ITTO20090269A1 - IMPROVEMENT IN TACTILE SENSOR DEVICES BASED ON INTEGRATION BETWEEN A PIEZOELECTRIC TRANSDUCER FILM AND A LOCAL CONDITIONING FET TRANSISTOR - Google Patents

Description

â € œPerfections in tactile sensor devices based on the integration between a transducer piezoelectric film and a local conditioning FET transistorâ €

DESCRIPTION

The present invention relates to tactile sensor devices, in particular sensor devices which use piezoelectric polymers as force / pressure transducers, directly coupled to field effect semiconductor transistor devices for local signal conditioning.

More specifically, the invention relates to a method for manufacturing a sensor device according to the preamble of claim 1, and a sensor device according to the preamble of claim 10.

In these devices the ability to generate an electric charge proportional to the force / pressure applied is exploited to detect and quantify a contact force, for example of a humanoid robot interacting with the environment in which it finds itself, for recognition and the manipulation of objects, or the exploration of an environment.

Typically, the design of arrays of tactile sensor devices intended for installation on a robot is affected by system constraints, including the limited space available on the fingertips of a mechanical hand, for example, the need for a rapid response, need to minimize wired connections. To meet these requirements and improve overall sensor performance, an innovative sensory architecture is being established, based on the concept of "on-site detection and processing", better illustrated in figure 1.

On a contact area, an array A of sensors SENS comprises a plurality of pairs of adjacent modules, respectively a TRASD transducer module for detecting a local force or pressure and a conditioning module PROC of the electrical signal representative of the detected physical quantity. The individual pairs of detection and conditioning modules are connected to each other and to a central electronic control and data processing unit (not shown) by means of respective buses made on the supporting substrate of the array.

In each SENS sensor module a film of piezoelectric material is able to generate an electric charge (or potential difference) proportional to the local mechanical force or stress applied to it, according to the relationship

Q = d F

where Q is the electric charge generated by the applied force F and d is the piezoelectric constant of the material.

By associating a piezoelectric film to a respective gate terminal of a field effect transistor, it is possible to modulate the charge in the channel region of the transistor as a function of the charge / voltage induced in the piezoelectric film, generating an electrical signal representative of the one directly detectable at the heads of the transducer element, for subsequent processing at a remote control unit.

The configuration currently adopted is shown in figure 2 and is based on an extended gate architecture.

On a substrate (typically of Silicon) a semiconductor field effect transistor FET is formed, of which in the figure the source S and drain D contacts are schematically represented, between which the conduction channel region C is formed, whose extension is controlled through a gate electrode G. The gate terminal is extended over a device-free substrate region, from which it is separated by an oxide layer OL common to the transistor structure. The extended gate metallization, indicated by EG in the figure, is therefore topped at least in part by a layer of piezoelectric material PL, for example formed by a polymeric film such as PVDF (polyvinylidene fluoride) or its copolymer P (VDF-TrFE) (poly (vinylidene fluoride - trifluoroethylene)), coupled to the extended gate EG by interposition of an adhesive epoxy layer EL. A superior ML metallization completes the transducer device associated with the FET transducer.

The extended gate architecture resulted in significant performance improvements over traditional approaches where the transducer and processing electronics were built as separate entities.

However, the advantages of this architecture are limited by drawbacks linked to the presence of the epoxy layer, to the high substrate capacity and to cross-talk effects between neighboring sensor devices.

Figure 3 shows the equivalent model of the device of Figure 2, in a common source transistor configuration.

It is evident that both the epoxy layer EL, used as an adhesive to bond the polymeric film to the extended gate EG, and the oxide layer OL below the extended gate EG introduce additional capacities in the equivalent circuit.

The relationship between the voltage Vg available at gate G and the voltage Vpolymerai terminals of the piezoelectric transducer, due to the pressure stimulus, is therefore given by the following relationship:

where It has the intrinsic voltage amplification / gain of the transistor.

It is evident that high capacities of Csube substrate of the Cadhesiv epoxy layer contribute to an undesired attenuation of the voltage at the gate. This effect is all the more significant when the substrate capacity is greater than the capacity of the Cpolymer polymer piezoelectric layer.

The present invention therefore has the aim of providing a satisfactory solution to the problems described above, by providing an improved tactile sensor device.

In particular, the object of the present invention is to increase the overall sensitivity of the tactile sensor device and its response speed to the stimulus, hence the range of operating frequencies, and the spatial resolution of sensor arrays based on the device.

A further object of the invention is to provide an innovative manufacturing process for an improved tactile sensor device.

According to the present invention, these objects are achieved thanks to a process for manufacturing a tactile sensor device having the characteristics recalled in claim 1, and an integrated tactile sensor device having the characteristics recalled in claim 10.

Particular manufacturing methods are the subject of the dependent claims, the content of which is to be understood as an integral or integral part of this description.

A further object of the invention is a tactile sensor array for a humanoid robot comprising a plurality of sensor devices according to the invention.

In summary, the present invention is based on the principle of realizing an integrated sensory architecture where the layer of piezoelectric transducer material is arranged in correspondence with the gate electrode of the associated signal conditioning transistor, substantially in correspondence with the active gate area or effective, or of dimensions congruent or similar with the channel region of the transistor, and not on a free substrate area adjacent to the transistor region. The transistor in turn has an extended dimension, even of the order of 0.25 -1 mm <2> (while in general it can have somewhat smaller dimensions), so as to realize a tactile sensor device having a spatial resolution corresponding to that of human touch, with conditioning characteristics of the transducer signal higher than those obtainable according to the traditional extended gate configuration, where the dimensions of the transistor channel region are smaller.

Advantageously, the confinement of the piezoelectric transducer area to the active or effective gate region on the substrate of an array of sensors allows the realization of an array of sensor devices close to each other, separated by distances of the order of 0.5 mm without effects. of cross-talk between adjacent devices, increasing the spatial resolution of a sensor array, in the example up to a spatial resolution of 1 mm.

Even more advantageously, the presence of the piezoelectric material only in correspondence with the gate electrode area, therefore on a lower surface than in the case of an extended gate device, with the same thickness of the piezoelectric layer, determines a lower effective capacitance between transducer and transistor, hence a time constant RC of the lower integrated device.

The interconnections necessary to connect the extended gate metallization with the gate electrode of the FET transistor are absent, which contributes to a further reduction of the time constant.

Ultimately, the foregoing contributes to achieving the purposes of the invention of realizing a tactile sensor device having a linear response over a large range of dynamic forces acting on it.

As an alternative to the greater concentration and sensitivity of detection, with the same size and separation of the sensor devices, the configuration object of the invention would allow to obtain a space saving on the substrate, useful for the integration of accessory devices.

Conveniently, the presence of temperature sensor elements in association with the pressure sensor devices of an array allows the measurement of the local temperature at the point of contact, making the array multifunctional and allowing to compensate - in a subsequent processing - the variations in sensitivity of the material. piezoelectric with temperature.

An integrated sensor device as claimed can be obtained through a manufacturing process characterized by carrying out the configuration of the piezoelectric transducer layer before its polarization, which is preferably a thermal polarization, through reactive ion etching.

Advantageously, this makes the configuration of the piezoelectric layer more precise and uniform and does not affect the polarization condition of the material.

These characteristics make the device object of the invention, and the arrays of such devices, suitable for robotic applications with cognitive capabilities for carrying out exploratory tasks in an environment and the accurate manipulation of objects.

Further characteristics and advantages of the invention will be set out in more detail in the following detailed description of an embodiment thereof, given by way of non-limiting example, with reference to the attached drawings, in which:

Figure 1 is a schematic representation of a sensory architecture according to the on-site detection and processing technology;

Figure 2 is a schematic sectional representation of a sensor device according to the prior art, including a polymeric transducer and an associated signal conditioning transistor with extended gate metallization;

figure 3 is an equivalent circuit of the device of figure 2;

Figure 4 is a schematic representation of a sensor device according to the invention, including a polymeric transducer integrated with an associated signal conditioning transistor;

figure 5 is an equivalent circuit of the device of figure 4;

figure 6 is a topographical view from above of a sensor device object of the invention;

figure 7 is a succession of sectional views illustrating the manufacturing process of a sensor device according to the invention; And

figure 8 is a graph of three different timing models for the application of a bias voltage of the piezoelectric transducer module of the sensor device object of the invention.

Figures 1-3, which respectively represent the logical principle scheme of an "on-site survey and processing" architecture, and the solution currently proposed in the literature, have been commented on in the introductory part of this description.

Figure 4 shows a SENS tactile sensor device object of the invention. Elements or components identical or functionally equivalent to those illustrated in Figure 2 have been indicated with the same references.

On a SUB substrate, in the example a p-doped silicon substrate, a semiconductor field-effect transistor FET is formed, of which in the figure the source S and drain D contacts are schematically represented, between which the conduction channel region C, whose extension is controlled through a gate electrode G possibly separated from it by an oxide layer common to the insulating layer OL, deposited laterally at the source and drain terminals for separation from the adjacent devices.

The gate electrode is surmounted by a layer of piezoelectric material PL, selected from the group comprising PVDF, PVDF-TrFE, ZnO, PZT, for example, preferably, a polymeric film of PVDF or P (VDF-TrFE), arranged substantially over the entire exposed gate metallization area (gate contact terminal). The piezoelectric material can be processed in situ, through a process of deposition, etching and polarization implemented locally according to an innovative sequence of operations, as described below, or a commercial polymeric film can be used, applied to the gate electrode and joined to it via adhesive. Polymeric piezoelectric materials are preferred due to the properties of flexibility and configuration in any desired shape. A superior ML metallization completes the sensor device.

Conveniently, the integrated device has a surface area of the order of 1 mm <2> or less and can be separated from adjacent devices by a distance of about 0.5 mm, forming an array reflecting fact, for spatial resolution and acuity. , the functional properties of human mechanoreceptors. In fact, human mechanoreceptors do not transfer raw data to the human brain, but carry out some sort of local preprocessing before data transmission for remote high-level processing.

A thin layer of polymeric material (for example, silicone) can be placed on top of the devices to protect them from damage in contact with objects, and contributes to the insulation of the piezoelectric material from the thermal variations of the external environment. The sensitivity of an array of tactile sensor devices thus protected can be increased if the polymeric layer has a structure with raised ribs on its internal side, i.e. facing the upper metallization layer of the transducers, to concentrate the stresses on the individual sensor devices. , in analogy with the structure of receptors buried in the human epidermis.

Such a device can be called POSFET (Piezoelectric Oxide Semiconductor Field Effect Transistor).

Figure 5 shows the equivalent model for small signal of the device of Figure 4, in a configuration of the common source transistor, which represents the worst case study. For simplicity, the source terminal and substrate are considered grounded.

The relationship between the voltage Vg available at gate G and the voltage Vpolymerai terminals of the piezoelectric transducer, due to the pressure stimulus, is therefore given by the following relationship:

The absence of the parasitic capacitances, present instead in the extended gate configuration of the known art, determines a greater ratio between the voltage Vg available at gate G and the voltage Vpolymerai at the ends of the transducer.

To obtain a greater voltage transfer from the transducer to the gate of the transistor, the capacitances of the transistor must be smaller than those of the transducer. The net capacitance seen by the transducer can be reduced by designing a transistor with a lower aspect ratio W / L (where W indicates the width and L the length of the channel zone), but a higher aspect ratio is instead desirable to obtain a higher voltage gain with reduced noise. It can be seen from the previous formula that the voltage transferred from the transducer to the gate also depends on the capacitance of the piezoelectric layer indicated herein Cpolymer. Higher voltage ratios can be obtained by increasing the capacitance of the polymer layer relative to the capacitance of the field effect transistor. Assuming an area WxL of the constant transistor region this can be achieved by reducing the thickness of the piezoelectric layer, which in turn leads to the reduction of the Vpolymer voltage.

A thin piezoelectric layer is also advantageous in the polarization operation of the piezoelectric material, if treated in situ, since the high voltages required for the polarization of thick layers can cause damage to the device as a whole.

Figure 6 is a topographical view from above of a sensor device object of the invention, in which the dimensions of the areas of the FET field effect transducer are highlighted.

A process for manufacturing an integrated sensor device according to the invention is subsequently described with reference to Figure 7.

The first six steps (7a-7f) are typical of a planar process according to the known art for the manufacture of n-MOS devices, comprising an alternation of lithographic steps for the definition of the surface configurations of the device regions and the subsequent surface treatment for doping or deposition of materials.

A transistor region comprising a p-doped well is defined on an n (n-Si) doped silicon substrate SUB (Figure 7a).

In the transistor region two regions of n diodes are defined (Figure 7b) suitable for forming the source and drain regions of a field effect transistor. The source and drain terminals are obtained by depositing metal (for example aluminum) within through holes made in an oxide layer OL, terminating in extended contact pads S and D (Figures 7c, 7d).

Subsequently (figures 7e, 7f), the source and drain pads are covered by a passivation layer PaL and a gate terminal is defined, in the particular example comprising a top gate pad G and a metal deposition inside a hole passing through the passivation and oxide layers and extending as far as proximate to the transistor region, so as to face out at the desired channel region.

Figures 7g and 7h represent the succession of the steps of the microfabrication process for the definition of the transduction module integrated in the field effect transistor, respectively the deposition of a layer of piezoelectric material PL on the metallized gate pad G, which thus acts from the lower electrode, and the realization of the metallization of the upper electrode ML of the transducer device, according to an innovative in situ treatment technique that will be described more fully below.

The layer of piezoelectric material, preferably a polymeric film of PVDF-TrFE in molar ratio 65:35, is first deposited by spin coating on the surface of the wafer on which the sensor array is manufactured. To meet the requirements of a subsequent polarization phase of the polymeric film in situ, this is deposited in a layer of the order of 2.5 microns and in any case not exceeding 7 - 8 microns.

The deposition of the polymeric film is followed by annealing, dry chemical etching and polarization in a rigorous temporal sequence, unlike the known technique where the polarization precedes the chemical etching step, with the drawback of a partial depolarization of the consequent polymer. to chemical attack and ultimately its lower sensitivity.

Furthermore, the adoption of a dry chemical etching technique, for example plasma etching, is advantageous compared to the wet chemical etching technique used up to now, since it allows to define the edges of the transducer device with greater precision. , and therefore, ultimately, it allows to obtain a better reproducibility and uniformity among several sensors of the same array.

Finally, the polarization is carried out through a thermal polarization technique, instead of the corona effect polarization of the known art, in order to obtain a more uniform polarization. To avoid damage to the devices, during the biasing operation, the gate, source and drain terminals of the transistor device and the substrate are kept at the same potential.

The top gate pad, corresponding to the lower electrode of the transducer device, is operationally connected to a potential reference only during the polarization operation of the piezoelectric material, while in the normal operating conditions of the integrated sensor device it is not electrically connected to other parts of the circuit for which the transistor device is used in the floating gate configuration.

In detail, to obtain polymeric films of uniform thickness on an array of sensor devices, a deposition by spincoating of a solution of PVDF-TrFE in RER 500 or in MEK with concentrations between 10% and 15% is carried out in three distinct phases per spin. rate and spin time, respectively a first phase at 500 rpm for 2s and a second phase at 600 rpm for 8s, in which the spin rate and spin time values are kept relatively low to achieve a uniform diffusion of the solution on the wafer, and a third phase at a rotation speed higher than 1000 rpm and proportional to the desired deposition thickness for a time of 30s. Note that the thickness of the polymer film decreases as the rotation speed increases. More generally, a deposition by spin-coating at speeds between 1000 and 4000 rpm for a total time of 30 seconds allows to reach thicknesses between 2 and 5 microns.

The heat treatment or annealing step is carried out before the metallization by evaporation of the upper electrode. It is effective for increasing the crystallization of the piezoelectric material, evaporating any residual particles of the solvent and removing local stresses generated during deposition. Preferably, the heat treatment takes place at a temperature between 100 and 120 ° C, reached in 1 hour, maintained for 3 hours with return to room temperature in 1 hour to slowly accommodate the stresses to the structure.

After the annealing operation, the wafer is preferably treated with HDMS to improve the adhesion of the polymer and the metal electrodes, typically Al / Cr or Au / Cr and preferably Au / Cr, deposited in vacuum and configured with standard etching procedures wet chemical or selective deposition through masks.

The phase of chemical etching and configuration of the polymeric film differs from the traditional wet one applied to PVDF-TrFE film, since this has the drawback of not defining the edges of the device with precision and neatness, resulting in variations in the area and of the aspect ratio between sensors of the same array and difficulty in the subsequent polarization operation of the material, especially with thicknesses of the order of a few microns. Furthermore, during polarization the presence of poorly defined edges causes the ignition of electric arcs that can cause damage to the structure of the film.

A dry chemical attack technique was adopted in the presence of oxygen (20 sccm) and helium (40 sccm) at 20 mBar, acting at a chemical attack speed of 1 micron per minute. The upper metallization electrode advantageously acts as a mask for the etching.

The polarization phase is finally necessary to improve the piezoelectric properties of the polymer film, but must be carried out with caution in order not to damage the underlying active device (transistor) during the application of a high electric field through the thickness of the film (up to a maximum of 100V / micron). This problem is characteristic of the integrated embodiment of the sensor device object of the invention, and does not arise for extended gate architectures where there is no active device (transistor) below the transducer region subjected to polarization.

Among the techniques reported in the literature, thermal polarization is preferred by applying a polarization voltage to the transducer electrodes, advantageously at a high temperature, for example in the order of 80-100 ° C, an interval within which the temperature falls. phase transition (from paraelectric to ferroelectric behavior) of PVDF-TrFE. Figure 8 shows three different models for applying the bias voltage. In model I a voltage of 250 V (for a 2.7 micron thick film) is applied incrementally in steps over a finite number of steps, each for a duration of 10 minutes, interspersed with a short circuit period. 5 minutes to neutralize any excess charges. Such a multi-stage process reduces the chances of electrical break down. In model II a voltage of 250 V is reached progressively in 15 minutes, is maintained applied for a duration of 10 minutes and is progressively canceled in 5 minutes. In model III, similar to the standard technique, a voltage of 250 V is applied directly for a duration of 10 minutes.

Advantageously, model I allows to minimize the occurrence of electric arcs along the edges and makes it possible to monitor the bias current during the process.

It should be noted that the embodiment proposed for the present invention in the preceding discussion has a purely illustrative and non-limiting character of the present invention. A person skilled in the art will be able to easily carry out the manufacturing process described for the production of sensor devices according to a traditional architecture with extended gate or of microelectromechanical devices.

Naturally, without prejudice to the principle of the invention, the embodiments and construction details may be widely varied with respect to what has been described and illustrated purely by way of non-limiting example, without thereby departing from the scope of protection of the invention defined by the appended claims.

Claims (14)

CLAIMS 1. Process for manufacturing a pressure sensor device (SENS) suitable for forming, in conjunction with similar devices on a common substrate (SUB), a tactile sensor complex (A) simulating in particular a human mechanoreceptor in a humanoid robot, and including: - pressure transducer means (TRASD) comprising a layer of piezoelectric material (PL) extended in correspondence with a predetermined contact area of the sensor device (SENS) with the environment, able to generate an electric charge signal indicative of the mechanical deformations undergone from the piezoelectric material; And - local signal conditioning means (PROC), which can be coupled to an external processing unit for controlling the sensor assembly (A), comprising a field effect semiconductor transistor device (FET) provided with a gate electrode (G) electrically coupled to said transducer means (TRASD), the procedure being characterized by the fact that it includes, in temporal sequence: - the realization on the substrate (SUB) of a transistor device (FET) having an exposed metalized gate area (G), suitable to constitute a first electrode of a piezoelectric cell associated with the device (FET), comprising said layer of material piezoelectric (PL); - the in situ deposition of a layer of piezoelectric material (PL) on at least said exposed gate area (G); - annealing of the deposited layer of piezoelectric material (PL); - subsequently, the deposition of a metallization layer (ML) on an area of predetermined configuration of the piezoelectric layer (PL), forming a second electrode of the piezoelectric cell; - subsequently, the dry chemical etching and removal of the excess piezoelectric material (PL) layer in correspondence of areas not covered by said metallization layer (ML); and finally - applying a uniform electric field between said first and second cell electrodes (G, ML) to obtain the polarization of the piezoelectric material (PL). 2. Process according to claim 1, in which said piezoelectric material is the copolymer P (VDF-TrFE), in the molar ratio 65:35 and its deposition occurs by spin-coating of a solution comprised between 10% and 15 % of said copolymer, in a first step at a speed between 500 and 600 rpm for a duration between 5 and 10 seconds, and in a second step at a speed between 1000 and 4000 rpm for a duration between 25 and 35 seconds. 3. Process according to claim 1, wherein said piezoelectric material is the copolymer P (VDF-TrFE), in the molar ratio 65:35 and its annealing takes place at a temperature between 100 ° C and 120 ° C for a duration between 2 and 4 hours. Process according to claim 1, wherein said metallization layer (ML) forming a second electrode of the piezoelectric cell forms a mask for the subsequent dry chemical etching step of the excess piezoelectric material (PL) layer. 5. Process according to claim 1, comprising a thermal polarization of the piezoelectric material carried out at a temperature corresponding to the paraelectric-ferroelectric phase transition temperature of the piezoelectric material. The method according to claim 5, wherein the bias includes connecting the electrically conductive terminals (S, D) of the transistor device (FET) and the first electrode of the piezoelectric cell (G) to a ground potential, and applying of a predetermined difference in polarization potential between the second electrode of the piezoelectric cell (ML) and the first grounded electrode (G), adapted to establish an electric field through the piezoelectric material. 7. Process according to claim 6, wherein the polarization includes the application of an electric field of increased intensity in discrete steps up to a predetermined maximum intensity as a function of the thickness of the layer of piezoelectric material (PL), each step lasting including between 7 and 10 minutes, interspersed with a short circuit period lasting between 3 and 5 minutes. 8. Process according to claim 6, in which an electric field of predetermined intensity is applied as a function of the thickness of the layer of piezoelectric material (PL) for a duration of between 10 and 15 minutes. 9. Process according to claim 6, in which an electric field of predetermined intensity is applied according to the thickness of the layer of piezoelectric material (PL), achieved progressively in a time comprised between 10 and 15 minutes, maintained for a duration comprised between 10 and 15 minutes and canceled progressively in a time between 5 and 10 minutes. 10. Integrated pressure sensor device (SENS), adapted to form in conjunction with similar devices on a common substrate (SUB) a tactile sensor complex (A) simulating, in particular, a human mechanoreceptor in a humanoid robot, including: - pressure transducer means (TRASD) comprising a layer of piezoelectric material (PL) extended in correspondence with a predetermined contact area of the sensor device (SENS) with the environment, suitable for generating an electric charge signal indicative of the mechanical deformations undergone from the piezoelectric material; And - local signal conditioning means (PROC), which can be coupled to an external processing unit for controlling the sensor assembly (A), comprising a field effect semiconductor transistor device (FET) provided with a gate electrode (G) electrically coupled to said transducer means (TRASD), characterized in that said layer of piezoelectric material (PL) is extended over an area congruent with the area of the gate electrode (G), said gate electrode (G) forming a first electrode of a piezoelectric cell associated with the device and comprising said layer of piezoelectric material (PL); the device (SENS) including a metallization layer (ML) forming a second electrode of the piezoelectric cell, superimposed on the layer of piezoelectric material (PL) and extended over a congruent area, whereby said electric charge signal generated by the piezoelectric cell is directly applied to the gate electrode (G) of the transistor device (FET). 11. Device according to claim 10, wherein said piezoelectric material is selected from the group comprising PVDF, PVDF-TrFE, ZnO or PZT. 12. Device according to claim 10 or 11, comprising a polymeric protective insulating layer arranged above said pressure transducer means, carrying a plurality of raised formations facing the piezoelectric cell. 13. Tactile sensor array (A), comprising a plurality of pressure sensor devices (SENS) according to any one of claims 10 to 12. 14. Tactile sensor array (A), comprising a plurality of pressure sensor devices (SENS) obtainable through a manufacturing process according to any one of claims 1 to 9.