Method of Making a Semiconductor Transducer

The present invention relates generally to transducers and more particularly to integrated circuit semiconductor transducers and a method for making such transducers.

It is known that silicon microstructures may be used to form transducers for the sensing of pressure and the like. Typically, a silicon transducer includes a glass substrate having a well formed in one surface thereof. A first surface of a silicon layer is bonded or hermetically sealed to the surface of the glass substrate to form an interior chamber. The opposite exposed second surface of the silicon layer is selectively etched away above the chamber to form a diaphragm of a selected thickness. In a pressure transducer, the diaphragm is deflected in response to the pressure differential across the diaphragm. In a tactile transducer, the diaphragm is deflected in response to a force applied thereto. In either the pressure or tactile transducer, the deflection of the silicon diaphragm within a predetermined deflection range is a linear function of the forces acting on the diaphragm. For example, a 100 micron thick diaphragm may have a deflection range of 0.5-2.0 microns to measure forces between 1-4 pounds.

The transduction may be performed by measuring changes in component values of circuit elements wherein the component values are a function of the deflection of the diaphragm. For example, the interior chamber of the transducer may form a variable gap of a capacitor, the capacitor having a fixed plate at the bottom of the well and a movable plate on the diaphragm. Alternatively, the diaphragm may include piezoresistive elements whose resistive values change as a function of the stress and strain on the diaphragm due to its being deflected.

It is also desirable to put a plurality of such transducers in an array to form a sensing element for robotic fingers. In such an array, it is desirable for each transducer to be addressable. The addressability of each transducer allows the output signal developed by each transducer to be multiplexed onto a data bus for processing by a remote processor. The processor may then determine that an object has been grasped by the fingers, as well as the force the fingers are exerting on the object and the orientation of the object within the fingers.

A disadvantage and limitation of prior art semiconductor tactile transducers utilizing piezoresistive elements is that regions of maximum stress and strain of the diaphragm are a function of the point where force is applied to the diaphragm. To optimize accuracy and linearity of the tactile semicodncutor transducer, it is desirable to place the piezoresistors within the regions of maximum deflection of the diaphragm. Thus, the alignment of the force vector of the force applied to the diaphragm with respect to the position of the piezoresistors is critical. The maximum stress and strain occurring within the diaphragm occurs at the clamp edge of the diaphragm about the periphery of the well. However, the piezoresistors must be offset from the clamp edge. Furthermore, since the piezoresistors are on the diaphragm when the silicon layer is bonded to the substrate, the piezoresistors are difficult to precisely align with respect to the periphery of the well which defines the clamp edge.

It is therefore an object of the present invention to overcome one or more of the disadvantages and limitations enumerated hereinabove.

It is a primary object of the present invention to provide a semiconductor transducer which is insensitive to misalignment of the force vector with respect to the piezoresistive sensing elements.

It is an important object of the present invention to provide a semiconductor transducer which is not sensitive to alignment of the piezoresistive sensing elements with respect to the periphery of the well.

It is a further object of the present invention to provide a semiconductor transducer which incorporates integrated circuit electronics within the diaphragm.

It is yet another object of the present invention which concentrates stresses and strains in the diaphragm in areas in which the piezoresistors are located.

It is another object of the present invention to provide a novel method for constructing a semiconductor transducer.

According to the present invention, a semiconductor transducer includes a substrate which has a well formed in one surface thereof and a semiconductor layer bonded at its first surface to the substrate about the periphery of the well to form a diaphragm. A second surface of the semiconductor layer, which is substantially parallel to the first surface, has a pedestal projecting therefrom. The pedestal is disposed above the well. The side walls of the pedestal are substantially orthogonal to the second surface of the semiconductor layer. Means are provided for sensing the deflection of the diaphragm as a function of the force applied to the pedestal.

A further feature of the present invention includes means for limiting the deflection of the diaphragm into the well. Accordingly, structures extend from the bottom of the well to stop the deflection of diaphragm into the well.

Another feature of the present invention includes a channel in the first surface of the semiconductor layer disposed above the periphery of the well. The channel determines the clamp edge of the diaphragm. The sensing means may then be accurately positioned with respect to the channel and relatively independent of the periphery of the well. Furthermore, the periphery of the well may include notches with the diaphragm extending over the notches. The sensing means may be located within the extended portions of the diaphragm wherein stresses and strains are concentrated.

In another aspect of the present invention, the diaphragm is micromachined by selectively sawing the semiconductor layer to a selected depth to form the second surface. The part of the semiconductor layer not sawed becomes the pedestal.

These and other objects, advantages and features of the present invention will become more apparent from the following description and appended claims when read in conjunction with and with reference to the drawings.

FIG. 1 is a plan view, partially broken away, of one embodiment of a semiconductor transducer constructed according to the principles of the present invention;

FIG. 2 is a cross section of the semiconductor transducer taken along line 2--2 in FIG. 1;

FIG. 3 is a cross section of a detail of the semiconductor transducer taken along line 3--3 in FIG. 1;

FIG. 4 is cross section of another detail of the semiconductor transducer taken along line 4--4 of FIG. 1;

FIG. 5 is a plan view, partially broken away, of another embodiment of a semiconductor transducer constructed according to the principles of the present invention;

FIG. 6 is a schematic circuit diagram of an integrated circuit integrated within the semiconductor transducer of FIG. 1 or FIG. 5; and

FIG. 7 is a schematic block diagram of an exemplary array of multiplexed semiconductor transducers.

Referring to FIGS. 1-4, there is shown a semiconductor transducer 10 having a substrate 12, a semiconductor layer 14 and a pedestal 16.

Substrate 12 has a well 18 formed in a surface 20 thereof. In one embodiment of the present invention, substrate 12 is glass and well 18 is formed by selectively etching surface 20. Well 18 has first opposite peripheral edges 22a and 22b and second opposite peripheral edges 24aand 24b. Substrate 12 does not need to be glass or any dielectric material to practice the present invention. It is within the scope of the present invention to provide semiconductor or metallic materials for substrate 12.

As best seen in FIG. 2, semiconductor layer 14 has a first surface 26 and a second surface 28 substantially parallel with first surface 26. First surface 26 is bonded at its edge portions to surface 20 of substrate 12 proximate to peripheral edge 22a-b and 24a-b of well 18. Semiconductor layer 14 forms a diaphragm 30 over well 18. In one embodiment of the present invention, diaphragm 30 has a thickness on the order of 100 microns.

Pedestal 16 projects outwardly from second surface 28 and has sidewalls 32 substantially orthogonal to second surface 28. Pedestal 16 deflects diaphragm 30 in response to forces applied to pedestal 16.

As hereinbelow described in greater detail, semiconductor transducer 10 further includes means 34 for sensing the deflections of diaphragm 30 in response to forces applied to pedestal 16.

According to the method of the present invention, pedestal 16 and diaphragm 30 are formed by micromachining semiconductor layer 14. A diamond saw, of the type used to separate integrated circuit chips within a silicon wafer, selectively cuts away semiconductor layer 14 to a selected depth to form second surface 28 and to leave pedestal 16. The sawing enables substantially orthogonal sidewalls 32 to be precisely machined. The importance of orthogonal sidewalls 32 in practicing the present invention is that such sidewalls 32 enhance the predictability of the regions of maximum stress and strain in diaphragm 30 for determining the most effective position of sensing means 34.

Semiconductor layer 14 further includes a channel 36 in first surface 26. Channel 36 is disposed over peripheral edges 22a-b and 24a-b of well 18. Channel 36 determines the location of a clamp edge of diaphragm 30 wherein maximum stress and strain occur in response to a force applied to pedestal 16. That is, as best seen in FIG. 3, the edge of channel 36 shown at 37 defines the clamp edge of diaphragm 30. This edge is independent of the position of the edges 22a-b and 24a-b of substrate 12. Without channel 36, the clamp edge of diaphragm 30 would occur where first surface 26 contacts peripheral edges 22a-b and 24a-b. In this latter case, should pedestal 16 not be accurately centered between peripheral edges 22a-b and 24a-b, an imbalance in sensing the stress and strain of diaphragm 30 at the clamp edge would occur. By fixing the clamp edge at the edge 37 of channel 36, a greater degree of tolerance is possible when positioning semiconductor layer 14 onto substrate 12 for bonding thereto.

Substrate 12 further includes a plurality of structures 38 extending from a bottom surface 40 of well 18. Structures 38 are formed in substrate 12 when surface 20 is selectively etched to form well 18. Structures 38 therefore have an upper surface 42 elevationally commensurate with surface 20. Semiconductor layer 14 further includes a plurality of recesses 44 disposed in first surface 26. Each recess 44 is associated with one structure 38. Recesses 44 may be formed by selectively etching first surface 26 to a selected depth. Recesses 44 are dimensioned to receive a portion of the associated one of structures 38. The depth of recesses 44 is selected to be commensurate with the maximum deflection of diaphragm 30 into well 18. Structures 38 and recesses 44 comprise means for limiting the deflection of diaphragm 30 into well 18. In one embodiment of the present invention, recesses 44 have a depth in the range of 0.5-2.0 microns.

Semiconductor transducer 10 further includes integrated circuit means 46 for transducing deflections of diaphragm 30 into an electrical signal as one particular embodiment of sensing means 34. Integrated circuit means 46 is formed in first surface 26 within diaphragm 30. Integrated circuit means 46 includes means 48 for sensing deflections of diaphragm 30, and means 50 responsive to sensing means 48 for developing the electrical signal.

Sensing means 48 includes diffused piezoresistors 52a and 52b, associated with peripheral edges 22a and 22b, and diffused piezoresistors 54a and 54b associated with peripheral edges 24a and 24b, respectively. Piezoresistors 52a and 52b are diffused to be parallel to the respective one of peripheral edges 22a and 22b to sense longitudinal stress and strain within diaphragm 30. Similarly, piezoresistors 54a and 54b are diffused orthogonally to the respective one of peripheral edges 24a and 24b to sense latitudinal stress and strain within diaphragm 30. Piezoresistors 52a-b and 54a-b are disposed substantially at the midpoint of the respective one of peripheral edges 22a-b and 24a-b and further disposed to be proximate to the clamp edge of diaphragm 30 to sense substantially the maximum stress and strain therein. The further description of the arrangement of piezoresistors 52a-b and 54a-b will become more apparent from the description hereinbelow in reference to FIG. 6.

Electrical signal developing means 50 is disposed in first surface 26 underneath pedestal 16. This region of diaphragm 30 is a region of minimum deflection due to the thickness of pedestal 18. Thus, the electrical signal developed by developing means 50 is substantially insensitive to the effects of such stress and strain occurring in diaphragm 30 underneath pedestal 16 and therefore accurately reflects just the stress and strain measured by piezoresistors 52a-b and 54a-b. Developing means 50 is described in greater detail hereinbelow with reference to FIG. 6.

Referring further to FIG. 5, a modified semiconductor transducer 10' is shown. To obtain maximum sensitivity, piezoresistors 52a-b and 54a-b are ideally disposed to intersect the clamp edge of diaphragm 30. Accordingly, peripheral edges 22a-b and 24a-b each include a notch 56. Channel 36 is etched to substantially conform to the contours of notches 56. Diaphragm 30 has extended portions 58 which extend over the portion of well 18 within notches 56. Piezoresistors 52a-b and 54a-b are disposed within associated ones of extended portions 58.

The "true" clamp edge of diaphragm 30 generally follows channel 36. However, in the region of notches 56, an effective clamp edge deviates from the path of channel 36 and the true clamp edge because the extended portions 58 are relatively rigid with respect to the main body of diaphragm 30. Therefore, the effective clamp edge tends to cut across extended portions 58 as illustrated diagrammatically at 60. Notches 56 and extended portion 58 thus provide means for disposing piezoresistors 52a-b and 54a-b to intersect the effective clamp edge to sense maximum stress and strain in diaphragm 30 to optimize sensitivity.

Electrical connections to integrated circuit means 46 are made through a deposited metallization layer on first surface 26 which is selectively etched to form interconnections and bonding pads. One such interconnection is exemplarily shown in FIG. 1 at 62 and one such bonding pad is exemplarily shown at 64. Surface 20 of substrate 12 also has a deposited metallization layer to form interconnections to provide electrical conduction between a bonding pad proximate to the periphery of well 18 and a further bonding pad disposed in a recess 66 positioned exteriorly of semiconductor layer 14 at the edge portions of surface 20. An exemplarily interconnction on surface 20 in recess 66 is shown in FIG. 1 at 68 interconnecting interior bonding pad 70 and exterior bonding pad 72.

As best seen in FIG. 4, when semiconductor layer 14 is bonded to substrate 12, bonding pad 64 makes an ohmic contact with interior bonding pad 70. Interconnection 68 traverses through the bond between semiconductor layer 14 and substrate 12 providing means for electrically conducting the electrical signals, such as the above described electrical signal developed by developing means 50 and further electrical signals described hereinbelow, to a point 72 exterior to semiconductor layer 14. The bond between substrate 12 and semiconductor layer 14 may be a hermetic seal. The means for forming such hermetic seals with electrical interconnection, such as interconnection 66, passing therethrough, is substantially described in commonly owned U.S. Pat. No. 4,525,766, which is incorporated herein by reference. However, it is within the scope of the present invention to provide a bond between substrate 12 and semiconductor layer 14 which is not a hermetic seal, i.e., a compression bond. The seal described in the referenced application is incorporated herein to describe one exemplarily means for obtaining a bond between substrate 12 and semiconductor layer 14.

Referring now to FIG. 6, there is shown one embodiment of integrated circuit means 46. Integrated circuit means 46 includes a first piezoresistor 74 and a second piezoresistor 76 connected in series as a half bridge, and a first transistor switch 78 for selectively coupling a current through the half bridge. Integrated circuit means 46 further includes a second transistor switch 80 for coupling the node voltage between piezoresistor 74 and piezoresistor 76 to one of the bonding pads 64 on first surface 26 of semiconductor layer 14.

To turn on transistor switch 78, a multiplexer control signal is applied to its gate. The control signal is inverted by an inverter 82 and is applied to the gate of transistor switch 80 to also turn on this transistor. Therefore, when transducer 10 is addressed by a multiplexer control signal, the node voltage to the half bridge between piezoresistor 74 and piezoresistor 76 is coupled to an output. A node voltage is a function of the force applied to pedestal 16.

Piezoresistor 74 is the circuit equivalent of a series connection between diffused resistors 52a and 52b. Similarly, piezoresistor 76 is a circuit equivalent of a series connection between diffused resistors 54a and 54b. As hereinabove described, diffused resistors 52a and 52b transduce stress and strain within diaphragm 30 which is longitudinal to the clamp ledge. Conversely, diffused resistors 54a and 54b transduce stress and strain latitudinally to the clamp edge. Therefore, in response to a force applied to pedestal 18, the change of resistive value of piezoresistors 74 and 76 are of opposite polarity. In other words, as the resistance of piezoresistor 74 increases, the resistance of piezoresistor 76 decreases. Transistor switch 78 selectively couples this half bridge between a first potential V.sub.cc and a second potential V.sub.ee. Piezoresistor 74 and piezoresistor 76 comprise sensing means 48 of integrated circuit means 46. Transistor switch 78, transistor switch 80 and inverter 82 comprise electrical signal developing means 50 of integrated circuit means 46.

Transistor switch 78 and transistor 80 are preferably n-channel and p-channel devices, respectively. However, this need not be the case. For example, transistor switch 80 may also be a n-channel device, which eliminates the need for inverter 82. In the preferred embodiment of the present invention, a p-channel MOSFET transistor switch 80 is preferred because it does not require an additional diffusion step when fabricating integrated circuit means 46.

The arrangement of piezoresistors 74 and 76 has been selected so that only forces applied normal to the top surface of pedestal 16 are transduced. Furthermore, by defining piezoresistor 74 to be the series equivalent of diffused resistors 52a and 52b and piezoresistor 76 to be the series equivalent of diffused resistors 54aand 54b, any alignment offset of pedestal 16 with respect to the diffused resistors is compensated for. For example, the resistance error in diffused resistor 52adue to alignment offset will be equal in magnitude but of opposite polarity to the resistance error in diffused resistor 52b. Such alignment offset of pedestal 16 could result from the effect of a slip force or a rotational force thereon.

Referring now to FIG. 7, there is shown an array 100 incorporating a plurality of semiconductor transducers, similar to transducer 10 or 10'. In the array 100, the transducers in a given row are simultaneously addressed by applying a multiplexer control signal to a selected one of the row control lines illustrated as control 1, control 2 and control 3. The output of the addressed transducers are coupled to an associated column output line, shown as output 1, output 2 and output 3. The output lines may be coupled to a data bus, and the output voltages digitized and processed by a conventional external processing means (not shown). Thus, by incorporating integrated circuit means 46 within the transducer, simplified multiplexing of an array 100 of transducers is possible for tactile or force sensing.

There has been disclosed novel apparatus and techniques for sensing of forces by integrated circuit transducers. It should be obvious to those skilled in the art that numerous uses of and modifications to the present invention may be made without departing from the inventive concepts disclosed herein. Accordingly, the present invention is to be limited solely by the scope of the following claims.