Semiconductor integrated capacitive acceleration sensor and relative fabrication method

Abstract: The acceleration sensor is formed in a monocrystalline silicon wafer (4) forming part of a dedicated SOI substrate (50) presenting a first (1) and second (4) monocrystalline silicon wafer separated by an insulating layer (2) having an air gap (3). A well (15) is formed in the second wafer (4), over the air gap (3), and is subsequently trenched up to the air gap to release the monocrystalline silicon mass (23) forming the movable mass (24) of the sensor; the movable mass (24) has two numbers of movable electrodes (28a, 28b) facing respective pluralities of fixed electrodes (29a, 29b). In the idle condition, each movable electrode (28) is separated by different distances from the two fixed electrodes (29) facing the movable electrode.

Description: The present invention relates to a semiconductor integrated capacitive acceleration sensor and relative fabrication method. As is known, acceleration sensors are used widely in the car industry: airbag, ABS, active suspensions, engine control and ASR (Anti Slip Rotation). In recent times, electromechanical silicon microstructures fabricated using microelectronics technology have been proposed for use as acceleration sensors, in view of the numerous advantages afforded as compared with traditional macroscopic inertial mechanical switches: low cost, high degree of performance and reliability, better signal/noise ratio, integration with memory circuits for forming intelligent sensors and on-line self-test systems, and greater reproducibility and sensitivity. Prototype silicon-integrated acceleration sensors are currently fabricated in six types substantially differing as to operation: piezoelectric: a mass, moved by acceleration, compresses or stretches a thin film of piezoelectric material, across which a voltage may be detected; piezoresistive: inertial displacement of a silicon diaphragm unbalances a Wheatstone bridge comprising piezoresistive elements diffused in the diaphragm; capacitive: acceleration induces displacement of a seismic mass forming the movable electrode of a single capacitor (absolute variation in capacitance), or of an electrode common to two electrically connected capacitors to vary the two capacitances in opposite directions (differential variation in capacitance); threshold: acceleration-induced inflection of a silicon microbeam closes an electric circuit; resonant: acceleration shifts the intrinsic frequency of an oscillated suspended micromechanical structure; tunnel-effect: acceleration varies the distance between two electrodes, one of which is movable, and hence the quantic tunnel current. The present invention relates to a differential capacitive acceleration sensor. Traditionally, integrated microstructures have preferably been fabricated using the bulk micromachining technique, whereby a silicon wafer is processed on both faces to exploit the excellent mechanical properties of monocrystalline silicon. Front-rear processing, however, and the need for particular handling of the wafers make bulk micromachining incompatible with current integrated circuit fabrication technology. In the mid - 80s, surface micromachining was therefore proposed, whereby the sensitive element is formed of polycrystalline silicon, and suspended structures are formed by depositing and subsequently removing sacrificial layers. Details of this are to be found, for example, in the article by W. Kuehnel and S. Sherman entitled "A surface micromachined silicon accelerometer with on-chip detection circuitry" in Sensors and Actuators A 45 (1994) p. 7-16, and in Patent EP-A-0 543 901 filed by Analog Devices Inc. Though compatible with planar microelectronics technology, the solutions described pose serious problems when releasing the suspended structures (i.e. detaching them from the rest of the semiconductor body) due to the tendency of the microstructures to collapse as a result of the capillarity and Van der Waals forces involved. (Details of this are to be found, for example, in "Stiction of surface micromachined structures after rinsing and drying: model and investigation of adhesion mechanisms" by R. Legtenberg, H.A.C. Tilmans, J. Elders and M. Elwenspoek, Sensors and Actuators A, 43 (1994), p. 230-238). Other highly specialized techniques, such as "wafer dissolving", provide for forming silicon microstructures by means of dedicated processes which are totally incompatible with standard planar microelectronics technology. In a sense, these "ad hoc" processes simply consist in transferring on to silicon what is currently done using other materials, and only provide for fabricating the sensitive portion, so that the processing and control circuit must be formed on a separate chip. For sensors of a different type, dedicated SOI (Silicon-on-Insulator) substrates have been proposed, wherein the starting wafer comprises a stack of Silicon-Silicon Oxide-Silicon with the oxide selectively removed at the sensor area to form an air gap. The trenches formed from the front of the wafer after contacting the air gap provide for forming the suspended structure. Details of this, relative to a shear stress sensor, are to be found, for example, in the article entitled "A Microfabricated Floating-Element Shear Stress Sensor Using Wafer-Bonding Technology" by J. Shajii, Kay-Yip Ng and M.A. Schmidt, Journal of Microelectromechanical Systems, Vol. 1, N. 2, June 1992, p. 89-94. The bonding technique used (excluding formation of the air gap) is also described in the article entitled "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, N. 8, August 1989, p. 1426-1443. EP-A-605 303, to which the preambles of claims 1 and 11 refer, discloses a sensor made from a SOI substrate wherein the upper wafer is only formed by P-type monocrystalline silicon, and the sensor region is isolated from the rest of the upper monocrystalline region by a trench, whereby the pads for biasing the sensor are formed within the sensor area and cannot brought outside. Furthermore, the know sensor has a SOI substrate manufactured according to the SIMOX technique, wherein an air gap is formed subsequently to the definition of the sensor by a time-controlled etching. EP-A-591 554 discloses an acceleration sensor made using a complex process that does not use junction isolation for isolating the sensor from the rest of the monocrystalline region. It is an object of the present invention to provide an acceleration sensor and relative fabrication method, designed to overcome the drawbacks typically associated with currently proposed solutions. According to the present invention, there are provided an integrated semiconductor capacitive acceleration sensor and relative fabrication method, as claimed respectively in Claims 1 and 11. A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: Figures 1-5 show cross sections of a semiconductor material wafer at successive steps in the fabrication method according to the present invention; Figure 6 shows a partial view in perspective of the Figure 1-5 wafer implementing the sensor according to the present invention; Figure 7 shows a larger-scale partial top plan view of the sensor according to the present invention; Figure 8 shows an enlarged detail of Figure 7. A silicon oxide layer 2 is formed, e.g. grown thermally, on a first wafer 1 of monocrystalline silicon (Figure 1); oxide layer 2 is masked and etched to selectively remove a portion and form an opening 3 for forming an "air gap" (Figure 2); and a second wafer 4 of monocrystalline silicon, P-type in the example shown, is bonded to oxide layer 2 using, for example, the method described in the above article by J. Hausman, G.A. Spierings, U.K.P. Bierman and J.A. Pals, to form a dedicated SOI substrate 50 in which the air gap 3 is delimited at the top and bottom by second and first wafers 4 and 1, and laterally by oxide layer 2. At this point, SOI substrate 50 is processed as usual for forming integrated circuit bipolar and MOS electronic components. More specifically (Figure 4), second wafer 4 has junction or dielectric isolating regions 5 extending from upper surface 6 of second wafer 4 to oxide layer 2, and a MOS transistor formed in an N-well 10 and including P-type source and drain regions 11, and a control gate region 12 insulated from second wafer 4 by a gate oxide region 14. When forming the electronic components, an N-well 15 is also formed extending deeply up to, and covering practically the whole area over, air gap 3. In practice, well 15 defines a region (intended for housing the sensor) laterally delimited by the rest (4') of second wafer 4, and at the bottom by air gap 3, and is preferably formed simultaneously with the N- or P-well implant step (in the example shown, when implanting well 10) using an appropriately patterned mask. A dielectric protective layer 16 (e.g. of silicon nitride or BPSG- Boron Phosphorous silicon Glass) is then deposited; the contacts are opened; and a metal layer is deposited and defined to form contact electrodes 17 over second wafer 4, and so obtain the intermediate structure shown in Figure 4. At this point, trenches are formed in second wafer 4 of monocrystalline silicon to release the movable mass of the accelerator, separate the fixed from the movable electrodes, and isolate the different-potential regions, by means of a phototechnical and chemical etching step to remove portions of dielectric layer 16 and of second wafer 4 over air gap 3, and so form trenches 20. The actual shape of trenches 20 is shown in the perspective and top plan views in Figures 6 and 7. More specifically, a first and second trench 20a, 20b define zig-zag electric isolation trenches for separating the movable from the fixed electrodes of the sensor; a third and fourth trench 20c, 20d define release trenches for releasing the movable structure indicated as a whole by 23; and trenches 20 all extend through N-well 15, thicknesswise from dielectric layer 16 (included) to air gap 3 (Figure 5), and lengthwise up to outer portion 4' of second wafer 4, beyond the PN junction, to ensure effective isolation of the electrodes, even though this is not strictly necessary in the case of trenches 20c, 20d. More specifically, movable structure 23 is H-shaped, and comprises a beam-shaped movable mass 24, and a pair of transverse arms 25 for anchoring movable mass 24 to substrate 50. Transverse arms 25 - known as "springs" by restoring movable mass 24 to the balanced position - are fixed to substrate 50 at the vertical end sides indicated by arrows 27 in Figure 7; which vertical sides 27 define the fasteners of movable structure 23, and are delimited laterally by trenches 20, at the top by the surface of the device, and at the bottom by air gap 3. A number of transverse walls 28a, 28b extend from each side of movable mass 24, define the movable electrodes of the capacitive sensor, and alternate comb-fashion with respective transverse walls 29a, 29b defining the fixed electrodes. Fixed electrodes 29a, 29b are biased at two different voltages V + and V - by respective contacts 17a, 17b; while movable electrodes 28a, 28b are biased by four contacts 17c adjacent to the ends of transverse arms 25 and at voltage V, where V - < V < V + . Each movable electrode 28a, 28b is separated by different distances from the two fixed facing electrodes 29a, 29b. As shown in the enlarged detail of an elementary cell in Figure 8, in the balanced condition, each movable electrode 28a is separated by distances d1, d2 (where d1 > d2) from the two fixed electrodes 29a respectively facing it (in Figure 8) in a first (upward) direction shown by arrow A, and in a second direction opposite the first; and each movable electrode 28b is separated by distances d2, d1 from the corresponding fixed electrodes 29b (i.e. respectively facing it in the first and second direction). Each movable electrode 28a, 28b therefore forms two parallel capacitors with the two fixed electrodes 29a, 29b facing it at distances d1 and d2; and, on account of the difference in the two distances, the effective capacitor for each movable electrode 28a, 28b is the one defined with the fixed electrode 29a, 29b at distance d2 (C1 to the left and C2 to the right in Figure 8). As such, the sensor comprises two capacitors connected in series between terminals 17a and 17b and having a common terminal 17c; and each capacitor in turn comprises the parallel connection of a number of elementary capacitors C1, C2 with the electrodes (at rest) separated by distance d1. In the presence of acceleration A (shown by the arrow in Figure 8) parallel to the direction of movable mass 24 and therefore such as to move it from the idle position, to an acceleration position, distance d1 of capacitor C1 decreases and distance d1 of capacitor C2 increases, thus increasing the capacitance of C1 and reducing the capacitance of C2; this variation in capacitance is detected by the known signal processing circuit integrated in the same chip, and is converted into a voltage for further processing according to the application. The advantages of the acceleration sensor and fabrication method according to the present invention are as follows. Forming the movable mass of monocrystalline silicon eliminates the tensile and compressive stress to which the sensor is typically subjected in the case of deposited, e.g. polycrystalline silicon, films; and, being formed in a monocrystalline silicon wafer, the movable mass is thicker as compared with deposited films, so that the sensor has a higher capacitance, is more sensitive, and involves fewer signal processing problems. The structure described presents a high degree of flexural rigidity, and, unlike structures formed by removing a sacrificial layer, is therefore less likely to collapse on to the substrate at the drying step following formation of the trenches. Fabrication of the sensor is fully compatible with planar microelectronics technology, and therefore provides for exploiting the same advantages in terms of reliability and reproducibility; the acceleration sensor and signal processing and control circuits may be integrated monolithically in one chip; and the method, as compared with currently used analog/digital fabrication processes, requires only two additional masks (i.e. for defining the air gap and forming the trenches) and is therefore also economical. Clearly, changes may be made to the sensor and fabrication method as described and illustrated herein without, however, departing from the scope of the present invention. In particular, the electronic components integrated in the same chip may be both bipolar and MOS; and the conductivity of the various regions may be other than as shown.

Claims: An integrated capacitive acceleration sensor comprising a semiconductor material body (50) defining a movable mass (24) having movable contacts (28a, 28b) facing respective fixed contacts (29a, 29b), said semiconductor material body (50) being formed by an SOI, i.e. Silicon-on-Insulator, substrate including a first semiconductor region (1) of monocrystalline material; a insulating material layer (2) over said first semiconductor region; and a second semiconductor region (4) of monocrystalline material over said insulating material layer (2); said second semiconductor region (4) comprising a monocrystalline sensor region (15) forming said movable mass, movable and fixed contacts, and said insulating material layer (2) being interrupted at an air gap (3) formed below said movable mass, movable and fixed contacts, characterized in that said sensor region (15) is isolated from the rest (4') of said second semiconductor region through junction isolation. A sensor as claimed in claim 1, characterized in that said SOI substrate (50) is of dedicated type and said first and second semiconductor regions (1, 4) are semiconductor wafers bonded together through said insulating material layer (2). A sensor as claimed in claim 2, characterized in that said second wafer (4) has a first conductivity type (P); and said sensor region comprises a well (15) of a second conductivity type (N), extending in said second wafer (4) between an upper surface (6) of said second wafer and said air gap (3), and laterally delimited by an outer portion (4') of said second wafer. A sensor as claimed in claim 3, characterized by a number of isolation and release trenches (20a, 20b, 20c, 20d) extending through said well (15) from said upper surface (6) to said air gap (3). A sensor as claimed in claim 4, characterized in that said isolation trenches (20a, 20b) have ends formed in said outer portion (4') of said second wafer (4). A sensor as claimed in claim 4 or 5, characterized in that said movable mass (24) is laterally delimited by said isolation and release trenches (20a, 20b, 20c, 20d), and is anchored to said well (15) by elastic arms (25) connected monolithically at end sides (27). A sensor as claimed in any one of the foregoing claims, characterized in that said fixed and movable electrodes (28a, 28b, 29a, 29b) comprise semiconductor material walls arranged comb-fashion; and in that each movable electrode (28a, 28b) is located, when idle, at two different distances (d1, d2) from two fixed electrodes (29a, 29b) facing said movable electrode (28a, 28b). A sensor as claimed in any one of the foregoing claims from 1 to 6, characterized in that said movable mass (24) is beam-shaped, and has a first and a second side respectively including a first (28a) and a second (28b) plurality of movable semiconductor material walls forming said movable electrodes and arranged comb-fashion with a first (29a) and a second (29b) number of fixed semiconductor material walls forming said fixed electrodes; in that each movable wall (28a) in said first plurality has a first distance (d1) from a fixed wall (29a) facing said movable wall (28a) in a first direction (A), and a second distance (d2) from a fixed wall (29a) facing said movable wall (28a) in a second direction opposite the first direction; and in that each movable wall (28b) in said second plurality has said second distance (d2) from a fixed wall (29b) facing said movable wall (28b) in said first direction (A), and said first distance (d1) from a fixed wall (29b) facing said movable wall (28b) in said second direction. A sensor as claimed in claim 8, characterized in that said first plurality of fixed walls (29a) is biased at a first voltage (V - ); said first and second pluralities of movable walls (28a, 28b) are biased at a second voltage (V) higher than the first voltage; and said second plurality of fixed walls (29b) is biased at a third voltage (V + ) higher than said second voltage. A sensor as claimed in any one of the foregoing claims, characterized in that said semiconductor material is silicon. A method of fabricating an integrated capacitive acceleration sensor, comprising the steps of: forming a semiconductor material body (50) as a SOI, i.e. Silicon-on-Insulator, substrate including a first semiconductor region (1) of monocrystalline material; a insulating material layer (2) over said first semiconductor region; and a second semiconductor region (4) of monocrystalline material over said insulating material layer (2); forming a monocrystalline sensor region (15) in said first semiconductor region; forming an air gap under said sensor region; and forming trenches (20) in to define a movable mass (24) having movable contacts (28a, 28b) and fixed electrodes facing respective fixed contacts (29a, 29b); characterized in that said step of forming a sensor region comprises the steps of forming a junction isolation isolating said sensor region (15) from the rest (4') of said second semiconductor region. A method as claimed in claim 11, characterized in that said step of forming a semiconductor material body comprises the steps of: forming an insulating material layer (2) on a first wafer (1) of monocrystalline semiconductor material; selectively removing a portion of said insulating material layer (2); and bonding a second wafer (4) of monocrystalline semiconductor material onto said insulating material layer, so as to define, together with said first wafer (1) and said insulating material layer (2), said air gap (3). A method as claimed in claim 12, characterized in that said second wafer (4) has a first conductivity type (P); and said step of forming a sensor region also comprises the step of forming a well (15) of a second conductivity type (N) in said second wafer; said well extending from an upper surface (6) of said second wafer to said air gap (3), and being laterally delimited by an outer portion (4') of said second wafer. A method as claimed in claim 13, characterized in that said step of forming a well (15) is followed by the step of selectively removing portions of said well to form a plurality of isolation and release trenches (20) extending from said surface (6) of said second wafer (4) to said air gap (3). A method as claimed in claim 14, characterized in that said isolation trenches (20a, 20b) present ends formed in said outer portion (4') of said second wafer (4); said movable mass being anchored to said well (15) by elastic arms (25) connected monolithically at end sides (27). A method as claimed in any one of claims from 11 to 15, characterized in that said semiconductor material is silicon.

Inventors: FERRARI PAOLO, FORONI MARIO, VIGNA BENEDETTO, VILLA FLAVIO, FERRARI, PAOLO, FORONI, MARIO, VIGNA, BENEDETTO, VILLA, FLAVIO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1996
Main classification: G01P15
IPC codes: G01P15, B81C1, H01L29, B81B3