Title of Invention

AN INTEGRATED PRESSURE AND TEMPERATURE SENSOR BASED ON POROUS SILICON AND PROCESS FOR MAKING THE SAME

Abstract

Porous silicon has been used to create thick oxide layers which act as an insulation between two sensors fabricated on the substrate. Porous silicon realized on silicon blunt emitters was also found to induce a field emission multiplication. However, it has not hitherto been used as the active pressure and temperature sensing material in micro electro-mechanical systems (MEMS). The present invention achieves such dual-purpose use and provides an integrated pressure and temperature sensor based on porous silicon comprising - a) p-type silicon (1) as the principal component; b) oxide layer (2) serving as the protective mask made from (a); c) metal contacts (3) for being attached / connected to the equipments for reading variations in parameters to be measured: d) bulk silicon (4) with established metal contacts and e) porous silicon bodies (5) used for observing variations in temperature and pressure. This invention also pertains to a process for making the aforesaid integrated pressure and temperature sensing device based on porous silicon.

Full Text

The present invention relates to an integrated pressure and temperature sensor based on porous silicon and a process for making the same. More particularly, this invention pertains to the fabrication of a smart pressure and temperature sensor offering enhanced sensitivity and financial advantages, thereby ensuring their wide application in various process industries. Heretofore porous silicon has been used in mechanical sensors as a sacrificial layer since micro-machining technique offers benefits of surface micro-machining along with the capabilities of bulk micro-machining. Porous silicon has been used to create thick oxide layers which act as an insulation between two sensors fabricated on the substrate. In recent times it has also found use for fabrication of field emitter pressure sensor suitable for high pressure measurements. The main advantage of porous silicon for field emission application is that even flat porous silicon surfaces exhibit field emission properties. Thus porous silicon realized on silicon blunt emitters was found to induce a field emission multiplication. However, it has not hitherto been used as the active pressure and temperature sensing material in micro electro-mechanical systems (hereinafter referred to as 'MEMS' for the sake of brevity) pressure sensors. It has been observed that sensitivity of piezoresistive sensors fabricated by prior art procedures were lower by almost three times in comparison to the sensors of the subject invention. Sensitivity has been reported to increase considerably by designing a variable thickness diaphragm with a rim around it, commonly known as 'ribbed and bossed diaphragm'. In the low pressure range, diaphragm has to be made thin (say, of the order 2.5 m) to increase the sensitivity. Such reduction in thickness, however, increases non-linearity. To compensate for this non-linearity and also to increase the sensitivity, 'ribbed and bossed diaphragms' have been designed. But fabrication of such diaphragms is quite a complex procedure, and they find use in ultra low-pressure range, e.g of the order of 10'3 torr. On the other hand, if the sensitivity could be increased by some other means, then one could obviate reduction in thickness of the diaphragm to a very low level, even for measurement of low pressure. Consequently, fabrication of complex "ribbed and bossed" diaphragms would no longer be a necessity for low-pressure application, thereby making the sensor more cost-effective. The piezoresistive property of porous silicon has not been studied in extenso but for the observance of pressure induced phase transition therein, which indicates that porous silicon undergoes a change in band structure induced by pressure. The present invention is based on the response of porous silicon membrane towards pressure in view of the improvement in piezoresistive coefficient due to quantum confinement in the porous silicon structure. This concept is supported by the fact that porous silicon is a natural nanomaterial with silicon column confined between silicon dioxide and voids. It has also been observed that in nanocrystalline silicon the piezoresistive co-efficient increases to the tune of around 54.8% (T. Toriyama, S. Sugiyama, Sensors and Actuators, Vol. 108, 2003, pp. 244-249). The principal object of the present invention is to provide an integrated pressure and temperature sensor based on porous silicon which overcomes the disadvantages experienced hereinbefore in the industry. This invention has also for its object to provide a simple, low-cost pressure and temperature sensor based on porous silicon. According to this invention there is provided an integrated pressure and temperature sensor based on porous silicon comprising- (a) p-type silicon as the principal component, (b) silicon oxide layer serving as the protective mark on top of the silicon surface, and (c) metal contacts for being attached/connected to the equipments for reading variations in parameters to be measured, characterised in that components (a), (b) and (c) are mounted on bulk silicon having porous silicon bodies with established metal contacts for observing variations in temperature and pressure. Usually for pressure sensors both the contracts are taken from the top surface on a different piezoresistor. In the present instance as porous silicon is highly resistive compared to that of bulk silicon, the current lines will effectively concentrate in the bulk silicon layer and the effect of porous silicon has the chance if being masked. Hence in such a configuration of metal contacts the bulk silicon parasites are likely to dominate. In order to study the response of porous silicon layer of different porosites and thickness on application of stress and to compare it with monocrystalline silicon, contacts have been taken from top surface of porous silicon layer and bottom surface of micromachined bulk silicon, whereby current lines are forced to pass through both porous silicon and silicon. Even though the bulk silicon layer acts as a parasitic component, still it is possible to get an estimate of the piezoresistive behaviour of porous silicon in this configuration of metal contacts. This invention also pertains to a process for making an integrated pressure and temperature sensor which comprises in combination the following steps :- (i) selecting a suitable p-type (100) silicon sample (1); (ii) oxidising the entire area by using a dry-wet-dry sequence to form an oxide layer (2) on top of the silicon surface (1); (iii) subjecting the sample from step (ii) to lithography by standard technique to expose a desired region for micromachining; (iv) micromachining the sample by using a wet etchant at room temperature to achieve desired thickness; (v) heating/filling the front region of the cell used in anodic etching with a mixture of HF and CH3OH, and immersing rear portion of said cell in KG solution thereby forming porous silicon; (vi) forming metal contacts (3) on top of porous silicon layer (4) by vacuum evaporation of aluminium and its subsequent heat treatment at around 500°C for 40-45 seconds and (vii) carrying out back contact metallisation either by nickel plating or by screen- printing with silver-aluminium paste followed by firing. The silicon wafer chosen as the starting material is a p-type nanocrystalline wafer (100) of resistivity of around 1-3 ohm-cm. After oxidation, the thickness of the oxide layer is around 0.6 m. Lithography of the sample exposes areas of around 2mm x 2mm membrane. Micromachining is done in KOH solution to avoid development of microcracks in the diaphragm. Formation of porous silicon by anodic etching is conducted in a cell specially designed for the purpose, and the area of the porous silicon layer is around 2mm x 2mm. The wafers actually act as a seal between the front and rear regions of the cell. The metal contact area on porous silicon layer is optimally around 1mm x 1mm. It has been found that in the course of formation of porous silicon, if the current density exceeds the critical current density for pore formation, electropolishing sets in instead of electrochemical etching. However, the critical current density depends on the surface morphology, temperature of formation and the concentration of HF. It has been observed that the critical current density for 48% and 24% HF concentrations are around 1450 and 513 mA/cm2 for oriented silicon wafer. The anodisation current density was varied from 10 to 50 mA/cm2 and HF concentration in methanol was varied from 24% to 48%. The range of porosites and thickness of the porous silicon sample have been obtained as 48%-70%, and 5m to 20m respectively. Thus in the course of formation of porous silicon layer, the chance of electropolishing of the surface is negligible. Furthermore, the thickness of the layer of porous silicon has been restricted to 20m on a p-type wafer since it was found that with increasing thickness, porosity increases and fabrication of ohmic and stable metal contacts becomes difficult. Freestanding porous silicon layer was not fabricated purposely, since it would have been extremely difficult to handle such fragile structure. On the contrary, bulk silicon gives mechanical support to the structure. The foregoing process leads to a twin MEMS porous silicon/silicon heterojunction structure with an isolation layer in between to be used as an integrated pressure and temperature sensor. It is observed that the piezoresistive coefficient of the porous silicon/silicon MEMS structure improves by about 50% with 63% porosity and 20m thick porous silicon layer leading to almost three times increase in sensitivity of pressure sensor. The same structure also displays an enhanced temperature sensitivity (about 10 times) compared to that of conventional sensors. The sensors developed in accordance with this invention is also helpful for high pressure range where the thickness of the diaphragm has to be made considerably thick to withstand the fracture stress, often by sacrificing sensitivity. Optimization of the doping concentration of the piezoresistors to increase the sensitivity and the signal to noise ratio as mentioned herein effectively overcomes the difficulty of low sensitivity of high range pressure sensors. Thus the subject design produces a cost effective sensor where the thickness of the membrane needs to be reduced to achieve high sensitivity since the sensitivity is increased by an optimisation process requiring simple fabrication steps. The invention will now be illustrated with the help of drawings accompanying the provisional specification, wherein - Fig. 1 shows graphically the improvement in piezoresistive coefficient and consequent increase in sensitivity in nanostructured porous silicon pressure sensors over commercially available pressure sensors; Fig. 2 shows graphically the increased temperature sensitivity of the porous silicon- silicon heterojunction compared to that of a commercial piezoresistor, and Fig. 3 gives a schematic view of an integrated pressure and temperature sensor of this invention. In Fig. 1 there is depicted variation of fractional change in resistance of porous silicon layer of thickness 20uxn and different porosities. The diaphragm thickness is 100m. Fig. 2 of the drawings depicts variation of fractional change in current with temperature for (a) reverse biased porous silicon-silicon heterojunction and (b) commercial silicon piezoresistor. Referring to Fig. 3 of the drawings, (1) is the porous silicon for isolation, (2) is the oxide layer, (3) represents metal contacts, (4) shows bulk silicon and (5) stands for porous silicon for sensing temperature and pressure variation. The integrated pressure and temperature sensor of this invention offers a number of distinct advantages which are, inter alia, as follows : i) enhanced sensitivity compared to that of a commercially available silicon sensor; ii) reduction in the number of fabrication steps with consequent cost saving; iii) precise monitoring of the temperature (due mainly to similar thermal mass of the pressure and the temperature sensor), iv) precise temperature compensation, if and when necessary. v) the sensors developed have simple structure and vi) the sensors are applicable for all pressure ranges. While the invention has been described in detail with specific embodiments thereof, and illustrated by means of the drawings accompanying the provisional specification, it will be apparent to one skilled in the art that various changes and modifications can be made therein without deviating or departing from the spirit and scope of the invention. Thus the foregoing disclosure include within its ambit the obvious equivalents and substitutes as well. "Having described the invention in detail with particular reference to the illustrative examples given above and drawings accompanying this specification, it will now be more specifically defined by means of claims appended hereafter". I claim: 1. An integrated pressure and temperature sensor based on porous silicon comprising— (a) p-type silicon (1) as the principal component, (b) silicon oxide layer (2) serving as the protective mark on top of the silicon surface (1), and (c) metal contacts (3) for being attached/connected to the equipments for reading variations in parameters to be measured, characterised in that components (a), (b) and (c) are mounted on bulk silicon (4) having porous silicon bodies (5) with established metal contacts for observing variations in temperature and pressure. 2. An integrated sensor as claimed in Claim 1, wherein the silicon wafer chosen as the starting material is a p-type (100) nanocrystallaine wafer of resistivity of around 1-3 ohm-cm, thickness of the oxide layer is around O.oum, metal contact area on porous silicon layer is 1 mm x 1 mm and area of the porous silicon layer is around 2 mm x 2 mm, respectively. 3. An integrated sensor as claimed in Claims 1 and 2, wherein the thickness of porous silicon layer is 20um and the diaphragm thickness is 100m, respectively. 4. An integrated pressure and temperature sensor based on porous silicon, substantially as hereinbefore described with particular reference to the drawings accompanying the Provisional Specification. 5. A process for making an integrated pressure and temperature sensor based on porous silicon, as claimed in Claims 1 and 4, which comprises in combination of the following steps: (i) selecting a suitable p-type (100) silicon sample (1) (ii) oxidising the entire area by using dry-wet-dry sequence to form an oxide layer (2) on top of the silicon surface (1); (iii) subjecting the sample from step (ii) to lithography by standard technique to expose a desired region for micromachining; (iv) micromachining the sample by using a wet etchant at room temperature to archive desired thickness; (v) heating/filling the front region of the cell used in anodic etching with a mixture of HF and CH3OH2 and immersing rear portion of said cell in KC1 solution thereby forming porous silicon; (vi) forming metal contacts (3) on top of porous silicon layer (4) by vacuum evaporation of aluminium and its subsequent heat treatment at around 500°C for 40-45 seconds and (vii) carrying out back contact metallization either by nickel plating or by screen-printing with silver-aluminium paste followed by firing. 6. A process as claimed in Claim 5, wherein an area of around 2 mm x 2 mm membrane is exposed by lithography of the sample. 7. A process as claimed in Claims 5 and 6, where HF concentration in methanol varies between 24% and 48% v/v and anodisation current density is varied between 10 and 50 mA/cm2, 8. A process as claimed in Claims 5 and 7, wherein the range of porosities and thickness of the porous silicon sample varies between 48%-70% and 5pm - 20m, respectively. 9. A process for making an integrated pressure and temperature sensor, based on porous silicon, substantially as hereinbefore described and illustrated in the drawings accompanying the Provisional Specification. Porous silicon has been used to create thick oxide layers which act as an insulation between two sensors fabricated on the substrate. Porous silicon realized on silicon blunt emitters was also found to induce a field emission multiplication. However, it has not hitherto been used as the active pressure and temperature sensing material in micro electro-mechanical systems (MEMS). The present invention achieves such dual-purpose use and provides an integrated pressure and temperature sensor based on porous silicon comprising - a) p-type silicon (1) as the principal component; b) oxide layer (2) serving as the protective mask made from (a); c) metal contacts (3) for being attached / connected to the equipments for reading variations in parameters to be measured: d) bulk silicon (4) with established metal contacts and e) porous silicon bodies (5) used for observing variations in temperature and pressure. This invention also pertains to a process for making the aforesaid integrated pressure and temperature sensing device based on porous silicon.

Documents:

329-kol-2004-granted-abstract.pdf

329-kol-2004-granted-claims.pdf

329-kol-2004-granted-correspondence.pdf

329-kol-2004-granted-description (complete).pdf

329-kol-2004-granted-drawings.pdf

329-kol-2004-granted-examination report.pdf

329-kol-2004-granted-form 1.pdf

329-kol-2004-granted-form 13.pdf

329-kol-2004-granted-form 18.pdf

329-kol-2004-granted-form 2.pdf

329-kol-2004-granted-form 3.pdf

329-kol-2004-granted-form 5.pdf

329-kol-2004-granted-gpa.pdf

329-kol-2004-granted-specification.pdf