WO1997019572A1

WO1997019572A1 - Method for the production of a system comprising a substrate and a capacitive sensor which is arranged on the substrate

Info

Publication number: WO1997019572A1
Application number: PCT/NL1996/000462
Authority: WO
Inventors: Michael Pedersen; Piet Bergveld
Original assignee: Stichting voor de Technische Wetenschappen STW
Current assignee: Stichting voor de Technische Wetenschappen STW
Priority date: 1995-11-23
Filing date: 1996-11-21
Publication date: 1997-05-29
Anticipated expiration: 1998-05-23
Also published as: NL1001733C2; AU7590896A

Abstract

The invention refers to a method for the production of a system comprising a substrate and a capacitive sensor which is arranged on the substrate and is provided with at least one movable membrane, which comprises polymer material, and at least two mutually separate electrodes, at least one of the electrodes being connected to the at least one membrane. The method according to the invention is characterized in that it comprises the steps of (i) forming the membrane in situ on the substrate by coating the substrate with a solution of the polymer material or a precursor thereof in a suitable solvent; and (ii) forming the membrane in a suitable manner by essentially removing the solvent.

Description

Method for the production of a svstem comprising a substrate and a capacitive sensor which is arranged on the substrate.

The present invention relates to a method for the production of a system comprising a substrate and a capacitive sensor which is arranged on the substrate and is provided with at least one movable membrane, which comprises polymer material, and at least two mutually separate electrodes, at least one of the electrodes being connected to the at least one membrane. The advantage of the integration of the sensor in the substrate is the resultant short signal path between the electrodes of the sen¬ sor and the electrical circuits integrated in the substrate. Said integration is particularly advantageous in the case of capacitive sensors, with which the electrical signal is a measure of the change in capacitance between the electrodes, which in general is very small. Moreover, the capacitive sensor has a high output impedance, as a result of which the electrical load of the parasitics in the electri¬ cal connection between the capacitive sensor and the integrated elec¬ trical circuits on the substrate is highly critical. A short signal path reduces said electrical load.

In general, use is made of an IC process, such as CMOS, for the manufacture of a substrate including integrated electrical circuits. IC processes of this type comprise one or more high temperature steps {T > 1000 °C) . The sensor material must be able to withstand these high temperatures in order to be able to arrange the sensor on the substrate before integration of the electrical circuits. Different problems again play a role in the case of integration of the manufac¬ turing process for the sensor in the IC process. Specifically, the majority of layers which are used in an IC process are not suitable as a component of mechanical sensors, in particular with regard to me¬ chanical stress and material parameters. On the basis of the preceding it must, therefore, be concluded that the sensor must be arranged on the substrate after the integrated electrical circuits have been com¬ pleted thereon, in order to obtain an optimum result. The use of polymer material in the membrane makes it possible to apply the sensor on the substrate without damage being caused to elec¬ trical circuits previously arranged thereon.

A method of the type mentioned initially is known from US Patent No. 5208789. In the known method a metallized diaphragm, consisting of a polyester film which is gold-metallized, is tensioned for bonding to the substrate by means of an adhesive. Once the diaphragm or membrane is bonded to the substrate further processing steps are necessary, e.g. to locally remove part of the membrane in order to make an elec¬ trical connection to the electronics on the substrate. Such additional processing steps are time consuming and heighten the production costs. These disadvantages are especially of importance when considering large production volumes. The aim of the present invention is to provide a method of the type described initially with which the sensor can be arranged on the substrate, which may or may not have been provided with integrated circuits, with the aid of a "post-processing" treatment step without having the disadvantages of the method known in the art. To this end the method according to the present invention is characterized in that it comprises the following steps:

(i) forming the membrane in situ on the substrate by coating the substrate with a solution of the polymer material or a precursor thereof in a suitable solvent; and (ii) forming the membrane in a suitable manner by essentially removing the solvent.

When compared to the known method the method according to the invention can be performed with a higher degree of accuracy, among others because micromachining technology can be used, thereby allowing for better product tolerances and/or smaller structural dimensions. The method according to the invention is furthermore extremely sui¬ table for the production of large volumes, among others since the membrane can be formed such that connection points are locally avai¬ lable for electrical connection to the electronics on the substrate. Additionally it must be noted that in the known method a silicon dioxide layer is applied to the substrate which will be charged and used as an electret to improve the properties of the sensor, which is necessary because of the relatively large air gaps present therein. In order to charge the electret a high-voltage has to be applied, which brings about the risk that the electronics already arranged on the substrate will be changed and/or destroyed. The use of an electret in combination with the required high-voltage charging step is not neces¬ sary when applying the method according to the invention. In an advantageous embodiment of the system according to the present invention, the polymer material is photosensitive polyimide. Photosensitive polyimide offers the advantage that it can simply be patterned on to the substrate. The treatment step involving arrange- ment of the sensor on the substrate is appreciably less complex as a result.

The invention will be explained in more detail below with refer¬ ence to the drawings, in which:

Figure 1 shows a side cross-sectional view of a first preferred embodiment of a sensor made by means of the method according to the present invention;

Figure 2 shows a side cross-sectional view of a second preferred embodiment of a sensor made by means of the method according to the present invention; Figure 3 shows a side cross-sectional view of a third preferred embodiment of a sensor made by means of the method according to the present invention; and

Figures 4 to 6 show a number of steps which form part of a pre¬ ferred embodiment of the method according to the present invention. In the figures, corresponding parts are designated by correspon¬ ding reference numerals.

Figure 1 shows a first preferred embodiment of a capacitive sen¬ sor 1 made by means of the method according to the present invention. In the preferred embodiment shown, sensor 1 comprises two membranes 5 and 8 made of polymer material. Membranes 5 and 8 are placed on a substrate 2 made of, for example, semiconductor material. Substrate 2 is provided with an insulating layer 3_t for example silicon oxynitri- de. Substrate 2 can further comprise electronic circuits (not shown). Membranes 5 and 8 are separated from one another by a gap 6. An elec- trode 4 and 7. respectively, is fixed beneath each of the membranes. Said fixing is such that at least one electrode is connected to each membrane. It will be clear to a person skilled in the art that the electrodes H and 7 can also be fixed on top of the respective mem¬ branes 5 and 8. Electrodes 4 and 7 preferably comprise a metal sand- wich, such as chromium-gold-chromium. The sensor electrodes can be connected in a known manner to electrical circuits (not shown) inte¬ grated in the substrate. Said connection can be made either during or after arrangement of the sensor on the substrate. The connection can, for example, be produced with the aid of 'bonding' techniques, such as welding of one or more connection wires at one end to one or more electrodes of the sensors and at the other end to the contact layers, present on the substrate, providing contact with the integrated cir- cults.

The capacitive sensor 1 functions as follows. When the sensor is in use, a voltage is applied between electrodes 4 and 7. which, on the basis of their mutual positioning, form a capacitor. Each mechanical movement of the membranes 5 and 8 can now be measured as a change in the capacitance between the two electrodes k and 7> It will be clear that such a change in capacitance can be measured in accordance with various, generally known methods. Each external force which causes a movement of the membranes 5 and 8 can thus be measured with the aid of sensor 1. In the embodiment shown, which is provided with two mem- branes, the sensor according to the present invention is outstandingly suitable for use as a microphone. In this case the relatively thinner membrane 5 will vibrate more than the relatively thicker membrane 8.

Figure 2 shows a second preferred embodiment of a capacitive sensor. In the case of sensor 20, the opening 12 in substrate 2 is smaller than that in the first embodiment. Consequently, the membrane 5 cannot be made to vibrate and only membrane 8 vibrates when the sensor is in use. Membrane 5 can therefore be replaced by an insula¬ ting layer of a suitable material, which does not have to comprise a polymer material. Figure 3 shows a third embodiment of a sensor according to the invention. Substrate 2 of sensor 30 now forms a whole without any opening. In use, only membrane 8 will vibrate. Membrane 5 can thus be replaced by an insulating layer, as has been described above with reference to Figure 2. Sensors 20 and 30 are outstandingly suitable as tactile or touch sensors and as acceleration sensors. Sensor 20 is, furthermore, also suitable as a pressure sensor.

In order to improve the associated frequency characteristics, the upper and/or the lower membrane of the sensor is preferably provided with holes 9- The sensor acquires a better response at different fre¬ quencies as a result. The holes 9 help to increase the sensitivity of the sensor because they lead to a reduced rigidity of the membrane in which they have been made. According to the invention, a preferred embodiment of a method for the production of a system as described earlier comprises the following steps:

(a) depositing a first electrode on the substrate; (b) depositing an insulating layer on the electrode;

(c) depositing an auxiliary layer on the insulating layer;

(d) depositing a second electrode, partially on the auxiliary layer and partially on the insulating layer;

(e) arranging a first membrane comprising a polymer material on the second electrode according to the following steps:

(i) forming the membrane in situ on the substrate by coating the substrate with a solution of the polymer material or a precursor thereof in a suitable solvent; and

(ii) forming the membrane in a suitable manner by essentially removing the solvent; and

(f) removing the auxiliary layer.

A system which has been produced in accordance with this method comprises an insulating layer, as described above with reference to Figures 2 and 3. instead of the first membrane 5. In a further embodiment the method according to the invention comprises the following steps:

(a) depositing a first electrode on the substrate;

(b) arranging a first membrane, comprising a polymer material, on the first electrode according to steps (i) and (ii) as described above;

(c) depositing an auxiliary layer on the first membrane;

(d) depositing a second electrode, partially on the auxiliary layer and partially on the first membrane;

(e) arranging a second membrane, comprising a polymer material, on the second electrode according to steps (i) and (ii) as described above;

(f) removing the auxiliary layer; and

(g) removing part of the substrate.

It will be clear that the first electrode 4 cannot be arranged directly on the substrate because of the risk of short-circuiting.

Consequently, an insulating layer 3 must first be arranged on the substrate. However, such an insulating layer is present as standard in the case of a substrate provided with integrated circuits. A preferred embodiment of the method according to the invention will be explained in more detail with reference to Figures 4 to 6.

Figure 4 shows a possible first step in the method for arranging sensor 1 on substrate 2. Substrate 2 can already have been provided with electronic circuits (not shown) . Substrate 2 has an insulating layer 3- Using an etching mask 11, which is known per se, substrate 2 is etched in the region where the sensor 1 is to be arranged. Etching can be carried out in a known manner and is preferably carried out with the aid of a chemical solution which contains potassium hydroxide (KOH) . During etching, the thickness of the substrate 2 is reduced in the sensor region from approximately 400 μm at 2 to approximately 50 μm at 2a. During this step the substrate retains sufficient stability for the subsequent treatment. It is pointed out that the procedural step described above can also be carried out as an intermediate step or as one of the final procedural steps.

Figure 5 shows a further step in the method according to the present invention. In this step a first electrode 4, preferably made of metal, is deposited on layer 3- A "lift-off" process with the aid of a photosensitive resist layer is preferably used for this purpose, instead of an etching process. Etching of the metal layer is avoided as a result. By making use of a layered construction of chromium-gold- chromium, resistive vapour deposition can be used for the deposition of electrode 4. A layer of polymer material is then applied to elec¬ trode 4 and patterned in the form of a membrane. According to the invention, the membrane can be produced from any suitable polymer, preferably thermoplastic material, which can be processed in a manner known per se to produce a membrane.

Examples of such materials will be known to those skilled in the art and/or are available commercially. Preferably, a thermoplastic polymer material will be used which has a softening temperature (T_s) higher than the envisaged use tempera¬ ture, preferably 100 °C higher than the use temperature. As the use temperature is generally room temperature, T_s is preferably > 115 °C.

A second requirement in respect of the polymer material used can be formulated on the basis of the curing temperature (T_c) . T_c must be sufficiently low to prevent any electrical circuits present on the substrate from melting or being otherwise adversely affected by the application of the sensor. A suitable limiting value temperature for T_c is therefore the lowest value of the melting temperatures of the materials from which the electronic circuits are made. When a CMOS process has been used for the integrated circuits, T_c is preferably < 660 °C: the melting temperature of aluminium. More preferentially, 0 °C < T_c < T_c' , where T_c' is the lowest value of the softening tem¬ peratures of the materials used in the integrated circuits.

The membrane can be produced and bonded to the substrate/arranged on the substrate in any suitable manner. Preferably, however, the membrane is formed in situ on the substrate by coating the substrate with a solution of the polymer material or a precursor thereof in a suitable solvent and then forming the membrane in a suitable manner by removing the solvent, for example by cross-linking, and/or forming the polymer material, for example by a chemical method by curing or a polymerisation reaction, optionally with the formation of cross-link- ages.

According to the invention, use is preferably made of polyimide material for applying the first and/or the second membrane. In a pre¬ ferred embodiment of the method according to the invention, the depo¬ sition of a layer of polyimide material comprises the following steps: (1) the application of a solution of the polyimide material in a suitable solvent on a substrate with the aid of a spinning move¬ ment;

(2) removal of the solvent;

(3) the application of a photosensitive resist layer on the layer of polyimide material with the aid of a spinning movement;

(4) patterning the photosensitive resist layer and the layer of poly¬ imide material;

(5) removal of the photosensitive resist layer; and

(6) curing the polyimide material. The following non-limiting example serves to illustrate this preferred embodiment:

A. In this example 'standard¹ polyimide material is used, preferably Probimide* from Ciba-Geigy or PI* from DuPont. First of all, in step 1, the polyimide material is applied in dissolved form, the solvent used preferably being N-methylpyrrolidone (NMP), to the substrate with the aid of a spinning movement. In this procedure, the polyimide solution is applied to the substrate while the substrate is rotated, preferably at about 4000 revolutions per minute for about 2 min. This gives rise to a uniform layer of polyimide material over the substrate. In step 2 the polyimide material is then heated to 80-100 °C, preferably 90 °C, for 20 to 30 minutes, preferably 2 minutes. In step 3 a photosensitive resist layer is then applied over the polyimide layer with the aid of a spinning movement, using a procedure analogous to that described in step 1. The photosensitive resist layer used is preferably the SlδXX series from Shipley. In step 4 the system is then heated for 10-20 minutes, preferably 1 minutes, to 80-100 "C, pre¬ ferably 90 °C. The photosensitive resist layer is then at least par- tially exposed to light in a controlled manner, preferably with the aid of a photolithographic mask. For this step use can be made of standard techniques which are known from IC technology. Under the influence of the light, the exposed areas of the photosensitive resist layer will start to depolymerise. The photosensitive resist layer is then developed. The developers used are preferably MF3 1 or MF312 from Shipley. It can be pointed out that the polyimide material also dis¬ solves in this developer, as a result of which the polyimide layer assumes approximately the same pattern as the photosensitive resist layer. In step 5 the photosensitive resist layer is then removed, preferably using acetone. As the polyimide material does not dissolve in acetone, the pattern thereof remains intact. Finally, in step 8, the polyimide layer is cured by baking the latter for 50 - 70 minutes, preferably 1 hour, at 300 °C < T_c < 400 °C, preferably T_c = 300 ^°C. This latter step is carried out to remove the water molecules from the layer and to force a chemical reaction, as a result of which very long polymer chains are formed. This process is also termed "curing".

As an alternative to 'standard' polyimide material, use can be made of photosensitive polyimide material. In a further preferred embodiment of the method according to the invention, the deposition of a layer of photosensitive polyimide material comprises the following steps:

(1) the application of a solution of the photosensitive polyimide material in a suitable solvent on a substrate with the aid of a spinning movement; (2) removal of the solvent;

(3) patterning the layer of photosensitive polyimide material; and

(4) curing the photosensitive polyimide material.

The following non-limiting example serves to illustrate this further preferred embodiment:

B. In this example photosensitive polyimide material is used, pre¬ ferably HTR3-200* from OCG Microelectronics. This is a solution which contains photosensitive polyimide material of the PMDA/ODA system and the solvent NMP. (PMDA/ODA = pyromellitic acid dianhydride/oxydianili- ne) . For application of HTR3-2O0 , first of all use is made of steps 1 and 2 which have been described above in relation to 'standard' poly¬ imide material. In step 3 the polyimide material is then first of all exposed to light. The techniques used for this purpose can be the same as those used in the case of the photosensitive resist layer in example A. The polyimide material is then heated to 80 C - 100 C, preferably 90 °C, for 8 - 12 minutes, preferably 10 minutes. This step is carried out to increase the polymerisation in the parts of the polyimide layer exposed to light. The polyimide material is then deve- loped. The developer used is preferably HTRD-2* from OCG Microelectro¬ nics. The parts of the polyimide material exposed to light are now rendered insoluble. Step 4, finally, comprises baking the polyimide layer. This latter step is analogous to curing step 6 in example A.

It can be pointed out that the application of a layer of photosensitive polyimide material is simpler than the application of a layer of 'standard' polyimide material. This is due, inter alia, to the fact that the photosensitive polyimide material can be patterned in a simpler manner. Both polyimide materials have the advantage that curing can take place at a relatively low temperature (T_c = 300 C) . An auxiliary layer 10 is then deposited on the polymer layer 5. Auxiliary layer 10 preferably consists of aluminium. The aluminium auxiliary layer can be applied with the aid of the known DC magnetron sputtering technique. This known technique is usually also used for the deposition of aluminium interconnection layers in CMOS techniques and is therefore directly compatible. Other materials, such as photosensitive resist, are also suitable in this context. In general, all materials which are removable without damage to the sensor or components thereof can be used for auxiliary layer 10.

Figure 6 shows yet further steps in the method according to the invention. In this figure it can be seen that a second electrode 7 has been deposited, partially on auxiliary layer 10 and partially on mem¬ brane 5- What has been described above with reference to the first electrode 4 applies in respect of the choice of material for the second electrode 7 and in respect of the process for the deposition thereof. A second layer of polymer material 8 is then applied, analogously to layer t in the form of a membrane on the second elec¬ trode 7. On the basis of Figure 6, the end product of the method according to the invention can finally be obtained (see Figure 1). With the aid of a further etching step, an opening 12 has been made in substrate 2, which opening is sufficiently large to render membrane 5 movable. Said further etching step can be carried out in a known manner, for example with the aid of reactive ion etching techniques, it being possible to use electrode 4 as etching stop.

Auxiliary layer 10 has been removed, as a result of which a gap 6 has been produced, such that the membranes 5 and 8 are able to move freely. The removal of auxiliary layer 10 can be effected with the aid of known etching techniques. It will be clear that only etching tech¬ niques which do not attack the sensor and, in particular, the polymer material can be used for this purpose. When auxiliary layer 10 is made of aluminium, etching solutions, such as a solution of H₃PO₄ in water, can safely be used. In general, at least one hole 9 s made in the second membrane 8 or in, respectively, the insulating layer or the first membrane for the purposes of removal of the auxiliary layer, which hole is such that an opening from the outside to the auxiliary layer 10 is pro¬ duced. Optionally, further holes can be made, as shown in Figures 1 to 3- When the system is in use, said further holes facilitate the outflow of air from gap 6 and consequently improve the functioning of the sensor according to the invention. Holes 9 can be made in a known manner, for example in the patterning step of the method described.

The method has been described above with a view to the production of a capacitive sensor having two membranes. In the case of the pro¬ duction of a capacitive sensor having only one membrane, the final step described, that is to say the removal of part of substrate 2, can be dispensed with. Layer 5 then has the function of an insulating layer and therefore does not necessarily have to be made of polymer material.

The two electrodes 4 and 7 can, if desired, consist of several electrodes in order, in this way, to refine the sensitivity of the capacitance measurement. Several capacitance values can be measured with a sensor of this type. This has the advantage that signal dis¬ turbances can be eliminated. In the case of a pressure sensor it is possible, for example, to eliminate signal disturbances caused by drift phenomena in this way. In the embodiments described insulating layer/membrane 5 prefe¬ rably has a thickness of < 1 um, whilst membrane 8 preferably has a thickness of 1 to 20 μm. More generally, the criterion for the thick¬ ness of the two layers 5 and 8 is preferably that said thickness is between 0.1 and 100 μm. Gap 6 preferably has a depth of 2 μm. In Fi- gure 1 the smallest horizontal dimension of opening 12 parallel to electrode 4 is preferably 2 mm. These dimensions are provided only for the purposes of illustration and are in no way intended to restrict the invention. For the sake of clarity, the associated figures have not been drawn to scale. Furthermore, it can be pointed out that the selection of a poly¬ mer material for the formation of one or more membranes in a capacitive sensor also appreciably simplifies the method for the pro¬ duction of said sensor, compared with the known method. The method according to the present invention is thus also advantageous in the case of the arrangement of a sensor on a substrate on which no elec¬ tronic circuits have been arranged.

Numerous embodiments and variants of this invention will occur to those skilled in the art. The invention is, for example, not restricted to the use of polymer material, but also comprises the use of materials of comparable mechanical and electrical properties. The present invention is, moreover, not restricted to the embodiments described and illustrated but comprises any embodiment which is con¬ sistent with the above description and the associated drawings and which falls within the scope of the appended claims.

Claims

1. Method for the production of a system comprising a substrate and a capacitive sensor which is arranged on the substrate and is provided with at least one movable membrane, which comprises polymer material, and at least two mutually separate electrodes, at least one of the electrodes being connected to the at least one membrane, characterized in that the method comprises the following steps:

(ii) forming the membrane in a suitable manner by essentially removing the solvent.

2. Method according to claim 1, comprising the following steps: (a) depositing a first electrode on the substrate;

(b) depositing an insulating layer on the electrode;

(c) depositing an auxiliary layer on the insulating layer;

(d) depositing a second electrode, partially on the auxiliary layer and partially on the insulating layer; (e) arranging a first membrane comprising a polymer material on the second electrode according to steps (i) and (ii); and

(f) removing the auxiliary layer.

3. Method according to claim 1, wherein the capacitive sensor comprises two membranes, at least one electrode being connected to each membrane, which method comprises the following steps:

(a) depositing a first electrode on the substrate;

(b) arranging a first membrane, comprising a polymer material, on the first electrode according to steps (i) and (ii) ; (c) depositing an auxiliary layer on the first membrane;

(e) arranging a second membrane, comprising a polymer material, on the second electrode, according to steps (i) and (ii); (f) removing the auxiliary layer; and

(g) removing part of the substrate.

4. Method according to claim 2 or 3_ι wherein at least one hole is made in the second membrane or in, respectively, the insulating layer or the first membrane for the purposes of removal of the auxiliary layer, which hole is such that an opening from the outside to the auxiliary layer is produced.

5. Method according to claim 3 or 4, wherein step g is carried out in such a way that the first membrane becomes movable.

6. Method according to claim 2, 3. 4 or 5. wherein the polymer material is polyimide material.

7. Method according to claim 6, wherein the arrangement of a membrane comprising polyimide material comprises the following steps:

(1) the application of a solution of the polyimide material in a suitable solvent on a substrate with the aid of a spinning movement;

(2) removal of the solvent;

(4) patterning the photosensitive resist layer and the layer of polyimide material;

(5) removal of the photosensitive resist layer; and

(6) curing the polyimide material.

8. Method according to claim 2, 3. 4 or 5. wherein the polymer material is photosensitive polyimide material.

9. Method according to claim 8, wherein the arrangement of a membrane comprising photosensitive polyimide material comprises the following steps: (1) the application of a solution of the photosensitive polyimide material in a suitable solvent on a substrate with the aid of a spin¬ ning movement;

(2) removal of the solvent;

(3) patterning the layer of photosensitive polyimide material; and

(4) curing the photosensitive polyimide material.