Combined laser drilling and plasma etching process for the production of a micromechanical device and micromechanical fixture

Abstract

Method for manufacturing a micromechanical device comprising the steps: (a) providing a micromechanical precursor product with a first substrate (11) and at least one first cavity (100), wherein the first cavity (100) is bounded at least by the first substrate (11), (b) applying an additional layer (9) to the first substrate (11), (c) laser drilling through the additional layer (9) and thus producing a mask, (d) laser drilling a first subsection (12) of the first substrate (11) through the mask, (e) plasma etching a second subsection (13) of the first substrate (11) through the mask and through the first subsection (12) such that an access (1) through the first substrate (11) to the cavity (100) is created, (f) laser melting of substrate material of the first subsection (12) and sealing the access (1) with melt.

Description

Prior ArtThe invention is based on a micromechanical device having a first substrate, having at least one first cavern, having a closed access to the first cavern, the access running through the first substrate.The document WO 2015 / 120939 A1 discloses a method for the targeted setting of an internal pressure in a cavity of a MEMS element. In this case, a narrow access channel to a MEMS cavity is produced in a cap wafer or in a sensor wafer. The cavern is flooded with the desired gas and the desired internal pressure via the access channel. The area around the access channel is then locally heated by a laser. The substrate material liquifies locally and hermetically seals the access channel during solidification.Such a method is usually used if one wishes to generate in a MEMS element having two caverns, wherein a different internal pressure is to be generated in the two caverns (FIG. 1 ).This is necessary if, for example, an acceleration sensor is to be combined with a rotation rate sensor. The acceleration sensor requires a high internal pressure for optimum mode of operation, whereas a rotation rate sensor rather requires a very low internal pressure. This method makes it possible to set the respective optimum internal pressure in each cavity.The method can also be used if a very low internal pressure in a cavern cannot be achieved, caused by outgassing during the process procedure, so this can be subsequently adjusted.In a discrete arrangement, in which only one MEMS element is realized and the evaluation circuit is provided separately, the generation of the access channel ( 1) can be made simultaneously with the clearing of the electrical contact surfaces ( 2). In such an arrangement, a relatively thin cap wafer is usually used. After the bonding of the cap wafer ( 3) to the sensor wafer ( 4), a narrow access opening to a first cavity and a large access opening to the contact surfaces can be produced in one step via a photomask and a trench method.German patent specification DE 102011103516 B4 proposes encapsulating a MEMS structure with a deposit a polycrystalline silicon layer and then producing an access channel into the polycrystalline silicon layer using a laser drilling process. A defined atmosphere is then set in the MEMS cavity and the access channel is closed by a laser closing process. The placement of the access channel is cost-effective compared to a pure trench process with which only one access channel is placed. A disadvantage is that in a laser drilling process smoke always arises which can affect the MEMS structures. It is also disadvantageous that the laser drilling process is not very selective for different materials. That is, regardless of the material, laser drilling not only creates an access hole through the polycrystalline silicon layer, but also simultaneously drill a hole into the underlying layer.Object of the InventionWhat is sought is a method or arrangement which allows an access channel to be produced cost-effectively in a thick substrate. The access channel should end in a cavern and there not generate any smoke and other contaminants in the production process. Also, once the method reaches the cavern, it is intended to stop and not drill further into the material lying under the cavern. Furthermore, the method is intended to produce very small access holes on the substrate surface, which access holes can be closed by a laser melting method.Advantages of the InventionThe invention is based on a micromechanical device having a first substrate, having at least one first cavern, having a closed access to the first cavern, the access running through the first substrate. The core of the invention consists in the fact that the access has a laser-drilled first subsection and a plasma-etched second subsection, the plasma-etched second subsection having an opening to the first cavity, the access in the first subsection being closed by a melt closure made of melt of at least the first substrate.An advantageous embodiment of the invention provides that the first substrate has an additional layer and the access is also closed by melting the additional layer.The micromechanical device is advantageously a hybrid integrated micromechanical device, the device having a second substrate having an ASIC circuit.The invention also relates to a combined laser drilling and plasma etching method for manufacturing micromechanical devices.Advantageously, the method according to the invention completely prevents smoke in the cavern. Advantageously, no expensive photomask is necessary for the plasma etching process. The laser drilling process generates the mask necessary for the trench process. Advantageously, the adjustment between the laser drilling process and the trench process takes place via the additional layer in a self-adjusting manner, an offset being ruled out. Advantageously, by combining two removal methods, namely laser drilling and trench etching, a higher overall aspect ratio for the access channel can be achieved. By suitable selection of the additional layer, it is advantageously possible to achieve smaller access openings in the laser drilling process. Advantageously, the additional layer can be used simultaneously to produce a more stable laser-research process, for example in that the material of the additional layer is easier to melt or forms a eutectic with the material of the substrate, in particular silicon. Advantageously, the combined laser drilling and plasma etching method according to the invention can be integrated with the defined introduction of a suitable atmosphere and the sealing at any point in the entire MEMS production process. In particular, this sequence of steps can also be integrated completely at the end of the wafer production process. Thus, the actual production process does not have to be changed.An advantageous embodiment of the method according to the invention provides that after step (c) the additional layer is removed. Advantageously, the additional layer can be removed immediately after step (c) or in a later method step.An advantageous embodiment of the method according to the invention provides that after step (e) an atmosphere with a specific composition and a specific pressure is set in the first cavern.An advantageous embodiment of the method according to the invention provides that in step (f) laser melting of material of the additional layer also takes place.Advantageously, the access is also closed by melting the additional layer.An advantageous embodiment of the method according to the invention provides that step (c) and / or step (d) is carried out substantially at atmospheric pressure.An advantageous embodiment of the method according to the invention provides that in step (c) the laser drilling is carried out with a first laser or also with first laser operating parameters, in particular with a very short wavelength or also very strongly focused or also with a very short pulse length, and that in step (d) the laser drilling is carried out with a second laser or also with second laser operating parameters which are different from the first laser operating parameters, in particular with a longer wavelength or also less strongly focused or also with a longer pulse length.An advantageous embodiment of the method according to the invention provides that in step (d) the laser drilling is first carried out with the first laser operating parameters to a certain depth and subsequently the laser drilling is carried out with the second laser operating parameters.DRAWINGFIG. 1 shows a micromechanical device having a cavity with a closed access in the prior art. FIGS. 2A to G show an exemplary embodiment of the combined laser drilling and plasma etching method according to the invention for producing a micromechanical device. FIG. 3 schematically shows the laser drilling and plasma etching method according to the invention for producing a micromechanical device.DESCRIPTION OF THE INVENTIONFIG. 1 shows a micromechanical device having a cavity with a closed access in the prior art. A micromechanical device having a MEMS wafer 4 and a cap wafer 3 is schematically illustrated. cap wafer 3 has an access channel 1, which is closed by a fusible closure 5. Electrical contact surfaces 2 are arranged in recesses of the cap wafer 3. The micromechanical device has a first cavity 100, which contains, for example, a rotation rate sensor, and a second cavity 200, which contains, for example, an acceleration sensor. The cavity 200 essentially contains an atmosphere with pressure and composition of the process gas during bonding of the MEMS wafer 4 to the cap wafer 3. The cavern 100 was evacuated through the access 1 and the access 1 was subsequently sealed by means of a melt seal 5. This ensures a high quality of the oscillator of the rotation rate sensor. The arrow indicates the direction of action of a laser closure process for producing the fusible closure 5.In a discrete arrangement of a micromechanical sensor or another micromechanical device, in which only one MEMS element is realized and the evaluation circuit or another electrical control circuit is provided separately, access channel 1 may be produced simultaneously with the clearing of electrical contact surfaces 2. In such an arrangement, a relatively thin cap wafer 3 is usually used. After bonding the cap wafer 3 to the sensor wafer 4, both a narrow access opening 1 to the first cavity 100 and a large access opening to the contact areas 2 can be produced in a common step via a photomask and a trench method.It becomes difficult to produce the access opening 1 to the first cavity 100 in the case of thick cap wafers. The access opening 1 must not become too large, since otherwise it is no longer possible to close it by local melting and subsequent solidification. Trench methods which produce very narrow accesses and at the same time extend very deeply, that is to say have a high aspect ratio, are difficult and become increasingly slower and more expensive as the aspect ratio increases.If the access 1 to the first cavity 100 is not produced simultaneously with the opening of the contact regions 2, a respective separate photomask must be provided for both processes. This is complicated and expensive and in some cases also technically difficult to implement.FIGS. 2A to G show an exemplary embodiment of the combined laser drilling and plasma etching method according to the invention for producing a micromechanical device. FIG. 2A first shows a provided wafer composite with a MEMS wafer and a substrate with an ASIC evaluation circuit. For applications of so-called hybrid integration, a second substrate 15 with an ASIC evaluation circuit 6 is directly combined at wafer level with a MEMS wafer 7 with a first substrate 11. Alternatively, a MEMS element is applied to the ASIC evaluation circuit and this wafer is encapsulated with a cap substrate. In order to obtain the most compact possible components, vias (trans silicon vias, TSVs) 8 can be provided through the ASIC substrate 15 or the MEMS substrate 11 in order to generate an electrical connection between the ASIC circuit, in particular a MEMS sensor evaluation circuit, and the environment. FIG. 2B shows the application of an additional layer 9 to the first substrate 11 A hole 10 is drilled into the additional layer 9 according to FIG. 2C using a laser drilling method with pulsed laser energy. Furthermore, as shown in FIG. 2D, the hole is then recessed into the material of the first substrate 11 below, likewise using a pulsed laser, and is advanced into a first subsection 12 of the first substrate 11. Before reaching the underlying first cavity 100, a plasma etching method, in particular a trench method, is switched over, as shown in FIG. 2E. The additional layer 9 serves as a mask for the plasma etching process. With this layer as a mask, the access channel 1 is etched by the plasma etching method through a second subsection 13 of the first substrate 11 as far as the first cavity 100, in which a MEMS functional element is located, for example. The arrows here identify the direction of action of a trench method. In the cavern, in particular on a wall of the cavern opposite the access channel 1, a stop layer 14 may optionally be provided, so that the plasma etching method no longer etches into the MEMS functional element or the ASIC circuit or into the second substrate.The cavern is then flooded with the desired gas and the desired internal pressure via the access channel. FIG. 2F shows a variant of the method according to the invention, in which the additional layer 9 is subsequently removed. The additional layer 9 can consist, for example, of oxide and be removed by etching. The arrows here identify the effect of an ore process for removing the additional layer 9. FIG. 2G shows how finally a region of the first substrate 11 around the access channel 1 is locally heated by a laser. The substrate material of the first substrate 11 liquifies locally and, on solidification, hermetically seals the access channel 1 in the laser-drilled first subsection 12 with a melt seal 5. Material of the additional layer 9 liquifies locally and, during solidification, likewise closes the access channel 1.FIG. 3 schematically shows the laser drilling and plasma etching method according to the invention for producing a micromechanical device, having the method steps: (a) providing a micromechanical precursor product having a first substrate 11 and at least one first cavity 100, the first cavity 100 being bounded at least by the first substrate 11, (b) applying an additional layer 9 to the first substrate 11, (c) laser drilling through the additional layer 9 and thus producing a mask, (d) laser drilling a first subsection 12 of the first substrate 11 through the mask, (e) plasma etching a second subsection 13 of the first substrate 11 through the mask and through the first subsection 12 in such a way that an access 1 is created through the first substrate 11 to the cavity 100, (f) laser melting substrate material of the first subsection 12 and closing the access 1 with melt.The essential process steps of the method according to the invention can be summarized as follows: 1. a MEMS wafer track is provided with at least one additional layer. 2. a blind hole is drilled into the additional layer and the substrate material by a laser drilling process. 3. With a plasma etching method, in particular with a trench method, the hole is etched further into the cavity. 4. Optionally, the additional layer can now be removed (FIG. 2F ). 5. the cavern is flooded with the desired gas and the desired internal pressure via the access channel. 6.The region around the access channel is locally heated by a laser, the substrate material liquifies locally and hermetically seals the access channel during solidification.Further Embodiments of the InventionIt is advantageous to carry out the laser drilling process with two different lasers or laser settings. The first laser drilling process is optimized to drill a hole in the overlay. For example, a laser with a very short wavelength can be deliberately used, which is very strongly focused and has a very short pulse length. Thus, a very small access hole can be formed in the additional layer. The second laser drilling process may be optimized to drill a hole in the substrate. In particular, the additional layer and the second laser can be combined in such a way that a part of the laser power is reflected. For example, a metal layer such as, in particular, aluminum can be used as an additional layer, which is combined with a laser wavelength, so that a large part of the light is reflected. Or a partially transparent layer such as oxide is used, but is selected in a thickness such that the laser light is largely reflected. By means of such an arrangement, the laser light can be spatially localized even further through the additional layer in addition to normal focusing, and it is thus possible to drill very small access channels even at a great depth. It is advantageous in this method that a laser with a larger wavelength and / or larger pulse length can also be used as the second laser drilling process, which permits faster drilling rates.Furthermore, it can be advantageous to drill a hole using the first laser drilling process not only into the additional layer 9, but also into the upper part of the first substrate 11, which is to be sealed later by the melting process. In the central part of the substrate, a second laser drilling process can then be used, which drill a larger opening and works faster for this. A narrow access opening in the upper substrate region is favorable for the subsequent closure process. It is therefore favorable to drill the first narrow bore into the substrate as deep or deeper than the depth of the melting region in the sealing process.It is further advantageous if the laser drilling process is first carried out in the air, i.e. at atmospheric pressure, in order to allow simple, cost-effective process management. The plasma etching process is then carried out in a vacuum chamber. Then, the wafer is directly sealed with the laser-residual process without bringing the wafer out of the vacuum apparatus. It is advantageous in this sequence that no impurities can enter the cavern during the subsequent aeration. Furthermore, moisture and other absorbing gases cannot enter the cavern. Some of these gases could only be removed by baking. Since no annealing is necessary in this process management, the method according to the invention can be carried out at any point, in particular also entirely at the end of the process chain for producing a micromechanical device. This is also possible, for example, if solder balls have already been placed on the wafer and therefore a temperature treatment is no longer possible.With the method according to the invention, the MEMS substrate can also be selected to be significantly thicker than the ASIC substrate. This is particularly favorable when the MEMS structures are mechanically coupled to the MEMS substrate.The method is particularly favorable for hybrid integrated MEMS elements.It is particularly favorable if a MEMS structure is provided on one substrate and an ASIC evaluation circuit is provided on a further substrate and the two substrates are bonded to one another. It is then advantageous to choose access through the substrate having the MEMS structure according to the described method. In this case, ASIC functional layers such as passivation layers (oxides) or wiring layers (Al or Cu) can be used as an etch stop layer for the plasma etching process without additional layers having to be applied in the system.In particular, the method is favorable for producing hybrid integrated MEMS elements with at least two caverns with different internal pressures.Furthermore, the method is favorable for producing hybrid integrated MEMS elements which are designed as bare die structures, i.e. are provided directly with solder balls and are not cast in plastic compound.List of reference characters1 Access channel 2 Electrical contact surfaces 3 Cap wafer 4 Sensor wafer 5 Fusible seal 6 ASIC wafer 7MEMS wafer 8 TSV 9 Additional layer 10 Laser-drilled hole in additional layer 11 First substrate 12 Laser-drilled first subsection of the access channel in the first substrate 13 Plasma-etched second subsection of the access channel in the first substrate 14 Stop layer 15 Second substrate 20 Solder ball 100 First cavity 200 Second cavity

Claims

Method for producing a micromechanical device, having the steps: (a) providing a micromechanical precursor product having a first substrate (11) and at least one first cavity (100), the first cavity (100) being bounded at least by the first substrate (11), (b) applying an additional layer (9) to the first substrate (11), (c) laser drilling through the additional layer (9) and thus producing a mask, (d) laser drilling a first subsection (12) of the first substrate (11) through the mask, (e) plasma etching a second subsection (13) of the first substrate (11) through the mask and through the first subsection (12), in such a way that an access (1) is created through the first substrate (11) to the cavity (100), (f) laser melting substrate material of the first subsection (12) and closing the access (1) with melt.Method according to Claim 1, characterized in that after step (c) the additional layer (9) is removed.Method according to claim 1, characterised in that after step (e) an atmosphere of a specific composition and a specific pressure is set in the first cavern (100).Method according to Claim 1 or 3, characterized in that in step (f) laser melting of material of the additional layer (9) is also carried out.Process according to any one of the preceding claims, characterized in that step (c) and / or step (d) is carried out substantially at atmospheric pressure.Method according to one of the preceding claims, characterized in that in step (c) the laser drilling is carried out with first laser operating parameters, in particular with a very short wavelength and / or very strongly focused and / or with a very short pulse length, and in that in step (d) the laser drilling is carried out with second laser operating parameters which are different from the first laser operating parameters, in particular with a longer wavelength and / or less strongly focused and / or with a longer pulse length.Method according to claim 6, characterised in that in step (d) the laser drilling is first carried out with the first laser operating parameters to a certain depth and subsequently the laser drilling is carried out with the second laser operating parameters.Micromechanical device having a first substrate (11), having at least one first cavern (100), having a closed access (1) to the first cavern (100), the access (1) running through the first substrate (11), characterized in that the access (1) has a laser-drilled first subsection (12) and a plasma-etched second subsection (13), the plasma-etched second subsection (13) having an opening to the first cavern (100), the access (1) in the first subsection (12) being closed by a melt closure (5) made of melt of at least the first substrate (11).Micromechanical device according to Claim 8, characterized in that the first substrate (11) has an additional layer (9) and the access (1) is also closed by melting the additional layer (9).Hybrid integrated micromechanical device according to either of Claims 8 and 9, characterized in that the device has a second substrate (15) with an ASIC circuit.

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