Patent Grant
8423311
The invention relates to a method for calibrating and operating a vacuum measuring cell arrangement, as well as to a method for operating a calibrated measuring cell arrangement.
At least three methods are known and feasible for the calibration of measuring cells. While within the metrology of a national standards institute, as for example the Physical-Technical Federal Institute (PTB, Germany), the methods with static and dynamic expansion are conventional, in the industrial manufacturing of measuring cells the approach of comparison with a transfer standard is preferably applied. The reasons are primarily the complexity of construction systems for the expansion methods, and particularly the time necessary to carry out a calibration using these methods. However the calibration method of the comparison with a transfer standard also requires taking into consideration some ancillary conditions, such as are stipulated in the standards DIN 28418 and DKD-R 6-2 or ISO/TS 3567:2005. The calibration of vacuum measuring cells is described for example in the literature citation Wutz-Adam-Walcher (Verlag Vieweg, ISBN 3-528-04884-0) in Chapter 11.8 Calibration of Vacuum Gauges.
It is known to measure pressures or pressure differences by pressurizing a thin diaphragm and measuring its deflection. A known and suitable method for measuring the deflection of such diaphragms comprises implementing the diaphragm arrangement as a variable electric capacitor, wherein, via an electronic measuring circuitry, the capacitance change is analyzed in known manner, which change correlates with the pressure change. The capacitor is formed by disposing the thin flexible diaphragm surface at a minimal spacing opposite a further surface and coating both opposing surfaces with an electrically conducting coating or developing them of an electrically conductive material. Upon pressure being applied to the diaphragm, the spacing between the two electrodes changes through the deflection leading to an analyzable capacitance change of the arrangement. Sensors of this type are produced of silicon in large piece numbers. The areal base body as well as also the diaphragm are herein often entirely comprised of silicon material. There are also designs with combined material composition, for example silicon with a glass substrate. The sensors can thereby be produced cost-effectively. As a rule, pressure sensors of this type are only applicable for higher pressure ranges in the range of approximately 10−1 mbar to a few bar. High resolution at lower pressures starting at approximately 10−1 mbar are no longer realizable utilizing silicon as the material. Sensors of this type are only conditionally suitable for typical vacuum applications. One of the reasons is that silicon reacts on its surface with the environment and in this way the sensitive sensor characteristic is disturbed. Water vapor contained in normal atmospheric air already leads to corresponding reactions on the surfaces. The problem is additionally exacerbated if the sensor is employed in chemically aggressive atmospheres, which is increasingly common in current reactive vacuum plasma processes.
One important application field in general are processes in the semiconductor industry. Here, semiconductors are produced utilizing, for example, the following techniques: chemical vapor deposition (CVD), physical vapor deposition (PVD), implanting and (dry) etching processes. Typical pressure ranges for processes in the semiconductor industry and pressure ranges of vacuum measuring cells typically operate in the range of 10−4 to 10 mbar. Typical process measuring cells for the applications are capacitive diaphragm measuring cells. In such processes, such as for example in vacuum etching methods, in particular especially aggressive media, such as fluorine, bromic acid and their compounds are employed. Due to such corrosion and resistance problems, the known silicon pressure sensors and diaphragm measuring cells with metallic diaphragms can only be employed to a limited extent.
For such applications there is increasing demand for being able to operate the diaphragm measuring cell at increased temperatures in order to be able to operate the measuring cell, for one, in a hot process environment and/or to avoid as much as feasible condensates in the measuring cell and to do this at high corrosion resistance.
There is expectation that the market demand for high-temperature diaphragm measuring cells will increase over the next years, for example due to the introduction of atomic layer deposition (ALD) in semiconductor production processes, which require pressure measurements at temperatures up to 300° C. or higher in certain applications. The apparatus structure for ALD processes is very similar to that of low pressure CVD (LPCVD) or CVD apparatus, which today are the most significant purchasers of measuring cells operated at increased temperatures.
A diaphragm measuring cell preferred for these applications is the capacitive diaphragm measuring cell (CDG). A capacitive diaphragm measuring cell, also referred to as capacitance diaphragm gauge (CDG), is based on the elastic deformation of a thin diaphragm, which is suspended over a solid, areal body and thus separates two volumes from one another. A pressure change in these volumes induces the diaphragm to move. The distance between the housing and the diaphragm changes. At high pressures the diaphragm is deflected more strongly than at low pressures. Metallic electrodes are disposed in the gap region on the diaphragm and on the base body which is located opposite the diaphragm. These two metal electrodes form a condenser capacitance. The capacitance change is consequently a measure of the pressure change. This measuring principle is independent of the type of gas.
It has therefore been proposed to produce measuring cells for vacuum pressure measurements of corrosion-resistant materials such as Al2O3. U.S. Pat. No. 6,591,687 B1 describes a capacitive vacuum measuring cell (CDG) which is substantially structured entirely of ceramic and thus is highly corrosion resistant. The content of this patent is herewith declared to be an integrated component of the present invention description. In order to measure, for example, very low pressures to 10−6 mbar with high accuracy, a very thin ceramic diaphragm of 25 μm to 100 μm thickness is preferably utilized, which is disposed substantially symmetrically in a ceramic housing. For the application at higher vacuum pressures up into the range of several 100 mbar, diaphragm thicknesses up to 950 μm are, for example, preferred. This diaphragm-based vacuum measuring cell is commercially highly successful and indicates a significant advance with respect to corrosion resistance.
A further preferred diaphragm measuring cell arrangement is based on the above described measuring cell of Al2O3 and utilizes a similar structure, wherein the degree of deflection of the diaphragm in this case takes place with the aid of optical means. In an optical diaphragm measuring cell, also referred to as optical diaphragm gauge (ODG), the pressure-dependent deflection of the diaphragm is measured in the sensor with the aid of an optical system, wherein the measured signal is conducted using fiber optics to the optical signal processing unit, which subsequently converts the optical signal into an electrical signal. The coupling-in of the light necessary for this purpose takes place directly onto the diaphragm via appropriately light-permeable regions on the housing of the sensor. From here the light is reflected back. The arrangement forms part of an interferometric Fabry-Perot system. In the associated interferometer through the signal analysis the degree of diaphragm deflection is measured, which is the measure of the obtaining vacuum pressure to be measured. The optical windows are advantageously produced of sapphire such that at least portions of the housing of the diaphragm vacuum measuring cell comprise sapphire. It is also advantageous if the diaphragm itself is comprised of sapphire. The optical signal can be conducted, for example, over large distances (even kilometers), with very low attenuation and without falsification through ambient disturbances, such as primarily electromagnetic interferences, vibrations and changes of ambient temperatures. Such a measuring cell can also be operated especially well as a heated measuring cell. A preferred disposition of an optical vacuum measuring cell has been described in the US application 2007 0089524 A1. The content of this patent application is herewith declared to be an integrated component of the present invention description.
A further improvement of the service life of such diaphragm measuring cells comprises that the connection regions between diaphragm and housing, as well as of the connection region for the connection fitting, and optionally the connection fitting itself, even when employed in aggressive process environments containing, for example, acids, halogens such as chlorine and fluorine, are covered and protected additionally with a thin corrosion-resistant layer. The deposition of such a protective layer, preferably of a metal oxide, is advantageously carried out with the aid of an ALD method, as is proposed in the patent application CH 01817/06. The content of this patent application is herewith declared to be an integrated component of the present invention description.
As already stated, in processes with aggressive gases, under especially high requirements made of measuring accuracy and long-term stability, heated measuring cells are preferably employed. Condensate depositions, for example, can thereby be decreased or avoided in regions within the measuring cell exposed to the process environment. Through the precise stabilization of the measuring cell temperature, instabilities through temperature effects can also be compensated. For this purpose correspondingly high complexities and costs are expended. For example, heating jackets, such as foil heating elements or heating tapes, are placed about the measuring cell which, in turn, are insulated in complex manner. The requisite electronic measuring circuitry, in turn, must be protected against these temperatures, for example by disposition at a spacing and through additional cooling measures, such as using ventilators and cooling bodies. Often additional heating elements, such as heating tapes, are utilized for heating the tubular inlets to the measuring cell. The temperatures are set to fixedly graduated values, such as for example 45° C., 100° C., 160° C. and 200° C., depending on the employment range for the processes to be measured. An arrangement especially suitable for heated diaphragm vacuum measuring cells with a heating system is described in the Swiss application CH 00985/07 by the same applicant. Herein a diaphragm vacuum measuring cell is disposed within a thermal container, which forms a heating configuration and thereby heats the measuring cell to the desired temperature, wherein the measuring cell connection for the vacuum pressure measurement is carried through the thermal container and in this region the thermal container is implemented as a thermal body in which a heat source is disposed. The thermal container is encompassed by an insulation jacket in order to insulate the heated thermal body against the environment and thereby to ensure the lowest feasible temperature gradients in the thermal container at low heat losses. Hereby a homogeneous temperature distribution on the measuring cell is made feasible at compact construction. The content of this patent application is herewith declared to be an integrated component of the present invention description.
Diaphragm measuring cells of aforementioned type supply a very small electric output signal which, accordingly, must be processed carefully. These measuring cells are also very sensitive, in particular to temperature changes. In the production, deviations occur with each measuring cell and especially so during operation at different increased temperature values. The more precisely the pressure measurements must be carried out, the more weight they carry and must therefore be taken into consideration correspondingly.
The measuring cells are therefore calibrated during their production and specially each in the operating points for which specified working application the measuring cell is intended. Calibration denotes the measurement of a test sample against a measurement standard or against a reference. Herein only the state is detected; the test sample is not, however, set to a standard state.
This setting is carried out in a separate step and is referred to as gauging. This gauging process means setting the test sample to a standard state. This action should subsequently preferably be followed by a further calibration. It is consequently important to differentiate between the two processes ‘calibration’ and ‘gauging’.
According to the above definition, the following steps are conventionally performed in a calibration means: measuring the test sample against a standard, storing the data, calculating the compensation values, gauging the test sample, calibrating the test sample with the simultaneous preparation of the calibration report.
Each measuring cell must in particular also be calibrated to the appropriate intended measuring cell temperature. The calibration process, as stated, comprises a comparison measurement of the measuring cell to be tested against a standard, thus against a reference measuring cell. Herein the state is detected and the deviations for the desired measuring range is recorded. The determined deviations can subsequently be utilized to correct or to gauge accordingly the measuring signals generated by the measuring cell.
Such measuring cells can thus always only be operated at this precisely defined temperature for which it has been gauged. For each value of a desired measuring cell temperature in each instance a measuring cell specifically calibrated to it must be employed by the operator at the vacuum process unit. Depending on the applied process and the requirements, if another working temperature of the measuring cell is required, another measuring cell must be employed in each case which has specifically been calibrated for this purpose. This increases the complexity considerably and the operating temperature of the measuring cell cannot be simply changed by the operator on site.
The present invention consequently addresses the problem of eliminating the disadvantages of prior art. The present invention addresses in particular the problem of realizing a method for operating and for calibrating a compact diaphragm measuring cell arrangement with an integrated heating means and electronic measuring circuitry, which can be stably operated over a wider temperature range and at various temperature values, wherein the same measuring cell can be employed by the operator for various selected temperature values.
The problem is resolved in the introduced methods according to the invention. The dependent patent claims relate to advantageous further method steps of the invention.
According to the invention a measuring cell arrangement with a heatable diaphragm vacuum measuring cell is calibrated after its production by being measured against a reference measuring cell at least at one, preferably at least at two, pressure point(s) and at least two temperature values of the working range, wherein from the detected deviations compensation values are determined, which are saved in a calibration data memory in the measuring cell arrangement and from here can be called up upon demand for the gauging of the diaphragm vacuum measuring cell for the desired working ranges.
The stored compensation values can be called up during measuring operation by the operator from the calibration data memory on the measuring cell arrangement according to the desired employment range in order to gauge the measuring cell arrangement on site to optimal values and to do so at various desired employment temperatures. With a single measuring cell arrangement, consequently, diverse applications at various desired employment temperatures can thus be covered. It is consequently no longer necessary to utilize for each selected working point or employment range a separate measuring cell arrangement set-up for it in the producer factory.
This makes feasible realizing not only high measuring precision at good reproducibility but also high flexibility and cost savings of the operator. This also makes feasible for the producer to cover additionally a wide application range using only a single measuring cell arrangement whereby the transaction of orders and the warehousing becomes significantly simplified. Potential delivery problems are thereby also significantly decreased.
For carrying out the method according to the invention a measuring cell arrangement is advantageously utilized in which a diaphragm vacuum measuring cell is disposed within a thermal container which forms a heating configuration. The measuring cell is thereby heated to the desired temperature, wherein the measuring cell connection for the vacuum pressure measurement is carried through the thermal container, and the thermal container is implemented in this region as a thermal body in which a heating source is located. The thermal container is encompassed by an insulation jacket in order to insulate the heated thermal body against the environment and to ensure thereby the lowest feasible temperature gradients in the thermal container at low heat loss. Hereby a homogeneous temperature distribution on the measuring cell is made feasible at compact construction.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure and are entirely based on the Swiss priority application no. 1173/07 filed Jul. 23, 2007 and International Patent Application PCT/CH2008/000256 filed Jun. 9, 2008, which is incorporated here by reference.
In the following the invention will be explained schematically and by example in conjunction with Figures.
In the drawing depict:
For the method according to the present invention for the calibration of a measuring cell arrangement 1, a measuring cell arrangement 1 is especially suitable which includes as the pressure sensor a diaphragm vacuum measuring cell 2 of the type such as has been described previously. A configuration for calibrating such a measuring cell is shown schematically in
The measuring cell arrangement 1 is connected to the outside with a signal line 20 across the measuring cell interface 8. With this signal line 20, on the one hand, the measured pressure signal can be output and utilized and, on the other hand, data can also be read into the measuring cell arrangement 1 in order to control and/or program this arrangement with the aid of the electronic measuring cell circuitry 4 according to the desired requirements and, for example, to calibrate it.
The calibration means 10 comprises a calibration device 12 and an electronic calibration circuitry or procedure control 11. The calibration device includes a vacuum volume 58 to which the measuring cell arrangement 1 is connected across its measuring connector 5. The vacuum volume 58 is also connected to a reference measuring cell whose signals can be compared with those of the measuring cell arrangement 1 to be calibrated. With a heating configuration 63, preferably a heating chamber, the one or several measuring cell arrangement(s) 1 can be brought to a desired presettable constant temperature in order to generate defined conditions at various temperature working points. The calibration process proceeds within a presettable period of time 65. The calibration procedure control 11 includes a calibration data memory 13, a calibration control 14, which comprises, for example, a processor, and a calibration interface 15 to which the measuring cell arrangement 1 is connected with the signal line 20.
A preferred procedure for the calibration process comprises the following steps:
The aforementioned implementation of a measuring cell arrangement 1 with its electronic measuring cell circuitry 4 and aforementioned structure with diaphragm measuring cell 2 and heating means 3 makes feasible, together with the method steps according to the invention, storing a set of compensation values which had been acquired during the calibration process, within the measuring cell arrangement 1 such that these, on demand for different desired applications with different working points, can be called up simply and thereby the measuring cell arrangement 1 is always set to optimal accuracy. The highly sensitive system of the diaphragm measuring cell technology can thereby be optimally utilized and at high economy very high, reproducible measuring accuracy over large working ranges can be attained. In addition, with the aid of the provided electronic measuring cell circuitry 4, interpolation methods can be employed between the acquired measuring points in order to increase further the accuracy and/or to expand the measuring range.
A calibration means 10 especially suitable for carrying out the method is schematically shown in
Before the calibration process, the vacuum volume 58 of the calibration means 10 should preferably be pumped down to a base pressure which is 5 orders of magnitude below the upper measuring range end of the measuring cell arrangement 1 to be calibrated. The measuring cell arrangement 1 should preferably cover a vacuum pressure measuring range of at least 2 to 4 orders of magnitude. It is advantageous if at least two different preset vacuum pressures of step c) are applied at the scale end value and at the scale beginning value of the measuring cell arrangement 1. Acquiring the measured values at more than two pressure points increases the accuracy of the measuring cell arrangement 1, 61 to be calibrated after the gauging step, wherein therewith the calibration complexity also becomes greater. Favorable conditions are attained if when performing step c) at least two to five, preferably five to ten, preset pressure values are acquired, and these are applied within the desired measuring range of the measuring cell arrangement 1 to be acquired.
It is often advantageous to carry out at least one further calibration step f) for at least one further pressure, analogously to the first step c), wherein this step serves as a control step and that potential determined value deviations between the values of the measuring cell arrangement 1 and the reference measuring cell 60 for the preceding measurement are retained in a memory for the further analysis.
The method makes feasible carrying out steps a) to e), preferably a) to f), for at least one further value of the heating temperature, preferably for three to six different heating temperature values. Hereby the same measuring cell arrangement 1 can be employed with high accuracy by the operator even at different measuring cell temperatures by simply calling up the stored calibration data sets with subsequent automatic gauging.
At the operator end the measuring cell arrangement 1 is utilized for vacuum pressure measurements in particular on process units for monitoring the vacuum at the corresponding vacuum chambers or vacuum generation systems. At a vacuum process unit often various process ranges must be monitored and several measuring cell arrangements 1 are often utilized.
In such cases advantageously one vacuum control unit 21 is employed which is connected with one or several measuring cell arrangements 1 such that they communicate for the purpose of data exchange, as is shown schematically in
A further application case of a measuring cell arrangement 1 comprises utilizing this arrangement without external controls 21, 22 and calling up directly at the measuring cell arrangement 1 via a control switch 23 the desired compensation data for undertaking the gauging of the measuring cell as is demonstrated schematically in
A preferred procedure for operating a measuring cell arrangement 1 for pressure measurements by the operator comprises the following steps:
| Number | Date | Country | Kind |
|---|---|---|---|
| 1173/07 | Jul 2007 | CH | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CH2008/000256 | 6/9/2008 | WO | 00 | 1/19/2010 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2009/012605 | 1/29/2009 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 4866640 | Morrison, Jr. | Sep 1989 | A |
| 5307683 | Phelps et al. | May 1994 | A |
| 6035721 | Krisch | Mar 2000 | A |
| 20060288758 | Woo et al. | Dec 2006 | A1 |
| 20070089524 | Walchli et al. | Apr 2007 | A1 |
| 20070108671 | Hong et al. | May 2007 | A1 |
| 20070185673 | Hubanks et al. | Aug 2007 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20100198545 A1 | Aug 2010 | US |
