Polysilicon bridges for the realization of tactile sensors

Semor3 and Actuators A, 25-27 (1991) 251-264

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Polysilicon Bridges for the Realization of Tactile Sensors M. R. WOLFFENBU-ITEL De@ University

and P. P. L. REGTIEN

of Technology,

Deparmenr

of Elecrrical

Mekelweg 4, 2628 CD Deyl (The Netherlands)

Engineering, Laboralory for Electronic Insmmentation,

Abstract The fabrication of a capacitive tactile sensor is examined. Therefore, the different transduction methods for converting tactile forces to an electrical signal, and the different manufacturing methods used by other researchers in the last 30 years will be summarized. We propose a sensor fabrication method using silicon surface-micromachining. This comparatively new microfabrication technology allows us to construct the tactile sense elements, with the integration of readout circuitry, upon a single silicon wafer in an entirely single-sided fabrication set-up, which also has the potential for reduced weakening of the supporting substrate. The structure and the fabrication of the sensor and the measured readout characteristic are described.

1. Introduction Small force sensors are suitable for controlling robotized assembly processes. Either the position of an object can be obtained using binary and scalar force sensors, or the identification of this object can be made using the contact pressure with an array of force sensors in the robot gripper. The second feature is of more interest when an object has to be manipulated by the gripper, during which process the control data concerning the orientation and shape of the object can only be deduced from the tactile image. Tactile sensors have been the subject of investigation for the last 15 years. Early devices use a solid, flexible, substrate to support a force-sensitive elastic layer, which measures a mechanical parameter using a well-known 09244247/91/$3.50

transduction mechanism [ 11. Some properties of the elastic sense layer, however, affect the sensor characteristics, and the signal-detecting circuitry is hybridized alongside the device on the supporting substrate. Therefore, a need for basic layers which carry even smaller structures and also the readout circuitry remained. Later, silicon was chosen as the basic material because it possesses remarkable mechanical properties and because it uses microelectronic techniques, which allows mass-production at low cost to be achieved. The undesired effects of creep and hysteresis in the elastic sense layer are avoided, as a thin diaphragm is etched in silicon after the electronics integration is performed. Stressing or bending of the etched diaphragms caused by applied forces is measured by the sense electronics. Improvements in these etching processes now allow us to fabricate very thin diaphragms. This paper reviews the wide variety of force transducers which have been developed to date. It touches upon examples of rubber- and silicon-supported devices. Two etching methods to realize small cavities in silicon wafers for use in piezoresistive and capacitive transducers are discussed. A careful evaluation of their applicability to the realization of highresolution tactile imagers is the basis of the design of a tactile sensor using surface micromachining. This technology is a most promising processing method to design mechanical structures on top of the wafer.

2. Rubber-basedTactile Sensors with Various Tramduction Principles [l-3] Figure l(a) shows the basic structure of a piezoresistive tactile sensor. The elastic layer 0 Elsevier Sequoia/Printed in The Netherlands

conductive elastomer /

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Fig. I. Rubber-based tactile sensors, using diKerent transduction mechanisms: (a) piezoresistive, (b) optical, and (c) magnetic tactile devices.

is carbon- or silver-impregnated rubber. Tactile forces imply a localized condensing of conductive particles, and also imply that the conductivity between two selected adjacent electrodes increases. Other devices use the pressure-dependent contact resistance between a conductive rubber layer and contact electrodes [4]. Various low-cost rubbers for a wide range of measuring purposes are available. The lateral stiffness of the rubber, however, causes crosstalk between the adjacent tactile elements, which limits the spatial resolution to a few mm. Moreover, such sensors exhibit non-linear resistance effects with unknown stress factors. These properties are difficult to describe and model for the sensor behaviour. Besides, hysteresis remains in the sensor characteristics, and the random nature of the contact behaviour between the conductive particles involves a fluctuation in the measured resistance in a new unloaded situation. The properties of this sensor layer are seen as being more suitable for use in a binary detector.

Piezoelectric tactile sensors are constructed from polyvinylidene fluoride film material, PVFZ, which generates localized charge at the surface when it is subjected to mechanical stresses. PVF2 has a large piezoelectric effect, a good linearity and low hysteresis with respect to piezoresistive elastomers. It is cheap, strong, chemically inert, very flexible, and any polarization direction can be obtained for compliant devices. However, charge leakage through internal resistances prevents the formation of an image of static tactile forces. Moreover, these layers are sensitive to changes and gradients in temperature. PVF2 has a narrow temperature and force range of operation, because the low Curie temperature of 100 “C and a depolarization of the layer at large forces change the piezoelectric properties. Figure l(b) shows an optical tactile transducer which conducts trapped light in a transparent glass plate under an elastic layer, using the condition of total internal reflection Little light leaves the plate, and a uniformly black area is seen from the bottom until the elastic layer is brought into contact with the glass layer as a consequence of tactile forces. Diffuse reflection rather than total internal reflection takes place, and illuminated areas are seen where the object touches the sensor. The elegant way in which large spatial resolutions are obtained is the major advantage of this structure. Two other devices measure tactile forces, when there is a change in the reflectivity of a flexible membrane with a reflecting surface [5], and when there is an obstruction of a light path [6]. Such sensors are robust and have an overload protection in most applications. The major drawback is the expense of the construction materials, and the skin-plate adhesion is an added limitation to the dynamic range. Most magnetic tactile sensors also require complex mechanical constructions, which limit the minimal size of each element to a few mm. Different approaches have been tested, for example the detection of changes in the magnetization pattern of a magnetostrictive layer as a consequence of a stress-dependent

259

anisotropy in the magnetic permeability. Figure l(c) shows the complexity of such a sensor. This magnetoresistive tactile sensor is composed of matrix-resistive Permalloy strips on an A1203 substrate, which is covered with a rubber skin with flat copper wires on top of it. The resistance of each Permalloy strip is sensitive to changes in the strength of the magnetic field which is provided by the flat copper wires. Tactile forces modulate the localized magnetic field, which is measured with a row-by-row sampling of the network. One limitation of the size of each tactile element in a rubber-based tactile sensor is dictated by the creep, hysteresis and lateral crosstalk properties of the elastic layer. The lateral stiffness presents a large spatial spread in the deformation at a localized force, which makes rubber-based tactile arrays less suited to be assembled in a robot arm designed to pick up high-resolution tactile information. Further, the viscoelastic properties of a rubber layer limit the dynamic range to 100 Hz, and electrical connections with a conductive rubber layer are sources of noise. In contrast to these rubber sensing layers, silicon possesses excellent mechanical properties. An elastic deformation over a considerable mechanical stress, and a negligible creep and hysteresis make silicon layers improved substitutes for the elastic rubbers. Moreover, microelectronics integration in the silicon supporting substrate resolves the drawback in rubber-based devices that a limited number of tactile elements and a minimal size is dictated by the large number of wires to the tactile array and a reduced sensor signal in a high-resolution sensor [7-91.

3. Silicon Tactile Sensors To realize the desired silicon sensor structure, bulk micromachining [lo], which etches bulk parts of the silicon from the rear, or surface micromachining, which deposits and etches structural layers on top of the wafer, is used. Both these techniques allow us to construct a silicon diaphragm in the wafer, which

is the basis of many sensors. The pressure sensors currently available are the capacitive and piezoresistive sensors. These elements are most suited to supply the sense of touch. In such devices, a mechanical measurand stresses and bends the etched membrane. The stress and bending are detected by strain gauges or by a second conductive plate in a capacitive measuring set-up. 3.1. Bulk-micromachined Devices Bulk micromachining, in which either chemical anisotropic or plasma etching is used, selectively removes parts of the silicon wafer to obtain a diaphragm or cantilever with a thickness of lo-30 pm. Anisotropic etchants show different etch rates in different crystalline orientations of the etching surface. In crystallographic silicon, the (100) and the (110) planes are etched at a considerable rate in EDP (ethylenediamine-pyrocatechol-water) [ 1I], hydrazine-water [ 121 or KOH [ 131etchants. The membrane thickness is controlled using the boron etch-rate reduction method or the ECE (electrochemically controlled etching) method. The first method reduces the etch rate when the etchant reaches parts of the silicon doped with a high boron impurity concentration. Its major drawback is the need for highly doped regions, because these are not accessible to integrated devices and they also introduce stresses into the wafer. The ECE method yields membranes with reduced mechanical stress. The microelectronic integration process in the medium-doped epilayer precedes this etching step, in which a reverse-biased junction with the p-substrate is performed. Unmasked substrate regions are etched in KOH [ 151or EDP [ 161from the back. When the etchant reaches the n-doped regions, a current oxidizes the etching surface as a consequence of the removed junction. This oxide layer protects the remaining epilayer thickness against the etchant. Plasma etching, a dry etching process, realizes cavities with almost vertical side walls, and the technique is not limited by the crystalline orientation of the etching surface. This

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simplifies the calculations of the shape of the etched cavity in the 300 pm thick wafer. High-energy ions in a plasma hit the Si surface perpendicularly, increasing the etching rate in the concave direction [17]. The major drawback is the dependence of the rate, anisotropy and the selectivity of the etchant, and the many physical process parameters, all of which require compromises to be made for the rate and the anisotropy. Low-cost, highly sensitive and reproducible sensors are fabricated using the above etching methods. After the electronics are integrated on the upper surface of the wafer, the cavity is etched in a double-sided processing from the rear. Because the cavity weakens the wafer [ 181, this structure should be bonded on a supporting substrate. 3.2. Surface-micromachined Devices Double-sided processing is not done on surface-micromachined silicon sensors. In this application, free-standing structures like cantilevers, bridges and micromotors [ 19-211 are processed on top of the silicon substrate. The process starts with the standard integration of the electronics in silicon, after which several layers are deposited on top. The key step in this process is the etching of a sacrificial spacer layer, which releases the etched microstructure in the crossover layers from the substrate. This step utilizes a single-side processed sensor structure, which allows the fabrication to take place without alignment problems and, if desired, the minimizing of a microstructure is performed to a few microns. No cavities are etched in the substrate, and thus no wafer weakening occurs. For highyield tactile arrays, however, beams with small residual stresses should be shaped. A thermal strain-relief cycle of 1050 “C, which fortunately has a minimal effect on the circuitry performance [22], is used after the structural layer is deposited on top of the sacrificial layer. Unfortunately, the phosphosilicate glass (PSG) sacrificial layer, which etched rapidly in hydrofluoric acid, flows at the prevailing temperature as it loses adhesion to the underlying nitride layer, and this

would damage the structural layer. A posite PSG-Si02 sacrificial layer [23] comes this difficulty, as the PSG remains attached to the underlying layer.

comoverlayer SiO,!

3.3. Comparison of Capacitive and Piezoresistive Transducers Semiconducting resistors in piezoresistive devices are composed of several deposited polysilicon strips, or diffused doping areas in the diaphragm. These semiconducting strain gauges possess a large piezoresistive effect, which results from a large variation in the carrier mobility. This depends on the crystallographic orientation of the membrane, and thus on the direction of each piezoresistor. Currently available piezoresistive sensors can have a high thermal sensitivity. A sensitivity of aR(O)/(RaT) = 5 x 1O-3/“C [24] is quite common. At maximum pressure, AR/R is about 0.01, and a small temperature variation would mask the pressure-dependent characteristics. The major cause of this dependence is the modification of the carrier mobility over the usual temperature range. A Wheatstone bridge configuration can compensate for this effect, but a temperature offset of several 1000 ppm/“C at the output remains as a consequence of a bridge imbalance, because the doping profile and the geometric dimensions of each resistor are also not strictly identical. A highly doped piezoresistor is used to decrease the temperature sensitivity, but this limits the minimal size of this resistor. Moreover, the comparison of the normalized responses indicates that the pressure sensitivity of piezoresistive sensors is less than that of capacitive sensors at comparable sensor dimensions [ 241. Currently available capacitive sensors have a small temperature drift. The origin is to be found in the mismatch of the different thermal expansion coefficients and the drift in the packaging of stray mechanical stresses, which is only one of the five temperature-drift mechanisms shown in piezoresistive sensors. To be more specific, a X/(C8T) < 100 ppm/“C has been reported [25]. This is a

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major advantage of a capacitive sensor. Moreover, the size can be reduced because changes in capacitance of a few ff can be measured. Although such a sensor exhibits a strong non-linear sensor characteristic, it is reproducible and easily calculated in most applications. Therefore, taking these aspects into consideration, we selected the capacitive transduction method as the detection mechanism for tactile forces.

4. Proposed Surface-micromachined Tactile Sensor The evaluation of the different sensing principles and the manufacturing methods previously described are of great value for the consideration of a new tactile sensor with the following requirements. The spatial resolution of the tactile array should be better than 1 mm, which requires very small and very closely spaced scalar tactile elements. Moreover, the sensor should exhibit negligible hysteresis and its elements should be sensitive to forces but not to other physical parameters like acceleration of the robot arm, creep, hysteresis and temperature fluctuations. Finally, the sensor must be built on a solid substrate which also carries the sense and the selection electronics. 4.1. The Device Structure Figure 2 shows the proposed mechanical sensor structure, which is built on a composite silicon dioxide and silicon nitride film on top of a silicon wafer. The nitride film is

SECOND POLY-LAYER\

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Fig. 2. Spatial view of the proposed tactile element structure.

required to protect the oxide from the HF etchant when the sacrificial layer is dissolved. A strongly doped area in the Si wafer and a 1 pm surface-micromachined free-standing doped polySi bridge on top of the wafer form two opposing plates, giving a capacitor that increases in value when it bends as a consequence of an applied force. The force is applied to the force transfer beam which is composed of a polynitride layer. The poly hubs at the border of a bridge force the plate to behave as a free-standing plate with two clamped edges. A significant sensor feature is seen in this approach. For tactile sensing applications, each bridge has to withstand a maximum force over which the two opposing plates touch each other. However, the fabrication of polysilicon bridges demands a limited layer thickness, which implies a limited beam length and a reduced nominal capacitance. Therefore, we introduced a multi-bridge tactile element, which is composed of several bridges in a small array. Each bridge is very thin, but in combination they can withstand a tactile force. This size is limited to prevent the buckling of several bridges, but their capacitances are connected in parallel, so that an increased nominal capacitance is achieved. 4.2. Fabrication of the Bridge Surface-micromachined technology is used to fabricate these bridges. Figure 3 shows the major steps in the proposed bridge fabrication. In Fig. 3(a), an n+ implantation in a p-substrate, which is covered with a thermal oxide layer and a LPCVD nitride passivation film, is indicated. After opening gaps for the doped bridge with the wafer substrate, a 1 pm CVD oxide-phosphosilicate glass (PSG) composite sacrificial layer is deposited and patterned, as shown in Fig. 3(b). A 1 pm polysilicon layer is then deposited and etched to form the bridge. This layer is doped from a second PSG layer (Fig. 3(c)). A second LPCVD nitride, a second polylayer and a 1 pm nitride layer are then deposited (Fig. 3(d)) and etched to form the force transfer beam and the clamping edges (Fig. 3(e)-(f)). The

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(9) Fig. 3. Cross sections of the fabrication sequences of the tactile element.

second nitride film on the polysilicon bridge is used as an etch stop so that the etchant for the clamping side walls does not attack this bridge layer. Once the central stiffener and the hubs for the side walls are formed, an HF etchant removes the sacrificial layers, undercutting the first polylayer. The complete structure is shown in Fig. 3(g) after deposition and patterning the Al interconnect.

4.3. The Readout Circuit: A Test Set-Up In order to measure the force which presses upon a bridge, the device needs a capacitance readout circuit. Either a voltage or a charge amplifier can be used to process the signal. We have made use of a charge amplifier for our tactile array, as the output signal is less dependent on parasitic capacitances of the unselected tactile elements. The circuit principally consists of a single-stage shunt-feedback circuit as is shown in Fig. 4(a). To compensate for the large bias voltage at the output of this amplifier, the balanced version of Fig. 4(b) is used. The change in capacitance rather than the nominal capacitance has to be measured, and, therefore, the nominal capacitance is compensated using the sinusoidal driving voltage. After compensation, the amplitude of the output voltage will be limited to the offset value when the mechanical measurand has a zero value, and it increases linearly with changes in capacitance. Moreover, as we have the intention of integrating the charge amplifier and the selection electronics at the border of the wafer, the bias resistors are grounded at one end. With this circuit configuration, it is possible to use a small bias resistor. SPICE simulations and breadboard measurements are carried out with the readout set-up described above. The SPICE a.c. analy-

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Fig. 6. (a) A more detailed diagram of the analog-signal process circuitry and (b) the measured characteristics.

sis shows the performance of the loop gain, the asymptotic gain for the transfer, and the direct transmission at high frequencies of the driving voltage. Experimental results have been obtained from the measurement set-up shown in Fig. 5, which consists of a dummy 4 x 4 sensor array and a capacitance readout circuit. In this set-up, the capacitance selection is performed by DIP switches. The analog-signal process circuitry in detailed perspective is given in Fig. 6(a), and the measured readout characteristic of this circuitry is given in Fig. 6(b). The output of the charge amplifier is processed in a coherent detector to enable an A/D conversion to be performed for computer processing.

In this paper, it has been shown that surface-micromachined polysilicon bridges are suitable components to consider in the design of a tactile array. A capacitance readout and the selection electronics can, therefore, be integrated in the silicon substrate, whereas the sensor structure can be fabricated on top of this same substrate. Experimental measurements have been made of the readout characteristic, using a breadboarded test set-up. A six-bit accuracy in the readout electronics is easily achieved. Having solved the problems of a possible sticking of the bridges to the substrate in high-yielded arrays, the undercutting of protective nitride films during the oxide removal, and the buckling of the bridges, a single-sided processed tactile sensor structure without alignment problems and compatibility with integration processing is to be realized. References I A. Pugh (ed.), Robot Sensors, Vol. 2, Tactile and Non-vision, IFS Publications, U.K./Springer,

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