Patent Application
20150028433
The invention relates to the field of encapsulation structures including one or more microdevices encapsulated in one or more cavities in hermetic fashion, for example in a controlled atmosphere. The invention advantageously applies to the encapsulation of resonant microdevices of the MEMS (electromechanical microsystem) type.
The term “microdevice” designates any microsystem of the MEMS type, NEMS (electromechanical nanosystem) type and MOEMS (opto-electromechanical microsystem) type, together with any type of electronic, optical or opto-electronic component or microcomponent.
There are currently two major families of techniques of encapsulation, or “pre-packaging”, of microdevices enabling these microdevices to be protected, and in particular enabling the moving portions of these microdevices to be protected, whilst handling them and during subsequently implemented steps (cutting, packaging, etc.):
Bearing in mind the handling operations to which the microdevices encapsulated in this manner are subject, it is important that the caps protecting these microdevices have satisfactory mechanical resistance in order to resist the stresses caused by these handling operations.
Several types of mechanical reinforcement have been proposed to increase the robustness of these thin or thinned caps, which are subject, for example, during overmoulding steps, to pressures equal to or greater than 100 bars, which generally cause major deformations, which may cause the caps subject to such pressures to fracture.
Document FR 2 901 264 describes the production of mechanical reinforcements in the form of protrusions from the top wall of a cap obtained by deposition of one or more thin layers. However, this method is compatible only with the production of caps made of thin layers and not with the production of caps by wafer cap packaging. In addition, with such reinforcements made outside the cavity, there is a high risk that delamination may occur between the reinforcements and the cap, since if the assembly defined by the protruding elements is more mechanically rigid than the cap the forces exerted on the cap can then cause delamination between the cap and the reinforcements. Lastly, the reinforcements described in this document are not sufficient for the encapsulation structure to be able to withstand pressures higher than or equal to approximately 100 bars, which are the pressures experienced when implementing a step of overmoulding of the microdevice encapsulated in this manner.
Document FR 2 955 999 describes the mechanical reinforcement of a cap obtained by deposition of one or more thin layers, via a localised deposition of a portion of mechanical reinforcement material covering the cap. Although the mechanical reinforcement obtained is satisfactory in terms of mechanical resistance given to the cap, the continuous reinforcement area produced in this manner over the entire surface of the cap prevents access to the cap or to the microdevice, for example preventing checks or optical measurements on the interior of the cap or on the encapsulated microdevice. In addition, the residual stresses which are located at the interface between the reinforcement material and the encapsulation layer can have a deleterious effect on the mechanical properties of the interface. Lastly, this method is compatible only with production of caps made of thin layers and not with the caps formed by wafer cap packaging.
Lastly, in both mechanical reinforcement solutions described above, no solution is described to control the atmosphere in which the microdevice is encapsulated.
One aim of the present invention is to propose an encapsulation structure the cap of which is mechanically reinforced, which is compatible with production by thin film packaging or by wafer cap packaging, and the atmosphere of which can easily be controlled.
To this end, the present invention proposes a structure for encapsulation of at least one microdevice produced on and/or in a substrate and positioned in at least one cavity formed between a substrate and a cap rigidly connected to the substrate, in which the cap includes at least:
The mechanical reinforcement portions incorporated in this manner in the cap strengthen and greatly consolidate the encapsulation structure, and in particular make it resistant to the final stages of handling and of packaging which the encapsulation structure is intended to undergo. In addition, by producing these mechanical reinforcement portions in the inner wall of the cavity, with a material the Young's modulus of which is higher than that of the material of the layer of the cap, the production of such a cap is compatible both with thin film packaging and with wafer cap packaging of an advantageously thinned substrate. This encapsulation structure uses only elements compatible with conventional microelectronic techniques.
In addition, the mechanical reinforcement portions have gas absorption and/or adsorption properties, i.e. they include a getter function which will enable the atmosphere in the cavity to be controlled, both in terms of the nature and of the partial pressure of the residual gases in the cavity.
Such an encapsulation structure is advantageously used to protect microdevices, for example MEMS of the inertial type including moving parts (free or embedded beams, membranes, seismic masses, microtubes, etc.), before they are included in a final packaging line. This is, in addition, implemented using low-cost collective manufacturing technologies. Such an encapsulation structure can also be transferred directly to a printed circuit or PCB (“Printed Circuit Board”).
The second material can be a metallic getter material. It is thus possible to produce mechanical reinforcement portions with a material having at once gas absorption and/or adsorption properties and high mechanical resistance properties.
Each of the mechanical reinforcement portions can include at least one first layer of the second material and at least one second layer of a metallic third getter material such that the first layer of the second material is positioned against the layer of the first material, and the second layer of the third material and/or is covered by the second layer of the third material.
By this means the production of each of the mechanical reinforcement portions with these different layers of materials enables the stresses relating to the maximum production thicknesses of the materials used to form the mechanical reinforcement portions to be overcome. Such a multi-layer structure of the mechanical reinforcement portions is advantageously used to produce mechanical reinforcement portions which are more than approximately 0.5 μm thick.
In addition, in this configuration, the first layer of the second material can be used as a sub-layer for adjusting the thermal activation temperature of the third metallic getter material. The second material can in this case be a metal such as Cr and/or Ti and/or Co and/or Fe and/or Mn and/or Mo and/or Ta and/or V and/or W, and/or advantageously Ru and/or Pt and/or Cr and/or Ni. The details for production of such an adjustment sub-layer are described, for example, in document FR 2 922 202.
Each of the mechanical reinforcement portions can include a stack of layers two at least of the said layers of which form a bimetallic strip exerting on the cap a force directed outside the cavity.
The first material can be made of Si and/or silica and/or silicon nitride, and the second material can be made of Cr and/or Ti and/or Co and/or Fe and/or Mn and/or Mo and/or Ta and/or V and/or W and/or Pd and/or Rh and/or Pt and/or Ru and/or Zr. When one of the metallic getter materials contains Zr this material can be associated with one of the other materials mentioned above, the Young's modulus of which is higher than that of the first material.
The metallic getter material(s) can be made of Cr and/or Ti and/or Co and/or Fe and/or Mn and/or Mo and/or Ta and/or V and/or W and/or Pd and/or Rh and/or Pt and/or Ru and/or Zr.
The expression “made of” may designate equally the material in question and one of its alloys.
Each of the mechanical reinforcement portions can be of roughly rectangular parallelepipedic shape, and the mechanical reinforcement portions can be positioned roughly parallel relative to one another. As a variant, the mechanical reinforcement portions can form a mesh on the said face of the layer of the first material.
These configurations of mechanical reinforcement portions enable these portions to play a positive role in the accidental case of the appearance of cracks in the layer of the first material, by diverting these cracks at the interfaces between the layer of the first material and the mechanical reinforcement portions.
A ratio r=h/H can be between approximately 0.6 and 0.9, and a ratio R=b/t can be higher than or equal to approximately 0.7, where:
h: thickness of the layer of the first material;
H: sum of h and of the thickness of one of the mechanical reinforcement portions;
b: width of one of the mechanical reinforcement portions;
t: period with which the mechanical reinforcement portions are positioned on the layer of the first material.
The encapsulation structure can also include electrically conductive vias produced all the way through the substrate and/or the layer of the first material, next to the cavity. Such vias can form, for example, electrical points connecting to the encapsulated microdevice.
The invention also relates to a microelectronic device including at least two encapsulation structures as described above, positioned one above the other, in which at least one electrically conductive via of a first of the two encapsulation structures is electrically connected to at least one electrically conductive via of a second of the two encapsulation structures, directly or through an interposer. It is thus possible, for example, to connect electrically at least two microdevices encapsulated in this manner, the encapsulation structures of which are superposed on one another. When the caps of these encapsulation structures are thin layers or thinned substrates the device obtained in this manner is thin and facilitates assembly in three dimensions of the encapsulation structures, i.e. the superposition of these encapsulation structures. The superposed microdevices can have different functions, one corresponding, for example, to a gyroscope and the other corresponding to an accelerometer. Such a microelectronic device can be transferred directly on to a PCB.
The invention also relates to a method of encapsulation of at least one microdevice, including at least the following steps:
and in which the mechanical reinforcement portions include at least one second material, the Young's modulus of which is higher than that of the first material.
Such a method of encapsulation by wafer cap packaging enables an encapsulation structure as described above to be obtained. Such a method also corresponds to a method of encapsulation by transfer of a cap formed by deposition of one or more thin layers in a mould.
The invention also relates to a method of encapsulation of at least one microdevice, including at least the following steps:
and in which the mechanical reinforcement portions include at least one second material, the Young's modulus of which is higher than that of the first material.
Such a method of encapsulation by TFP enables an encapsulation structure as described above to be obtained.
Each of the mechanical reinforcement portions can be produced at least by a deposition of a first layer of the second material and a second layer of a metallic third getter material such that the first layer of the second material is positioned between the layer of the first material, and the second layer of the third material and/or is covered by the second layer of the third material.
Each of the mechanical reinforcement portions can be produced by a deposition of a stack of layers two at least of the said layers of which form a bimetallic strip exerting on the cap a force directed outside the cavity.
The method can also include, after rigid attachment of the cap to the substrate, or etching of the portion of sacrificial material, production of electrically conductive vias all the way through the substrate and/or the layer of the first material, next to the cavity.
Lastly, the invention relates to a method of manufacture of a microelectronic device, including implementation of a method of encapsulation as described above, and forming at least two encapsulation structures, where the said encapsulation structures are positioned one above the other, where at least one electrically conductive via of a first of the two encapsulation structures is then electrically connected to a least one electrically conductive via of a second of the two encapsulation structures directly or through an interposer.
The present invention will be better understood on reading the description of example embodiments given purely as an indication and in no sense restrictively, making reference to the appended illustrations in which:
Identical, similar or equivalent parts of the various figures described below have the same numerical references, to make it easier to move from one figure to another.
The various portions represented in the figures are not necessarily represented at a uniform scale, in order to make the figures more readable.
The various possibilities (variants and embodiments) must be understood as not being mutually exclusive, and being able to be combined with one another.
Reference is firstly made to
Encapsulation structure 100 includes a substrate 102 in which a microdevice 104 is produced. This microdevice 104 is, for example, an MEMS of the inertial type including one or more moving portions (beams, membranes, seismic masses, microtubes, etc.). A cap 106 is rigidly attached to substrate 102 through a sealing bead 108 which delimits laterally a cavity 110 in which microdevice 104 is encapsulated. Cap 106 includes a layer 112 of a first material corresponding to a thinned substrate, which is, for example, between approximately 5 μm and 100 μm thick. A face 114 of layer 112 forms an inner wall of cavity 110, in this case an upper wall of cavity 110. Lastly, mechanical reinforcement portions 116 having gas absorption and/or adsorption properties, and including at least one metallic getter material, are produced against face 114 of layer 112, having the dual function of gas pumping and mechanical reinforcement of cap 106.
A first example embodiment of cap 106 is described in connection with
In this first example embodiment each of mechanical reinforcement portions 116 is of rectangular parallelepipedic shape, and is positioned against face 114 of layer 112. These mechanical reinforcement portions 116 are positioned next to one another, and parallel to one another (and are aligned parallel to axis Y in the example represented in
In order that mechanical reinforcement portions 116 play both their role of mechanical reinforcement of cap 106 and that of gas pumping, these portions 116 are produced, in this first example embodiment, with a metallic getter material the Young's modulus of which is higher than or equal to that of the first material of layer 112 (where this first material is in this case silicon, the Young's modulus of which is equal to approximately 110 GPa). Thus, in this first example embodiment, each of mechanical reinforcement portions 116 includes a portion of one or more of the following materials: chromium, molybdenum, tantalum, tungsten, vanadium, cobalt, iron, manganese, nickel, copper, ruthenium, platinum. When layer 112 is made of silica, and contains for example glass such as glass of the Pyrex® type, the Young's modulus of which is equal to 70 GPa, or deposited SiO2 (for example in the case of a cap formed by thin layer deposition), the Young's modulus of which is also 70 GPa, mechanical reinforcement portions 116 can also include hafnium and/or niobium and/or titanium.
Mechanical reinforcement portions 116 are advantageously dimensioned and positioned such that a ratio r=h/H is between approximately 0.6 and 0.9, and such that a ratio R=b/t is greater than or equal to approximately 0.7. More precisely, considering a parameter M=E(X)/E(Y), with E(X) the Young's modulus of the material of mechanical reinforcement portions 116 and E(Y) the Young's modulus of the first material of layer 112, a parameter O=H/b and a ratio of moments of inertia Dy/D=2, where Dy is equal to the moment of inertia of cap 106 (layer of first material 112+mechanical reinforcement portions 116) and D is equal to the moment of inertia of the layer of first material 112 on its own, i.e. without mechanical reinforcement portions 116, ratios r and R can be chosen as indicated in the table below:
As an example, for a layer 112 including silicon and mechanical reinforcement portions 116 including tungsten, mechanical reinforcement portions 116 can be dimensioned such that:
In the case of a layer 112 produced by thin layer deposition (with the thickness for example less than or equal to approximately 10 μm) of SiO2, the associated Young's modulus of which is 70 GPa, the following dimensioning rules can be followed in producing mechanical reinforcement portions 116:
As a variant, mechanical reinforcement portions 116 can include several layers, or portions, of different materials superposed on one another. Thus, in
In each of these mechanical reinforcement portions 116 formed from several layers of different materials, it is possible to use a metallic getter material to produce the upper layer (layer 120 in the examples of
In the case of a silicon layer 112, layer 118 can include one or more metals such as chromium, molybdenum, tantalum, tungsten, vanadium, cobalt, iron, manganese, nickel, copper, ruthenium or platinum. In the case of a layer 112 made of silica and comprising for example SiO2, in addition to the previously mentioned materials, it is possible for layer 118 to include hafnium and/or niobium and/or titanium.
As a variant, each of mechanical reinforcement portions 116 can include more than two layers of different materials and/or include at least two layers of metallic getter materials, notably different from one another.
In the example represented in
When mechanical reinforcement portions 116 include several superposed layers of materials, the material used to produce the lower layer (layer 118 in the examples of
Mechanical reinforcement portions 116 can also allow cap 106 to be pre-stressed through the formation of intrinsic tension stresses in mechanical reinforcement portions 116. Several methods of deposition of materials used in microelectronics and applied here for the production of mechanical reinforcement portions 116 enable this to be obtained (cathodic sputtering, hardening by polarisation of the substrate, etc., in order to control the density and microstructure of the deposit). It is thus possible, for example, to “pre-deform” cap 106, during production, in the direction opposite to a deformation which cap 106 will undergo in a subsequent step, such as a step of overmoulding.
Similarly, when mechanical reinforcement portions 116 contain several superposed layers of materials, materials having different expansion coefficients (for example to produce layers 118 and 120) can be used for production of mechanical reinforcement portions 116. Each mechanical reinforcement portion 116 then forms a bimetallic strip which, under the action of the temperature, and in particular during the assembly operations, will increase the mechanical resistance of cap 106. Indeed, the bimetallic strip which is not deformed at ambient temperature will be able to oppose the external stresses acting on cap 106. To produce such a bimetallic strip, a pair of materials is chosen to produce layers 118 and 120 of mechanical reinforcement portions 116 such that the material with the higher thermal expansion coefficient is deposited against face 114 of layer 112, while the second element of the bimetallic strip (the getter material), which has a lower thermal expansion coefficient, is deposited on this first material. Such a bimetallic strip effect can, for example, be obtained by making first layer 118 of nickel and by making second layer 120 of titanium. In addition, the use of nickel to produce first layer 118 also has the benefit that it can easily be deposited by electrolytic deposition (ECD) up to thicknesses of approximately 10 μm to 20 μm. Such a pair of materials therefore enables the previously defined design rules to be met.
As a variant, a bimetallic strip effect at ambient temperature enabling a force directed towards the exterior of cavity 110 to be exerted can be caused by choosing a pair of materials such that the material deposited against face 114 has the lower thermal expansion coefficient of these two materials, on which the getter is deposited at a temperature higher than that of the first deposit. As an example, to reinforce a cap made of silicon, it is possible firstly to deposit tungsten and/or molybdenum and/or chromium on the cap, and then to deposit titanium and/or zirconium. To reinforce a thin film cap the same materials can be used, where the getter effect material is deposited first on the sacrificial layer, at a temperature higher than the one used to produce the second deposit.
As represented in
Several encapsulation structures similar to those represented in
As represented in
As represented in
A sacrificial layer is then deposited on substrate 102, notably covering microdevice 104. This sacrificial layer is then shaped, for example by etching, in order that a remaining portion 130 of sacrificial layer, positioned on microdevice 104, occupies a volume corresponding to that of cavity 110 in which microdevice 104 is intended to be encapsulated (
As represented in
To accomplish this, mechanical reinforcement portions 116 can be directly produced on portion of sacrificial material 130 by deposition of one or more materials, depending on whether mechanical reinforcement portions 116 contain one or more superposed layers of materials. The deposited material or materials can then be structured by photolithography followed by chemical etching, forming mechanical reinforcement portions 116. It is then possible to deposit, between mechanical reinforcement portions 116, a sacrificial material, for example of a similar nature to that of portion 130, which will be etched simultaneously with portion 130 during the subsequent formation of cavity 110. This sacrificial material can in this case be deposited and then planarised, stopping at mechanical reinforcement portions 116.
As a variant, it is possible to etch in portion of sacrificial material 130 positions for mechanical reinforcement portions 116, and then to deposit the material(s) of these mechanical reinforcement portions 116 in the etched positions.
When the thickness of mechanical reinforcement portions 116 is less than or equal to approximately 2 μm, the material of these portions 116 can be deposited directly by PVD (Physical Vapour Deposition) on portion of sacrificial material 130. The thickness of mechanical reinforcement portions 116 can be increased by using several successive PVD deposits, for example of different materials.
When the thickness of mechanical reinforcement portions 116 is greater than approximately 2 μm, it is possible to deposit firstly a germination layer by PVD, and then to deposit the material or materials of mechanical reinforcement portions 116 by ECD. It is thus possible, for example, to produce a first deposit of titanium and of nickel, of thickness of between approximately 10 nm and several hundred nanometres, by PVD through a resin mask, and then to deposit nickel by electrolysis through the mask over a thickness, for example, of between 1 micron and several tens of microns. When the thickness of the material of mechanical reinforcement portions 116 deposited by ECD is substantial, for example greater than approximately several tens of microns, the ECD deposit is advantageously extended beyond the thickness of the resin mask. Indeed, once this height has been exceeded, the deposited material also grows laterally, by this means giving mechanical reinforcement portions 116 a profile with a “mushroom” shape, i.e. having an upper portion which is wider than the remainder of the portion, which limits the spaces between mechanical reinforcement portions 116 which are intended to be plugged by layer 112 which is then deposited. In the above example, the first titanium deposit forms upper layer 120 of mechanical reinforcement portions 116, whereas the deposited nickel forms layer 118 of these mechanical reinforcement portions 116. The deposited titanium thickness is advantageously greater than approximately 100 nm. The resin mask is then removed and the portions of the germination layer between mechanical reinforcement portions 116 are then etched, stopping at portion of sacrificial material 130.
As represented in
Lastly, sacrificial layer 130 is etched, for example through an aperture made through layer 112, thus forming cavity 110 in which microdevice 104 is encapsulated and the upper wall of which is formed by face 114 of layer 112, against which mechanical reinforcement portions 116 are positioned.
As a variant it is possible to produce an additional mechanical reinforcement of cap 106 by depositing, on layer 112 (on the face of layer 112 opposite face 114), a continuous layer, for example made of metal, and by PVD with a thickness of approximately 1 μm, or by ECD, to obtain a thickness of material which can range from several microns to several tens of microns. This continuous layer is advantageously also deposited at the periphery of cap 106, on substrate 102, which enables the rigidity of this layer to be improved, bearing in mind a vault effect obtained with such a layer.
As represented in
Cap 106 is then produced from a layer 112 corresponding to a substrate, or wafer, made for example of glass or silicon (
A recess 132 intended to form at least a portion of cavity 110 of encapsulation structure 100 is etched in layer 112, by this means revealing face 114 of layer 112 which will form the upper wall of cavity 110 (
Mechanical reinforcement portions 116 are then produced against face 114, in recess 132 (
This deposition is, for example, accomplished through patterns made in a photosensitive film positioned above the germination layer. Several layers of materials can be deposited through this film, notably terminating by the deposition of a metallic getter material, in order that the latter is exposed in cavity 110. The material or materials deposited between portions 116, on the photosensitive film, can be removed simultaneously with this film by lift-off. The portions of the germination layer located between mechanical reinforcement portions 116 are then etched, for example by chemical means. Prior to this etching it is possible to protect mechanical reinforcement portions 116, and in particular the getter material, by nitrogen hardening or oxidation, using a dry method, of this or these materials, by this means limiting the chemical attack of this or these materials during the chemical etching of the germination layer.
As a variant, when the material of mechanical reinforcement portions 116 cannot be deposited by ECD this material can be deposited by PVD. In this case the different layers of mechanical reinforcement portions 116 are firstly deposited, and then mechanical reinforcement portions 116 are obtained by shaping these layers, for example using photolithography and lift-off or chemical etching of these layers.
Cap 106 obtained in this manner is then rigidly attached to substrate 102, directly or through a sealing bead, depending on the nature of the materials of layer 112 and of substrate 102. Microdevice 104 is then encapsulated in cavity 110 in which mechanical reinforcement portions 116 are located in the area of the upper wall of cavity 110 (
As represented in
As a variant of the methods described above in connection with
| Number | Date | Country | Kind |
|---|---|---|---|
| 13 57386 | Jul 2013 | FR | national |
