Introduction

The Dense Plasma Focus X-ray Source prototype developed by Science Research Laboratory, Inc., Somerville, Massachusetts, under a contract with a Fortune 100 client (DPF Source), is a photolithography tool for semiconductor manufacturing of the next decade. The tool uses high-voltage high-energy gas discharges to generate X-rays. The tool includes multiple subsystems and has a footprint of approximately 20m². Solidus Integration, Inc. was contracted to develop a control system for the tool. The main requirements for the control system were:

• Control of triggering sequences for X-ray generation;
• Data acquisition from X-ray digitizer and energy calculation;
• Control and monitoring of the state of multiple subsystems;
• Extensive event and data logging for diagnostics purposes.

Control System

For a control system of DPF Source, we chose an industrial computer running Windows NT and LabVIEW development environment. Our choice of LabVIEW was based on two requirements: a need for a fast development environment and a need to quickly implement modifications reflecting changes in the prototype, which was also under development.

The control system consisted of three top-level loops running in parallel:

• A fast, "soft real-time" loop for X-ray triggering and energy calculations;
• A slow loop for "housekeeping" – subsystem control and monitoring;
• An auxiliary loop for support of interaction with the user.

The fast loop had to support generation of a complicated real-time pulse sequence for triggering gas discharges at a rate of up to 150 discharges per second, data acquisition from an X-ray digitizing unit and X-ray statistics calculations. The discharges were started by the user and had to stop upon delivering a desired X-ray dose.

Each discharge involved generating five pulses with various delays and duration and with a real-time precision (10 microseconds). For this purpose, we chose the PC-TIO-10 plug-in board. It was important for us that multiple counters of this board could be programmed individually, but assigned to the same group and started with one command.

The X-ray digitizer is an instrument developed and built by Science Research Laboratory. The instrument detects signals from two X-ray detectors (PIN diodes) and digitizes the signals into 10 bits each, deriving an analog trigger from one of the signals. The digitized signals are stored in the output registers until the arrival of an external reset pulse. For interaction with this instrument, we chose the AT-DIO-32F board which has enough lines for reading the required 20 bits and supports handshaking. The X-ray digitizer did not provide a request pulse upon completion of digitization, so we had to implement "a one-handed handshaking": the request pulse for the DIO board was generated by one of the spare counters of the TIO board after a discharge, and the acknowledge pulse from the DIO board was sent to the digitizer in order to reset its sample-and-hold circuitry.

The slow loop of the control system was responsible for controlling all the auxiliary subsystems (charging, cooling, etc.) and monitoring the overall system health. The specific requirements that we had to deal with were a large number of analog and digital input and output channels – approximately 200 lines. A combination of the PCI-MIO-16E-1 board and an assortment of SCXI modules were used to address the requirements. The SCXI modules provided high channel counts and optical or galvanic isolation of the front end and the MIO board was used for digitization of analog signals and for controlling the multiplexed SCXI modules.

Finally, the auxiliary loop provided all user dialogs. A separate loop was required here, so that waiting for the user response did not suspend the operation of the rest of the program.

User Interface

While under most circumstances it is advisable not to clutter the user interface with too many controls and indicators, in the case of the DPF Source prototype tool is was important that the development engineers have immediate access to all the data collected by the system. The user interface of the control system spanned over two 21-inch monitors (see Fig. 1).

Figure 1. User interface of prototype DPF Source control system

Controls and indicators were grouped by subsystems, green/red LED’s denoted system status, gray/yellow LED’s showed on/off state of subsystems, and charts showed trends of various analog parameters, like temperature and pressure. While the interface is mostly mouse-driven, emergency shutdown could be initiated by pressing the Escape key.

Further Development

At the time of this writing, the assembly of a production system is in its final stage. The control system of the production model has a few distinctions from that of the prototype. In order to improve system robustness in case of computer or control program failure, it was decided to implement control over "housekeeping" functions using a FieldPoint network and a PLC, both of which have watchdog timer and fault detection capabilities and provide isolation of their front ends from the controller. The choice of PLC fell on Allen Bradley SLC-500. The PLC communicates with the PC over Ethernet. Support for LabVIEW is provided by using National Instruments Industrial Automation Servers software. For improved noise immunity, analog control and acquisition functions were performed by a network of FieldPoint modules located in proximity to the signal sources and communicating with the PC over the RS-485 interface (at the time of development, Ethernet FieldPoint network modules were not available).

An additional requirement for the production unit is that it should be capable of working both in a stand-alone configuration and as a slave to another instrument (a mask aligner). The communication between instruments was implemented using TCP/IP over Ethernet.

The user interface of the production tool also had a facelift. The interface is implemented as a hierarchy of screens. The main screen (see Fig. 2) shows only the X-ray exposure information and subsystem status summary. A password-protected utility screen shows more detailed system information and leads to individual subsystem screens.

Figure 2. Main screen of the production DPF Source instrument

Future development plans involve integration of the tool into the client’s semiconductor production environment including production database connectivity.

Acknowledgements

The author would like to express his gratitude to Dr. Dennis Reilly, Dr. Rodney Petr and Mr. Nelson Orozco of Science Research Laboratory for their help in developing the DPF Source control system.