to the chamber. MFCs represent another closed-loop control system where an input signal is translated into a specific mass flow of gas. A widely misunderstood component, a typical tool may use up to 100 MFCs. In this set of exercises, the students understand the operation and proper use of flow controllers, calibration and verification techniques, and basic troubleshooting.

  • MFC flow verification using the pressure rate-of-rise technique. This is a 2 part exercise that reproduces a typical in situ flow verification sequence. The students learn how the technique is performed, limitations on accuracy and how the technique embodies the ideal gas law.
  • Developing a gas correction factor. MFCs are gas sensitive and correction factors have to be developed for the various gases used in processes.
  • MFC troubleshooting with a breakout connector. This is a non-intrusive technique that can often diagnose problems even before they reach the state of catastrophic failure.

High Vacuum System Structure and Operation

Students add a high vacuum (turbo-drag) pump to the system and learn the proper use of a high vacuum pumping system.

Use of the Residual Gas Analyzer (RGA)

Partial pressure measurement using the RGA serves as an important troubleshooting and diagnostic tool. RGAs may be used as part of maintenance routines, during process development and for process monitoring and control.

The RGA exercises introduce the student to the use of the instrument through a series of structured exercises that cover set up, various vacuum system configurations, and operational modes.

  • Initial set up
  • Using a sampling system with differential pumping. RGAs are high vacuum instruments but the processes that they may monitor operate at much higher pressures. A sampling system is used to provide the required pressure drop from chamber to RGA.
  • Performing a mass scan in analog mode
  • Pressure vs time mode (often used for leak detection or process monitoring)
  • The library function. The software tools that are used to identify the gases that are associated with the various peaks.
  • Establishing a baseline to examine changes from the baseline condition.
  • Outgassing and permeation. Observing and identifying the gases that are released from various materials that may be found in a vacuum system.

Student Worksheets

Austin Community College has developed a number of student lab worksheets for the VTS-1B. These may be downloaded as an Adobe AcrobatTM file.

Download Student Worksheets

Get Acrobat

More Information from MKS:

Link to the MKS Instruments Education page (http://www.mksinst.com/train.html). Information on educational materials and hardware and article reprints that may be of interest to the educator. The data sheet for the VTS-1B Vacuum Training System is also available here.

Link to the MKS Instruments home page (link to http://www.mksinst.com/)

Contact MKS Instruments' Training Dept. (mailto:train@mksinst.com)

 


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