GB2372331A

GB2372331A - A mechanism for in situ calibration of pressure sensors in plasma processing systems

Info

Publication number: GB2372331A
Application number: GB0101270A
Authority: GB
Inventors: Glenn Frazer Counsell; Alistair Mark Edwards
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-01-18
Filing date: 2001-01-18
Publication date: 2002-08-21
Also published as: GB0101270D0

Abstract

A mechanism consisting of a method and apparatus (Fig. 1), for <I>in situ</I> calibration of a plasma processing system pressure sensor (8). Measurements from the sensor (8) are compared to those from a closely coupled, calibrated sensor (6), over a continuous pressure range obtained by flowing an appropriate gas into the plasma processing system (Fig.2) from a suitable, <I>in situ</I>, mass flow controller (10). The flow rate is low enough such that the actual pressure difference between the sensors (6, 8) is kept minimal. The calibration accuracy is enhanced by comparing measurements from the sensors at four static pressures obtained by allowing gas to move to and from an additional chamber of known volume (4). The apparatus can be isolated and disconnected from the plasma processing system and is kept at low pressure when not in use by means of an integral vacuum pump (3).

Description

MECHANISM FOR IN SITU CALIBRATION OF PRESSURE SENSORS IN PLASMA PROCESSING SYSTEMS.

Plasma processing systems are used in a variety of manufacturing environments, including those found in the production of semiconductors and the surface treatment of materials. Accurate pressure measurement and the precise control of gas flow rates are necessary to obtain a high quality of output from these systems. Although pressure sensors and gas flow controllers with sufficient range and sensitivity are commonplace, their calibrations are often inadequate and drift with use. Replacement of the sensors and controllers with newly calibrated devices can involve lengthy periods in which the processing system is unavailable for production. It is therefore advantageous to check the calibrations regularly, In situ, to minimise unscheduled device replacement.

Gas flow controllers are often calibrated using a'rate of rise'procedure in which actual flow rates are determined by measuring the corresponding pressure rises in a known volume. This procedure can be used for In situ calibration of gas flow controllers in plasma processing systems provided that the plasma processing system pressure sensor is itself accurately calibrated. Mechanisms for in situ pressure sensor calibration exist but are intrinsically time consuming, require an additional gas supply and are unwieldy as a result of the need for a separate vacuum pump. The invention described here is a mechanism consisting of a method which enables rapid, In situ pressure sensor calibration and an apparatus requiring neither an additional gas supply nor a separate vacuum pump.

Accordingly, this invention calibrates a plasma processing system pressure sensor by comparing measurements from the sensor to those from a closely coupled, calibrated pressure sensor (reference sensor), over a continuous pressure range obtained by flowing an appropriate gas into the plasma processing system from a suitable, in situ mass flow controller (MFC), at a low enough flow rate such that the actual pressure difference between the sensors is kept minimal. The calibration accuracy is enhanced by comparing measurements from the closely coupled sensors at four static pressures, obtained by: stopping the flow of gas into the plasma processing system (first static pressure); reducing this pressure by allowing the gas in the plasma processing system to flow into an evacuated chamber (reference chamber) of known volume (second static pressure); further reducing the pressure by isolating the reference chamber, evacuating the plasma processing system and then allowing the gas in the reference chamber to return into the plasma processing system (third static pressure) ; evacuating the plasma processing system (fourth static pressure).

Ideally, the reference sensor and reference chamber are kept at low pressure when not in use, by means of an integral vacuum pump. This protects both the invention and the plasma processing system from possible contamination and provides a constant pressure (by eliminating the effect of leaks) which can be used to determine any drift In the zero level of the reference sensor.

A preferred implementation of the invention will now be described with reference to the accompanying drawing in which: Fig. 1 shows a schematic of the apparatus.

Fig. 2 shows a schematic of the components of a typical plasma processing system used with the apparatus.

As shown in Fig. 1, the apparatus comprises a calibrated pressure sensor 6 which can be connected to a plasma processing system Fig. 2 by a short pipe 15 and optionally isolated from the pipe by valve 7. The sensor 6 is also connected to a chamber of known volume 4 and optionally isolated from the chamber by valve 5. The volume 1 comprising sensor 6, valve 5, valve 7 and their interconnections is kept as small as possible. Chamber 4 is also connected to a vacuum pump 3 and optionally isolated from the pump by valve 2.

The method uses the apparatus Fig. 1 together with components of a plasma processing system Fig. 2 to calibrate pressure sensor 8 as follows: i. Valves 2,5, 7, 11, 14 are closed. ii. Apparatus Fig. 1 is connected to plasma processing system Fig. 2 by pipe 15. ill. Valves 11, 12,14 are opened.

IV. The pressures measured by sensors 6,8 are continuously recorded. v. Chamber 9, sensor 8 and pipe 15 are evacuated using pump 13. vi. Valve 11 is closed. vii. Valve 7 is opened.

Vlll. Volume I is evacuated using pump 13.

IX. Valve 5 is opened. x. Chamber 4 is evacuated using pump 13.

XI. Valves 7,12 are closed. xu. A period of a few minutes is allowed to elapse. xui. Valves 7,12 are opened. xiv. Valve 5 is closed. xv. Chamber 9, sensor 8, pipe 15 and volume I are evacuated using pump 13.

XVI. Valve 7 is closed jcw. A period of a few minutes is allowed to elapse. xviii. Valves 5,7 are opened.

XIX. Volume I and chamber 4 are evacuated using pump 13. xx. Valve 5 is closed.

XXI. Valve 11 is opened. xii. The pressures measured by sensors 6,8 are allowed to settle until the rate of pressure change in both sensors is approximately constant. xxiii. Valve 12 is closed. xxiv. MFC 10 is set to allow dry nitrogen gas into chamber 9, sensor 8, pipe 15 and volume I at a sufficiently low flow rate such that the pressure difference between sensors 6 and 8 is kept minimal. xxv. Valve 11 is closed once either of the pressures measured by sensors 6,8 has reached a value slightly below the corresponding sensor's full scale deflection. xxvi. MFC 10 is set to zero flow. xxvii. The pressures measured by sensors 6,8 are allowed to settle until the rate of pressure change in both sensors is approximately constant.

xxviii. Valve 5 is opened allowing gas to flow into chamber 4. xxix. The pressures measured by sensors 6,8 are allowed to settle until the rate of pressure change in both sensors is approximately constant. xxx. Valve 7 is closed. xxxi. Valve 12 is opened. xxxii. Chamber 9, sensor 8 and pipe 15 are evacuated using pump 13. xxxiii. Valve 12 is closed. xxxiv. Valve 7 is opened allowing gas to flow back into pipe 15, chamber 9 and sensor 8. xxxv. The pressures measured by sensors 6,8 are allowed to settle until the rate of pressure change in both sensors is approximately constant. xxxvi. Valve 12 is opened. xxxvii. Chamber 9, sensor 8, pipe 15, volume 1 and chamber 4 are evacuated using pump 13. xxxviii. Valves 7,14 are closed. xxxix. A value for the leak rate in the coupled volume 1 and chamber 4 is calculated from the pressures measured by sensor 6 during the period in step xii. xl. A value for the leak rate in the coupled chamber 9, sensor 8 and pipe 15 is

calculated from the pressure measured by sensor 8 during the period in step Xli. xli. A value for the leak rate in volume 1 is calculated from the pressures measured by sensor 6 during the period in step xvii.

xlu. A value for the leak rate in chamber 4 is calculated using the leak rates calculated in steps XXXIX and xlz.

Xllll. A calibration for sensor 8 is calculated using the pressures measured by sensors 6, 8 during the period in step XXIV, the static pressures measured by sensors 6, 8 immediately following steps XXII, xxvu, XXIX and xxxv and the leak rates calculated inxl, xlz andxlu.

Claims

CLAIMS 1. A mechanism for in situ calibration of a plasma processing system pressure sensor in which measurements from the sensor are compared to those from a closely coupled, calibrated sensor (reference sensor), over a continuous pressure range obtained by flowing an appropriate gas into the plasma processing system from a suitable, in situ, mass flow controller, at a low enough flow rate such that the actual pressure difference between the sensors is kept minimal.
2. A mechanism as claimed in claim 1 where measurements from the closely coupled sensors are compared at four static pressures obtained by: stopping the flow of gas into the plasma processing system (first static pressure); reducing this pressure by allowing the gas in the plasma processing system to flow into an evacuated chamber (reference chamber) of known volume (second static pressure); further reducing the pressure by isolating the reference chamber, evacuating the plasma processing system and then allowing the gas in the reference chamber to return into the plasma processing system (third static pressure); evacuating the plasma processing system (fourth static pressure)
3. A mechanism as claimed in claim 1 where the appropriate gas is dry nitrogen.
4. A mechanism as claimed in claim 1 and claim 2 where the reference sensor and reference chamber can be isolated and disconnected from the plasma processing system.
5. A mechanism as claimed in claim 1 and claim 2 where the reference sensor and reference chamber are kept at low pressure when not in use, by means of an integral vacuum pump.
6. A mechanism as claimed in claim 5 where the constant low pressure provided by the integral vacuum pump is used to determine any drift in the zero level of the reference sensor.
7. A mechanism as claimed in claim 1 and claim 2 where any pressure rise measured at either of the closely coupled pressure sensors and not associated with the flow of gas from the mass flow controller is determined by continuous measurement of pressure during periods when the flow of gas is set to zero.
8. A mechanism substantially as described herein with reference to the figures 1 and 2 of the accompanying drawing.