Process for manufacturing integrated semiconductor devices comprising a chemoresistive gas microsensor

Abstract: To manufacture integrated semiconductor devices comprising chemoresistive gas microsensors, a semiconductor material body (1, 2) is first formed, on the semiconductor material body are successively formed, reciprocally superimposed, a sacrificial region (14a) of metallic material, formed at the same time and on the same level as metallic connection regions (14b, 14c) for the sensor, a heater element (21), electrically and physically separated from the sacrificial region (14a) and a gas sensitive element (25), electrically and physically separated from the heater element (21); openings (16, 28) are formed laterally with respect to the heater element (21) and to the gas sensitive element (25), which extend as far as the sacrificial region (14a) and through which the sacrificial region (14a) is removed at the end of the manufacturing process.

Description: The invention relates to a process for manufacturing integrated semiconductor devices comprising a chemoresistive gas microsensor. As is known, chemical sensors detect the presence of gas thanks to a chemical reaction which takes place between the molecules of the gas and a sensitive film. The chemical reaction depends significantly on the operating temperature which influences the effects of adsorption, desorption and diffusion of the gas in the film. Consequently, temperature is an important factor in optimizing the performance of the sensor, particularly as regards sensitivity, selectivity and response time. To guarantee optimum operation, therefore, the sensors are provided with means for regulating and controlling temperature. Recently, integrated chemoresistive gas microsensors, the manufacture of which makes use of microelectronics techniques, have been proposed and produced. These microsensors have the following advantages: reduced manufacturing costs, low energy consumption in operation, high response times and integrability with the temperature control and output signal processing circuit. Integrated gas microsensors using chemoresistive membranes based on tin oxide are appearing on the market; on the surface of such membranes, deposited on a wafer of semiconductor material machined using the technique of "bulk micromachining", described below, a chemical reaction takes place between the oxygen of the membrane and the gas to be detected which has the effect of changing the resistance of the film and thus enables the presence of the gas to be detected. In view of the fact that in order to operate correctly, such sensors must be maintained at temperatures of approx. 400 °C, they are provided with heater elements and must be thermally insulated from the rest of the chip, integrating the signal processing and control circuit. Various techniques for isolating the sensitive part from the rest of the chip are known in literature. The technique used historically consists of "bulk micromachining", consisting of producing the sensitive part on top of or inside a dielectric layer deposited on a solid silicon wafer and removing a portion of solid silicon from the back of the wafer with wet etching methods. The dielectric layer performs the dual task of mechanically supporting the sensor and thermally insulating the sensor from the wafer of solid silicon. In the context of this technique prototypes have been produced with partial removal of the silicon from the area of the sensor, in which the excavation is carried out only on part of the thickness of the wafer, and prototypes which provide the total removal of the silicon at the area of the sensor (the etching reaches as far as the dielectric layer carrying the sensor element). As regards this second solution, reference may be made for example to the article entitled "Basic Micro-Module for chemical sensors with on chip heater and buried sensor structure" by D Mutschall, C Scheibe, E Obermeier. On the other hand the technique of bulk micromachining requires the presence of front-back machining processes and comprises particular demands for handling the chips which are such that it proves to be incompatible with current integrated circuit manufacturing methods. Another proposed technique consists of "front micromachining" on the basis of which the wafer of solid silicon is etched from the front and a dielectric layer mechanically supports and thermally insulates the sensor element. In this respect, for the production of a different type of sensor, reference may be made for example to the article by D Moser and H Baltes entitled "A high sensitivity CMOS gas flow sensor based on an N-poly/P-poly thermopile", DSC-Vol. 40, Micromechanical Systems, ASME, 1992; furthermore, for a survey of the techniques of bulk and front micromachining, reference may also be made to the article entitled "Micromachining and ASIC technology" by Axel M Stoffel in Microelectronics Journal, 25 (1994), pages 145-156. EP-A-0 751 385 describes a process for forming a semiconductor sensor of the front-micromachining type, having the features of the preamble of the main claims. WO-A-96 36869 discloses a diode sensor including a suspended electrode obtained by removing a sacrificial layer made of, for example, metallic material. This technique for producing suspended structures does, however, require the use of etching phases that are not very compatible with the current manufacturing processes used in microelectronics and does not therefore permit sensors and the related control and processing circuitry to be obtained on a single chip. Furthermore, the use of dedicated SOI (Silicon On Insulator) substrates has been proposed, in which the starting wafer comprises a stack of silicon/silicon oxide/silicon, with the oxide selectively removed at the sensor area, forming an air gap. The excavations made from the front of the wafer at the end of the process phases to contact the air gap enable the sensor to be thermally insulated. In this respect, for a shear stress sensor, reference may be made for example to the article by J Shajii, Kay-Yip Ng and M A Schmidt entitled "A Microfabricated Floating-Element Shear Stress Sensor Using Wafer-Bonding Technology", Journal of microelectromechanical systems, Vol. 1, No. 2, June 1992, pages 89-94. The method used for bonding (apart from the formation of the air gap) is further described in the article "Silicon-on-Insulator Wafer Bonding-Wafer Thinning Technological Evaluations" by J Hausman, G A Spierings, U K P Bierman and J A Pals, Japanese Journal of Applied Physics, Vol. 28, No. 8, August 1989, pages 1426-1443. Finally, the use of a dedicated SOI substrate is also described (in EP-A-0 822 578, corresponding to the European patent application No. 96830436.0 filed on 31.7.96 in the name of this applicant. The object of the invention is to provide a manufacturing process and a chemoresistive gas sensor which do not have the disadvantages of the current techniques. According to the invention, there are provided a process for manufacturing an integrated semiconductor device comprising a chemoresistive gas microsensor and an integrated device comprising a chemoresistive gas microsensor, as defined in claims 1 and 9 respectively. For an understanding of the invention a preferred embodiment will now be described, purely by way of non-exhaustive example, with reference to the accompanying drawings, in which: Fig. 1 shows the transverse section of a wafer of semiconductor material in an initial phase of the manufacturing process; Fig. 2 shows a portion of the wafer of Fig. 1 on an enlarged scale; Fig. 3 shows the same section of Fig. 2 in a different phase of the manufacturing process; Fig. 4 shows a top view of the portion of Figs. 2 and 3 in a successive phase; Figs. 5 and 6 are views similar to those of views 2 and 3, corresponding to the section lines V-V and VI-VI of Fig. 4; Figs. 7 and 8 are views similar to those of views 5 and, respectively, 6, in a successive phase; Fig. 9 shows a top view similar to Fig. 4 in a subsequent phase; Figs. 10 and 11 are views similar to those of Figs. 7 and 8, corresponding to lines X-X and XI-XI of Fig. 9; and Figs. 12 and 13 are views similar to those of Figs. 7 and 8, in a final phase of the manufacturing process. Initially a P-type single crystal silicon substrate is subjected to standard process phases for forming electronic components, whether bipolar or MOS, of integrated circuits. With reference to the numbering of Fig. 1, in particular, an N-type epitaxial layer 2 is grown on substrate 1; on the surface of the substrate 1, in the definition phase of the active areas, a field oxide layer 10 is caused to grow; in the epitaxial layer 2, P-type junction isolation regions 3 are formed to define N-type pockets 4, 4', 4", ..., inside which the active components of the device are formed. In greater detail, the first pocket 4'' (above which the sensor will successively be formed) is completely covered by the field oxide layer 10 whereas the pocket 4' houses an NPN-type transistor 7 forming part of the temperature control and output signal processing circuit (not shown). The transistor 7 has a collector region formed by the pocket 4' and by the N+ region 8, a P-type base region 9 and an N+ type emitter region 5. Subsequently, a protective dielectric layer 11 (such as silicon nitride or BPSG, that is boron phosphorus silicon glass) is deposited over the entire surface. The contacts are then opened (Fig. 1) and a first metallic layer 14 is deposited. The first metallic layer 14 is then defined so as to form at least one sacrificial region 14a of rectangular shape, situated on the area of the wafer intended for forming the sensitive element, two first contact regions 14b (only one of which is visible in Fig. 1) for forming the metallic connections of the heater, two second contact regions 14c (visible in Fig. 6) for forming the metallic connections of the sensitive element, as described below, and further regions 14d constituting the contact electrodes of the regions 5, 8, 9 of the transistor 7. The disposition of the regions 14a and 14b is easier to see in the enlarged scale view of Fig. 2. Then (Fig. 3) a intermetallic dielectric layer 15 is deposited and defined so as to form etching openings 16 through which the sacrificial region 14a will successively be removed, openings 17a for the contacts of the sensitive element (visible in Fig. 6), openings 17b for the contacts of the heater as well as the openings (not shown) required for the control circuitry. As deducible from the subsequent Fig. 4, the openings 16 are disposed in the vicinity of (and inside) the vertices of the rectangle formed by the sacrificial region 14a. On the intermetallic dielectric layer 15 is deposited a second metallic layer 20 which will fill the openings 16, 17a and 17b in the intermetallic dielectric layer 15. The second metallic layer 20 is preferably formed by a triple layer of titanium/platinum/chromium, which permits operating temperatures of the finished device which are greater than those which can be tolerated in the case of aluminium metallizing. As shown in Figs. 4-6, the second metallic layer 20 is then defined so as to form, above the sacrificial region 14a, a heater 21 of the coiled type, having contact electrodes 21a extending as far as the openings 17b, and filling them, so as to connect the heater 21 to the contact regions 14b. Contact electrodes 22 for the sensor are also formed, extending from the sides of the heater 21 (but separated from it) as far as the openings 17a, and filling them, so as to form an electrical contact with the contact regions 14c, as well as regions 27 at the openings 16. A dielectric passivation layer 23 (Figs. 7 and 8) is then deposited which is subsequently removed above the regions 27 (openings 28), at the ends, close to the heater 21, of the contact electrodes 22 (openings 29) and at the contact pads (not shown) of the device. As shown in Figs. 9, 10 and 11, a tin oxide film 24 is deposited (by "sputtering", for example) on top of the dielectric passivation layer 23. A catalyst layer (not shown), of platinum/palladium for example, having the purpose of reducing the activation energy of the film 24 and facilitating the chemical reaction between the molecules of gas and the tin oxide, may optionally be deposited on top of the film 24. Alternatively, a certain percentage of catalyst metal may be incorporated directly in the tin oxide film 24. The tin oxide film 24 and the optional catalyst layer are then defined by means of masking, so as to produce a sensitive element 25 extending over (but isolated from) the heater 21 and having contact regions 30 passing through the openings 29 and in direct contact with the uncovered portions of the contact electrodes 22, producing an electrical connection between the sensitive element 25 and the contact regions 14c. Subsequently, the structure is masked so as to cover the entire surface apart from the zone at the regions 27; wet etching of the regions 27 and, through the etching openings 16, of the sacrificial region 14a, is then carried out. After the removal of the masking layer, the suspended structure shown in Figs. 12 and 13 is obtained, in which an air gap 26 with the function of thermal insulation is present in the place of the sacrificial region 14a. In practice, in the final integrated device, the sensitive element 25 and the heater 21 are supported by the dielectric layer 15 and are disposed above the air gap 26, which insulates them thermally from the regions underneath. The advantages which can be obtained with the manufacturing process and the sensor which have been described are as follows. Firstly, the process described is completely compatible with planar microelectronics technology, enabling use to be made of its well-known advantages in terms of reliability, reproducibility and costs. Furthermore, the monolithic integration in a single chip of the sensor and of the associated control and signal processing circuits is possible. The sensor described has superior spatial integration compared with the known solutions which use techniques of anisotropic etching from the front or the back of the substrate. As a result of this greater integration, the sensor is smaller and requires, for its operation, a smaller amount of energy than the known sensors. The air gap which is present beneath the sensor, thanks to its low thermal conductivity, considerably increases the thermal resistance of the chemoresistive film with respect to the substrate, enabling the sensitive element 25 to reach the desired operating temperatures without detriment to the other parts of the device. Furthermore it will be clear that modifications and variants may be introduced to the process and the sensor described and illustrated here without thereby departing from the protective scope of the invention. In particular, the fact is emphasized that the isolation regions in the epitaxial layer may be formed in a different manner; for example, they may be dielectric instead of junction in type; the electronic components integrated in the chip may be both of the bipolar type and of the MOS type; the type of conductivity of the various regions may vary with respect to that shown.

Claims: Process for manufacturing integrated semiconductor devices comprising chemoresistive gas microsensors, comprising the steps of: forming a sacrificial region (14a) above a semiconductor material body (1, 2); forming a heater element (21) above said sacrificial region (14a) and electrically and physically separated from it; forming a gas sensitive element (25) above said heater element (21) and electrically and physically separated from it; forming, laterally with respect to said heater element (21) and to said gas sensitive element (25), openings (16, 28) extending as far as said sacrificial region (14a); and removing said sacrificial region (14a) through said openings (16, 28), characterized in that said sacrificial region (14a) is made of metallic material, and in that said step of forming a sacrificial region (14a) comprises the steps of: depositing a first metallic layer (14) above said body of semiconductor material (1, 2); and selectively removing portions of said first metallic layer to delimit said sacrificial region (14a) as well as contact regions (14b, 14c) electrically and physically isolated from said sacrificial region (14a). Process according to claim 1, characterized in that said step of forming a heater element (21) comprises the steps of: forming a first insulating layer (15) above said first metallic layer (14), said first insulating layer having first windows (17a, 17b) at said contact regions; depositing a second metallic layer (20) above said first insulating layer, said second layer having electrode portions (21a, 22) in direct contact with said first metallic layer at said first windows (17a, 17b); selectively removing parts of said second metallic layer (20) to define said heater element (21) and said electrode portions (21a, 22). Process according to claim 2, characterized in that said electrode portions comprise first electrode portions (21a) placed in direct continuation of said heater element (21) and second electrode portions (22) electrically separated from said first electrode portions and from said heater element. Process according to claim 3, characterized in that said step of forming an gas sensitive element (25) comprises the steps of: forming a second insulating layer (23) above said second metallic layer (20), said second insulating layer having second windows (29) at said second electrode portions (22); forming a region sensitive to gas (25) extending above said second insulating layer and at said second windows (29) for direct electrical connection of said region sensitive to gas (25) to said second electrode portions (22). Process according to claim 4, characterized in that said step of forming openings (16, 28) comprises the step of selectively removing parts of said first insulating layer (15), second metallic layer (20) and second insulating layer (23) as far as said first metallic layer (14). Process according to claim 5, characterized in that it comprises the step of: cutting said first insulating layer (15) to form third windows (16) facing said sacrificial region (14a) before forming said second metallic layer (20); and in that : said step of selectively removing parts of said second metallic layer (20) comprises the step of forming regions of metallic interconnection (27) inside and above said third windows (16); said step of forming said second insulating layer (23) comprises the step of forming fourth windows (28) above said regions of metallic interconnection (27) and in that the step of removing said regions of metallic interconnection (27) is carried out before said step of removing said sacrificial region (14a). Process according to claim 6, characterized in that said steps of removing said regions of metallic interconnection (27) and said sacrificial region (14a) are carried out with a single step of chemical etching. Process according to one of claims 2-7, characterized in that said step of depositing a second metallic layer (20) comprises the step of depositing a triple layer of titanium, platinum and chromium. Integrated semiconductor device comprising a substrate (1, 2) of semiconductor material and a chemoresistive gas microsensor (21, 25) disposed above said substrate (1, 2) and including a heater element (21) and a gas sensitive element (25), wherein an air gap (26) is interposed between said microsensor (21, 25) and said substrate (1, 2); and a first insulating layer (15) supporting said microsensor (21, 25) is interposed between said air gap (26) and said microsensor, characterized by : first and second metallic contact regions (14b, 14c), disposed on the same level as and laterally with respect to said air gap (26), said first and second metallic contact regions being electrically and physically isolated with respect to said air gap . Device according to Claim 9, characterized in that said first insulating layer (15) has first (17b) and second (17a) windows in a position laterally offset with respect to said heater (21); in that it comprises first electrode portions (21a) extending on said first insulating layer in continuation of said heater (21), said first electrode portions (21a) being electrically connected to said first metallic contact regions (14b) through said first windows (17b); second electrode portions (22) extending on said first insulating layer (15) and electrically isolated with respect to said first portions with dielectric (21a), said second electrode portions (22) being electrically connected to said second metallic contact regions (14c) through said second windows (17a); a second insulating layer (23), interposed between said heater element (21) and said gas sensitive element (25), said second insulating layer (23) having third windows (29); said gas sensitive element (25) comprising connection portions (30) extending through said third windows (29) and in direct electrical connection with said second metallic contact regions (14c). Device according to claim 10, characterized in that it comprises openings (16, 28) extending laterally with respect to said heater element (21) and to said gas sensitive element (25) through said first and second insulating layer (15, 23) as far as said air gap (26). Device according to one of claims 9-11, characterized in that said heater element (21) extends according to a coiled line.

Inventors: MASTROMATTEO UBALDO, VIGNA BENEDETTO, MASTROMATTEO, UBALDO, VIGNA, BENEDETTO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1997
Main classification: G08B1
IPC codes: B81C1, G01N27, B81B1