GB2399170A - Chemical sensor with temperature differential between measurement and reference SAWs - Google Patents
Chemical sensor with temperature differential between measurement and reference SAWs Download PDFInfo
- Publication number
- GB2399170A GB2399170A GB0304895A GB0304895A GB2399170A GB 2399170 A GB2399170 A GB 2399170A GB 0304895 A GB0304895 A GB 0304895A GB 0304895 A GB0304895 A GB 0304895A GB 2399170 A GB2399170 A GB 2399170A
- Authority
- GB
- United Kingdom
- Prior art keywords
- saw
- chemical species
- measurement
- sensor
- saws
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
- G01N29/44—Processing the detected response signal, e.g. electronic circuits specially adapted therefor
- G01N29/4409—Processing the detected response signal, e.g. electronic circuits specially adapted therefor by comparison
- G01N29/4436—Processing the detected response signal, e.g. electronic circuits specially adapted therefor by comparison with a reference signal
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
- G01N29/02—Analysing fluids
- G01N29/022—Fluid sensors based on microsensors, e.g. quartz crystal-microbalance [QCM], surface acoustic wave [SAW] devices, tuning forks, cantilevers, flexural plate wave [FPW] devices
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
- G01N29/22—Details, e.g. general constructional or apparatus details
- G01N29/24—Probes
- G01N29/2468—Probes with delay lines
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
- G01N29/22—Details, e.g. general constructional or apparatus details
- G01N29/30—Arrangements for calibrating or comparing, e.g. with standard objects
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
- G01N29/22—Details, e.g. general constructional or apparatus details
- G01N29/32—Arrangements for suppressing undesired influences, e.g. temperature or pressure variations, compensating for signal noise
- G01N29/323—Arrangements for suppressing undesired influences, e.g. temperature or pressure variations, compensating for signal noise compensating for pressure or tension variations
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2291/00—Indexing codes associated with group G01N29/00
- G01N2291/02—Indexing codes associated with the analysed material
- G01N2291/025—Change of phase or condition
- G01N2291/0253—Condensation
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2291/00—Indexing codes associated with group G01N29/00
- G01N2291/02—Indexing codes associated with the analysed material
- G01N2291/025—Change of phase or condition
- G01N2291/0255—(Bio)chemical reactions, e.g. on biosensors
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2291/00—Indexing codes associated with group G01N29/00
- G01N2291/02—Indexing codes associated with the analysed material
- G01N2291/025—Change of phase or condition
- G01N2291/0256—Adsorption, desorption, surface mass change, e.g. on biosensors
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2291/00—Indexing codes associated with group G01N29/00
- G01N2291/04—Wave modes and trajectories
- G01N2291/042—Wave modes
- G01N2291/0423—Surface waves, e.g. Rayleigh waves, Love waves
Abstract
A chemical sensor comprises at least one measurement SAW 1 and at least one reference SAW 2. The measurement SAWs have coatings which are specifically sensitive to the chemical species to be monitored by the sensor and the reference SAWs have a chemically inert coating. A temperature differential (eg 20-40{C) is maintained between the measurement and reference SAWs by heating or cooling control devices 4,5 (eg nichrome wire, Peltier block, thermocouple, thermostat). The temperature differential encourages adsorption of the monitored species onto the measurement SAW whilst discouraging adsorption onto the reference SAW. The sensor can be used in a range of different pressure environments. The SAWs are mounted on a vacuum flange 3, and RF measurement circuitry 6 is mounted on the surface of the flange 3 external to the vacuum chamber.
Description
B: e: .:
- - -
2399 1 70
PRESSURE COMPENSATED CHEMICAL SENSOR FOR USE IN A
VACUUM ENVIRONMENT
This invention relates to the detection of chemical species. In particular the invention relates to a novel sensor arrangement particularly useful in detecting chemical species in low pressure environments.
In, for example, the semiconductor manufacturing industry, it is important to control the atmosphere in which wafers are manufactured. Such products are desirably manufactured in a vacuum or an inert purged environment.
Undesirable or varying levels of contaminant, such as water vapour can result in wafers having variable characteristics and often poor quality Various practices are adopted to reduce the levels of contaminant in the manufacturing environment including the use of in-line filters and purging of the environment. Nevertheless, undesirable levels of some contaminants occasionally find their way into gas lines or other equipment used in the manufacturing process. Thus it is desirable to be able to monitor the processing environment for the presence of contaminants.
In gaseous environments it is known to employ solid-state sensors to monitor the presence of certain gaseous species. One example of such a solid-state sensor is the SnO2 (tin oxide) type of sensor. Such sensors operate by measuring a change in the resistance caused by the target gas reacting with adsorbed oxygen on the sensor material surface. The selection of the metal-oxide material for the sensor is driven by the requirement to maximise the reaction with the target gas whilst minimising any reaction with other molecules which may be present in the surrounding environment so as to avoid false measurements. Such sensors work adequately under normal atmospheric conditions but have reduced accuracy at extremes of ambient pressure such as vacuum environments.
Conventionally, the presence of contaminant gases in vacuum environments is monitored using very different techniques to those used at normal atmospheric pressure. One example is a residual gas analyser (RGA). Such e ce e . . À À À À À À À À À À devices are effective at pressures below about 10-4 mbar but are increasingly unreliable at higher pressures. Many modern manufacturing process are carried out in inert purged environments at pressures of the order of 10-3 mbar or above. Conventional methods are, consequently unreliable in these manufacturing processes.
Thus, prior art sensors are not versatile in their application, they do not retain their accuracy when used in a range of different pressure environments. Consequently, there is a need for a chemical species sensor which can operate reliably at extremes of pressure and in environments where large pressure and/or temperature variations occur. The present invention aims to provide such a sensor.
In accordance with the present invention, there is provided a chemical species sensor comprising; at least one reference surface acoustic wave device (SAW) and at least one measurement surface acoustic wave device (SAW), the or each measurement SAW having applied thereto a chemically selective coating which is specifically sensitive to chemical species to be monitored by the sensor, the or each reference SAW having a chemically inert coating applied thereto, and means for providing a temperature differential between the or each reference SAW and the or each measurement SAW.
Surface acoustic wave devices (SAWs) have been used to monitor the presence of certain chemical species in a controlled environment. Such devices typically comprise an inter-digitated electrode pattern generated on a suitable substrate, for example, a piezoelectric crystal. Application of a voltage between alternately connected electrodes causes a periodic strain field to be generated in the crystal producing a standing surface acoustic wave. The standing surface acoustic wave gives rise to propagating waves that are launched from the input transducer and detected at the receiving À À À À À À À À cto À e transducer. The properties of the wave are affected by factors such as pressure, temperature and adsorbed mass of chemical species.
In use, a temperature differential is applied between the reference and measurement SAWs of the sensor of the invention. The at least one reference SAW is maintained at an elevated temperature relative to the at least one measurement SAW. Typically, the measurement SAW is cooled relative to the reference sensor and ambient environment. This temperature differential encourages adsorption of the monitored chemical species onto the chemically selective coating while discouraging adsorption of species (from any substance present in the environment) onto the reference electrode.
The probability that a molecule of the monitored species will adsorb on the cooled measurement SAW surface is given by the sticking coefficient. The sticking coefficient is determined from the impingement rate], of the monitored chemical species onto the measurement SAW (i.e. the number of molecules coming into contact with the surface per unit area per unit time) as follows: J_ Pv i - I, [I] where - Impingement Rate [molecules m2 s-1] m - Molecular Mass [kg] k - Boltzmann's Constant T - Temperature [K] Pv - Vapour Pressure [N me] Hence the adsorption rate, Rads is S], where S is the sticking coefficient. The surface coverage Nads, (in atoms/unit area) is given by: Nads = |Rads At [2] I cc -I . À À À À À À and thus the sticking coefficient S is given by: S-,/ 1 dNads PV dt [3] The optimum temperature differential to be applied may vary depending on the species to be monitored and the composition of gases in the ambient environment. The skilled artisan is no doubt capable of determining an optimum differential given the relevant information on the monitored species and composition of the ambient gaseous environment, without departing from the invention.
Adsorption of chemical species on the surface of a measurement SAW will affect the velocity and frequency of the wave produced by the device over and above any changes resulting from pressure fluctuations in the ambient environment. The reference SAW is relatively unaffected by adsorption of chemical species at the temperature at which it is maintained but does respond to changes in the ambient pressure. Frequency changes between the reference and measurement SAWs are monitored. From these frequency changes, the quantity of the monitored chemical species adsorbed on the measurement SAW and detected in the environment can be estimated. Pressure fluctuations in the monitored environment will be compensated by the fact that both the measurement SAW and the reference SAW are exposed to the same pressure fluctuations within the monitored environment. Hence, pressure fluctuations within the monitored environment will be compensated whilst the ability of the device to quantify the amount of the monitored chemical species in the environment will remain unaffected by background (i.e. permanent gases) pressure fluctuations.
À À À . À À À À À À À À a À a À À Where a plurality of measurement SAWs are included in the sensor, different measurement SAWs may have applied thereto different chemically selective coatings. In such an arrangement, the sensor can be used to monitor more than one chemical species A single or multiple reference SAWs may be employed in such an embodiment.
The means for providing a temperature differential may comprise separate temperature control devices associated with the reference SAW(s) and the measurement SAW(s). Where a plurality of measurement SAWs is employed, each may have its temperature controlled by a separate temperature control device so that each may be maintained at the most suitable temperature for maximising the probability of adsorption (i.e. the sticking coefficient) of the species to be monitored by that SAW. Suitable temperature controller devices may comprise an electrically heated wire, for example a nichrome wire. Optionally, the temperature may be monitored and controlled by means of a temperature control device such as a thermocouple. It is to be understood that as well as a heating device, the temperature controller device may also be a cooling device, for example a Peltier type cooling device. It will be understood that heat conducting materials may used both to heat and to cool.
Species detectable by the sensor may include inorganic species and organic species.
The reference and measurement SAWs may be mounted on a vacuum flange to allowing mounting of the sensor in a vacuum or controlled gaseous environment with a suitable seal. Frequency measurement circuitry may be located adjacent the SAWs, optionally in a sealed capsule. The circuitry may be connected with power and communications equipment via a vacuum feed-through. Alternatively, the circuitry may be located externally of a vacuum or other controlled gas environment and connected with the SAWs via a vacuum feed-through.
À À c e À À : : : : 1 À In another aspect, the invention provides a method for monitoring the presence of a selected chemical species in a low pressure gaseous environment comprising; coating a first SAW with a chemically inert coating, coating a second SAW with a chemically selective coating, increasing the temperature of the first SAW with respect to the second SAW, monitoring changes in the frequencies of the wave produced by each SAW and comparing the frequencies.
For the purposes of exemplification, an embodiment of the invention will now be further described with reference to the following Figure in which; Figure 1 shows schematically a sensor in accordance with the present invention As can be seen from the figure, a sensor in accordance with the invention comprises a first SAW 1 which acts as a reference SAW, and a second SAW 2 which operates as a measurement SAW. The SAWs 1, 2 are mounted on the surface of a vacuum flange 3 to allow mounting of the sensor in a vacuum chamber. The temperature of the measurement SAW 2 is controlled by a first temperature control device 4 and that of the reference SAW 1, by a second temperature control device 5. RF measurement circuitry 6 is mounted on a surface of the vacuum flange 3 externally of the vacuum chamber. Each of the SAWs is provided with a surface coating (not shown), the measurement SAW with a chemically selective coating selected for its sensitivity to a chosen chemical species. The reference SAW is provided with a chemically inert coating (inert with respect to any chemical species likely to occur in the monitored environment).
In use, a temperature differential is provided between the SAWs 1,2 by means of the temperature control devices 4,5. Typically, the reference SAW is heated to a temperature significantly higher than that of the # 1 À measurement SAW. The elevated temperature is desirably selected to be higher than the condensation temperature of the species to be monitored. A temperature differential of around 60 C or higher may be used, although a temperature differential of between about 20 to about 40 C is expected to be adequate for most purposes. It is envisaged that for many applications, the reference SAW will be maintained at or around normal room temperature (10 to 20 C), whilst the temperature of the measurement SAW will be cooled to and held at around (-10 to -20 C).
The temperature differential is selected and maintained to optimise the sticking coefficients such that there is a high probability for the species of interest to adsorb to the chemically selective coating and a low probability for the species of interest to adsorb to the chemically inert coating, while a low sticking coefficient for permanent (i.e not of interest) species to adsorb to the chemically selective coating is also maintained. The optimum temperature differential will no doubt vary with the characteristics of the environment to be monitored by the sensor.
Claims (14)
1. A chemical species sensor comprising; at least one reference surface acoustic wave device (SAW) and at least one measurement surface acoustic wave device (SAW), the or each measurement SAW having applied thereto a chemically selective coating which is specifically sensitive to chemical species to be monitored by the sensor, the or each reference SAW having a chemically inert coating applied thereto, and means for providing a temperature differential between the or each reference SAW and the or each measurement SAW.
2. A chemical species sensor as claimed in claim 1 wherein the measurement and reference SAWs are mounted on a vacuum flange.
3. A chemical species sensor as claimed in claim 1 or claim 2 wherein the means for providing a temperature differential comprises at least one electrically heated device.
4. A chemical species sensor as claimed in claim 3 wherein the electrically heated device is a nichrome wire.
5. A chemical species sensor as claimed in any preceding wherein means for providing a temperature differential comprises at least one cooling device.
6. A chemical species sensor as claimed in any preceding claim wherein the means for providing a temperature differential comprises a Peltier block.
7. A chemical species sensor as claimed in any preceding claim wherein the means for providing a temperature differential includes a thermocouple or a thermostat.
'.e a ate À À À a a a a aceÀ e À À a À a a
8. A chemical species sensor as claimed in any preceding claim further comprising an RF circuit electrically connected with the SAWs. I
9. A chemical species sensor as claimed in claim 8 wherein the RF circuit is encapsulated in a sealed capsule.
10. A chemical species sensor as claimed in claim 8 or claim 9 wherein the RF circuit is electrically connected with a vacuum feed-through.
11. A chemical species sensor as claimed in any preceding claim wherein the sensor comprises a plurality of measurement SAWs coated with different chemically selective coatings each sensitive to a different chemical species.
12. A chemical species sensor as claimed in claim 11 wherein each of the plurality of measurement SAWs has associated therewith a separate temperature control device, whereby each measurement SAW can be maintained at a different temperature.
13. A method for monitoring the presence of a selected gaseous chemical species in a low pressure gaseous environment comprising; coating a first SAW with a chemically inert coating, coating a second SAW with a chemically selective coating, increasing the temperature of the first SAW with respect to the second SAW, monitoring changes in the frequencies of the wave produced by each SAW, and comparing the frequencies.
14. A method as claimed in claim 12 wherein the temperature differential between the first and second SAW after increasing the temperature of the first SAW is between about 20 C and about 40 C.
Priority Applications (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB0304895A GB2399170A (en) | 2003-03-05 | 2003-03-05 | Chemical sensor with temperature differential between measurement and reference SAWs |
| PCT/GB2004/000732 WO2004079360A1 (en) | 2003-03-05 | 2004-02-24 | Pressure compensated chemical surface acoustic wave sensor for use in a vacuum environment |
| TW093105900A TW200510718A (en) | 2003-03-05 | 2004-03-05 | Pressure compensated chemical sensor for use in a vacuum environment |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB0304895A GB2399170A (en) | 2003-03-05 | 2003-03-05 | Chemical sensor with temperature differential between measurement and reference SAWs |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB0304895D0 GB0304895D0 (en) | 2003-04-09 |
| GB2399170A true GB2399170A (en) | 2004-09-08 |
Family
ID=9954066
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB0304895A Withdrawn GB2399170A (en) | 2003-03-05 | 2003-03-05 | Chemical sensor with temperature differential between measurement and reference SAWs |
Country Status (3)
| Country | Link |
|---|---|
| GB (1) | GB2399170A (en) |
| TW (1) | TW200510718A (en) |
| WO (1) | WO2004079360A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2006043299A1 (en) * | 2004-10-22 | 2006-04-27 | Saes Getters S.P.A. | Surface acoustic wave gas sensor with sensitive getter layer and process for its manufacture |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US10600952B2 (en) * | 2016-05-20 | 2020-03-24 | Pulmostics Limited | Surface acoustic wave sensor coating |
Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5012668A (en) * | 1989-08-22 | 1991-05-07 | The Boeing Company | Inclined electrode surface acoustic wave substance sensor |
| US5465608A (en) * | 1993-06-30 | 1995-11-14 | Orbital Sciences Corporation | Saw vapor sensor apparatus and multicomponent signal processing |
| US5795993A (en) * | 1995-11-29 | 1998-08-18 | Sandia Corporation | Acoustic-wave sensor for ambient monitoring of a photoresist-stripping agent |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5918258A (en) * | 1996-07-11 | 1999-06-29 | Bowers; William D. | High-sensitivity instrument to measure NVR in fluids |
| US5992215A (en) * | 1997-05-29 | 1999-11-30 | Sensor Research And Development Corp. | Surface acoustic wave mercury vapor sensors |
| US6295861B1 (en) * | 1999-01-28 | 2001-10-02 | Advanced Technology Materials, Inc. | Quartz crystal microbalance sensors and semiconductor manufacturing process systems comprising same |
-
2003
- 2003-03-05 GB GB0304895A patent/GB2399170A/en not_active Withdrawn
-
2004
- 2004-02-24 WO PCT/GB2004/000732 patent/WO2004079360A1/en active Application Filing
- 2004-03-05 TW TW093105900A patent/TW200510718A/en unknown
Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5012668A (en) * | 1989-08-22 | 1991-05-07 | The Boeing Company | Inclined electrode surface acoustic wave substance sensor |
| US5465608A (en) * | 1993-06-30 | 1995-11-14 | Orbital Sciences Corporation | Saw vapor sensor apparatus and multicomponent signal processing |
| US5795993A (en) * | 1995-11-29 | 1998-08-18 | Sandia Corporation | Acoustic-wave sensor for ambient monitoring of a photoresist-stripping agent |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2006043299A1 (en) * | 2004-10-22 | 2006-04-27 | Saes Getters S.P.A. | Surface acoustic wave gas sensor with sensitive getter layer and process for its manufacture |
Also Published As
| Publication number | Publication date |
|---|---|
| WO2004079360A1 (en) | 2004-09-16 |
| TW200510718A (en) | 2005-03-16 |
| GB0304895D0 (en) | 2003-04-09 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |