WO2010031987A2

WO2010031987A2 - Encapsulation method

Info

Publication number: WO2010031987A2
Application number: PCT/GB2009/001861
Authority: WO
Inventors: Changhai Wang; Jun Zeng
Original assignee: Heriot-Watt University
Priority date: 2008-09-19
Filing date: 2009-07-29
Publication date: 2010-03-25
Also published as: GB0817309D0; GB201106000D0; GB2476209B; GB2476209A; WO2010031987A3; CN102216201A

Abstract

The invention provides a cavity-containing package (1 ) for a MEMS device (17) comprising a first substrate (3) having a surface (4) facing the cavity (15), a second substrate (5) having a surface (6) facing the cavity (15) and a loop of composite material (7), the loop having a surface (8) facing the cavity (15), a surface (10) facing away from the cavity (15) and two substrate-abutting surfaces (12, 14) and comprising a layer of polymeric material (11, 13, 29) and a layer of metal (9, 99) each of which layers' edges face the cavity-facing surfaces (4, 6) of the substrates, said cavity-facing surfaces (4, 6) of the substrates (3, 5) being connected to each other by said loop of composite material (7), wherein the substrate-abutting surfaces (12, 14) abut the cavity-facing surfaces (4, 6) continuously along the length of said loop (7), whereby said cavity-facing surfaces (4, 6) of the substrates (3, 5) and the loop (7) define the cavity (15).

Description

ENCAPSULATION METHOD

FIELD OF THE INVENTION

This invention relates to a method for the encapsulation of microscale electrical, optical, RF, fluidic devices and sensors, and the manufacture of drug delivery devices such as receptacles for aerosol formulation used in medical inhalers. More particularly the method relates to a method for improving the hermeticity of a polymer seal when encapsulating such devices, by making use of a polymer seal comprising an additional barrier material.

BACKGROUND OF THE INVENTION

Microelectromechanical systems (MEMS), Microoptoelectromechanical systems (MOEMS) and other microelectronic components are commonplace in devices such as sensors, actuators, digital light projectors and gyroscopes. Microelectronics components of this type can be fabricated from any number of suitable materials using multiple-stage processing techniques such as micro-moulding, embossing, micromachining or photolithographic techniques to achieve sufficient resolution, functionality and high volume production.

MEMS devices are used in many products. For example, MEMS devices are used to provide miniature accelerometers that have applications in accident prevention, medical apparatus, telecommunications and in the provision of low-cost printing. Whilst the devices themselves have changed and advanced to the extent that they are present in many commonplace products, the packaging of the devices has not advanced to the same extent.

Microelectronics manufacturing can also include a plethora of additive or subtractive steps where material is either added or removed by known processes.

As a consequence of their small size, MEMS and related devices are often fragile and can be easily damaged by external agents and this damage can impair functionality or cause failure. To protect such sensitive devices they are often housed within a protective capsule or cavity. Cavity manufacture or "packaging" is thus an essential manufacturing step within the microelectronics industry and often contributes a significant portion of the cost of the finished device. One problem associated with the packaging of MEMS devices, and with the semiconductor industry in general, is the ability to bond surfaces together properly whilst minimising or obviating the damage caused to the surface and the MEMS and related similarly sized devices during bonding.

Whilst soldering, welding, glass frit and anodic bonding etc are well-known bonding methods used in packaging methods in the microelectronics industry, and allow the preparation of hermetic cavities when correctly performed, they often require high temperatures or strong electrical fields to effect the bonding process. This has the disadvantage that resultant thermal or electrical stresses can cause failure of MEMS and related devices whilst they are being packaged. This leads to increased reworking costs and/or lower device yields.

Alternative low or ambient temperature bonding processes for MEMS devices are known such as processes making use of radiation-curable materials, thermally curably materials, thermoplastic materials or self-adhesive materials. These can be advantageous since the materials and processes used can be low in cost, offer low or zero thermal stress during packaging of MEMS devices, potentially reducing failures and increase yields. However, the resultant seals can be non-hermetic, or not of sufficient hermeticity, allowing ingress of external agents such as liquids or vapours into the cavity.

A previous method directed towards the provision of hermetic packaging using polymer- based seals, sometimes referred to as wafer-level adhesive bonding, uses a thin metal or dielectric coating produced by vacuum deposition techniques (J. Oberhammer, F. Niklaus and G. Stemme, Sensors and Actuators A, 110(2), 407-412, 2004; and US Patent Application No. 11/113,545 (publication number US2005/0263866)) outside the cavity. A similar method is also described in WO 2004/025727. The processes described in these documents can be cumbersome and not always sufficiently reliable as a consequence of the cracks in the thin film coating resulting from the rough surface of the sidewall of the polymer seal.

Another method is described in WO 2004/025727 in which first and second wafers are joined together by a bonding material. Additionally, and separately to the bonding material, a diffusion barrier is provided and is described as being formed from a wide variety of sealing materials and different to the bonding material. The diffusion barrier is described as being provided inside or outside a ring of adhesive.

SUMMARY OF THE INVENTION

We have surprisingly found that, by providing a metal barrier as an integral part of a composite material that serves to bond two substrates together, suitable hermetic sealing may be achieved without the disadvantages associated with deposition of a metal diffusion barrier onto an already packaged device, or the complexity of providing a discrete diffusion barrier that is present in the package as a layer separate to the material used to bond substrates together, or possibility of damage to such a discrete diffusion layer when forming the package. Advantageously, by using a composite material derived from a composite material precursor comprising one or more layers of metal, and one or more layers of adhesive, an improved hermetic seal is provided in which the adhesive layer(s) serve(s) to support the metallic layer(s) during and after bonding of the substrates together, thereby minimising cracking or other damage to the metallic layer(s), whilst conferring to the resultant seal an improved hermeticity (of a factor of between 10 and 100,00) resultant from the presence of the metal layer(s) that are present in addition to the presence of polymeric material resultant from curing of the adhesive. Other advantages and benefits of the invention will become apparent from the discussion that follows.

Viewed from one aspect, therefore the invention provides a cavity-containing package for a MEMS device comprising a first substrate having a surface facing the cavity, a second substrate having a surface facing the cavity and a loop of composite material, the loop having a surface facing the cavity, a surface facing away from the cavity and two substrate-abutting surfaces and comprising a layer of polymeric material and a layer of metal each of which layers' edges face the cavity-facing surfaces of the substrates, said cavity-facing surfaces of the substrates being connected to each other by said loop of composite material, wherein the substrate-abutting surfaces abut the cavity-facing surfaces continuously along the length of said loop, whereby said cavity-facing surfaces of the substrates and the loop define the cavity.

Viewed from a further aspect the invention provides a method for forming a cavity- containing package of the invention comprising: - A -

(i) providing a first substrate having a first surface and providing a second substrate having a second surface; (ii) providing a loop of curable adhesive on the first surface and a loop of metal onto the first surface or the second surface; and (iii) bonding the first surface of the first substrate to the second surface of the second substrate through said loops whereby to form said package in which the loops of metal and cured adhesive form the composite material.

Other embodiments of the invention will be apparent from the discussion that follows below.

BRIEF DESRIPTION OF THE FIGURES

Figure 1 shows, schematically, and in cross-section, a package of the invention for a microelectronics device with a device present within the cavity.

Figure 2(a) shows, schematically, a plan view of two concentric regions of adhesive deposited onto a substrate, the regions of adhesive serving to pattern the substrate.

Figure 2(b) shows, in cross-section, the section X-X drawn on Figure 2(a)

Figure 2(c) shows a plan view of a polymer seal with an embedded metal barrier for improved hermeticity for encapsulation of devices and systems such as MEMS, RF, optical, microfluidic devices and sensors. The polymer-metal composite seal can be a continuous ring structure of any shape (circle, square, rectangle or any user defined geometry). Multiple barrier layers may be used to improve hermeticity.

Figure 3 shows a cross-sectional view of two substrates bonded together using a polymer seal with a metal barrier.

Figure 4 shows a cross-sectional view of two substrates bonded together using a polymer seal with an inner metal barrier. Figure 5 shows a cross-sectional view of two substrates bonded together using a polymer seal with an outer metal barrier.

Figure 6 shows a fabricated polymer seal with metal barrier on the cover. The polymer- metal composite structure can be fabricated for example using polymer surface micromachining by photolithography or dry etching and electroplating or electroless process for metal deposition.

Figure 7 shows a fabricated polymer seal with metal barrier on the substrate. The polymer-metal composite structure can be fabricated for example using polymer surface micromachining by photolithography or dry etching and electroplating or electroless process for metal deposition.

Figure 8 shows a fabricated polymer seal structure on a first substrate and the corresponding metal barrier on a second substrate. The polymer structure can be fabricated for example using polymer surface micromachining by photolithography or dry etching and the metal structure by electroplating or electroless processes.

Figure 9 shows a fabricated polymer seal structure on the second substrate and the corresponding metal barrier on the first substrate. The polymer structure can be fabricated for example using polymer surface micromachining by photolithography or dry etching and the metal structure by electroplating or electroless process.

Figure 10 shows a fabricated polymer seal structure on the first substrate. The metal structure is slightly above the surface polymer seal structure. A thin layer of liquid polymer is produced on the polymer-metal structure for example by contact printing. The liquid polymer acts as the joining material to produce a strong bond. The liquid polymer can flow easily to allow the surface of the metal ring in contact with the surface of the second substrate after bonding.

Figure 11 (a) shows a plan view of what may be considered to be either a composite material or composite material precursor of this invention provided with two layers of metal and four electrical interconnects. Figures 11(b) and 11(c) show a cross section of the same composite material (within packages of the invention) attached to which are conductor lines (Figure 11(b) or bumps for electrical interconnects (Figure 11(c)). Figures 11(d) and 11(e) show further variations in the cross section of the same composite material (within packages of the invention) attached to which are conductor lines and bumps for electrical interconnections whereby substrate 3 serves as an interposer.

Figure 12 (a)-(d) depicts schematically an example of fabrication steps that lead to the package depicted in Figure 11(c) and, in Figure 12(e), the use of such packages of the invention in flip-chip assembly or packaging of encapsulated MEMS on a board by reflow of solder bumps or thermocompression or thermosonic bonding.

Figure 13 shows a schematic embodiment of the invention wherein the bonding of the first and second substrates is achieved through the use of laser bonding.

Figure 14 shows an optical picture of a fabricated ring of composite material made of benzocyclobutene (BCB) with a nickel wall of 100 μm width in the middle of the BCB track of 400 μm width.

Figure 15 shows a schematic representation of the hotplate bonding setup.

Figure 16 shows an optical picture of a glass substrate bonded to a silicon substrate in accordance with this invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides the use of a composite material as described encapsulation of wide variety of microscale electrical, optical, RF, fluidic devices and sensors, and the manufacture of drug delivery devices such as receptacles for aerosol formulation used in medical inhalers.

By microscale devices is meant herein devices having at least one dimension, and typically all dimensions, in the range of about 0.01 μm to 10,000 μm, more typically 1 μm to 100 μm. Examples of microscale devices include MEMS₁ MOEMS and other microelectronic components well known to those of skill in the art. The nature of the device is not of particular consequence and so references to the nature of the device, typically to MEMS, herein are intended to be exemplary of and not limitative to the invention. The package of the invention is made from two substrates that are attached together through a loop of composite material, the substrates and loop of composite material serving to define a cavity of similar size to that of microscale devices within which a MEMS or other microscale device may be housed. An example of such a package 1 is depicted in cross-section in Fig. 1 , showing substrates 3 and 5 with associated cavity- facing surfaces 4 and 6 respectively, loop of composite material 7 having an embedded layer of metal 9 flanked by two layers of cured adhesive 11 and 13. Within the cavity 15 is housed a MEMS device 17.

The substrates 3 and 5 may be of any convenient material and may, for example be chosen from materials made of, or comprising, one or more of silicon (Si), silica (SiO₂), germanium (or other semiconductor), metals, glasses, polymers, ceramics or other materials. The second substrate can be from the same material or a different material as the first substrate. It will be understood that modifying films (organic, inorganic or metal) may also be deposited on one or both substrates, for example to improve the adhesion strength of the adhesive bonding.

For some applications, e.g. optical applications, it may be advantageous to have a device resting on or attached to any convenient first substrate with the second substrate formed of glass allowing the passage of light therethrough and onto the device within the package. Other materials for substrates will be selected according to the requirements of the user of the devices. For example, materials allowing the transmission of other, invisible, electromagnetic radiation may be appropriate, such as germanium allowing the transmission of infrared light, may be selected acording to the requirements of the device, or use or application to which it is to be put.

In many embodiments of this invention, the device to be housed within the package of the invention or by the method of this invention will be provided on the first substrate and second substrate provided with the composite material and applied onto the first substrate whereby to package the device.

However, as described herein, all components from which the composite material is formed need not be, although typically are, provided on one of the two substrates prior to forming the package: for example a metal layer may be provided on one substrate and the adhesive provided on the other. The composite material may be formed by bringing the two substrates, and so the components of the composite layer, together, whereby to form the composite layer after curing of the adhesive.

By composite material is meant herein a material formed from two or more different materials combined together that retain distinct and detectable. In this invention, the composite material is formed by curing curable adhesive present in a composite material precursor, which composite material precursor is formed by bringing into contact at least a layer of metal and at least a layer of curable adhesive.

Materials suitable for use as the curable adhesive are well known and include monomers, pre-polymers or polymeric materials with a viscosity suitable for deposition on a substrate by methods such as spin-coating, contact- or screen-printing, or other methods known to those skilled in the art. As is known, the choice of curable adhesive is informed by the desire to use a substance that exhibits as many as possible of the following properties: minimal outgassing; low moisture absorption; excellent dielectric properties; a low dielectric constant; and good electrical insulation; low bonding/curing temperature; ease of processing and patterning; and low cost.

Particularly suitable materials are curable adhesives such as thermoplastic or thermosetting polymers. Examples include benzocyclobutene (BCB) available from DOW Chemical Company or SU-8 photopolymer (CAS number 221273-01-4). Different types of BCB are commerciailly available from Dow, suitable for either dry-etching or photoablation as a consequence of photosensitivity. These have suitable softening points to enable formation of a strong adhesive bond. Other monomer, pre-polymer or polymer materials may offer advantageous properties such as lower viscosity, lower softening points etc and these may also be used.

Other appropriate adhesives may be selcted by those skilled in the art. An example of a useful class of adhesives include epoxy-based materials that may be applied as dry films and that develop adhesiveness upon curing. Appropriate materials are commercially available from Du Pont sold under the PerMX trade name.

Examples of suitable metals that may be included in the composite material and composite material precursor are nickel, gold, silver, copper, tin, titanium, lead or alloys or mixtures thereof. For example the metal layers may comprise these metals, for example nickel, copper or gold, or may consist of them or consist essentially of them. In certain embodiments of the invention the composite material 7 is provided as a triple- layered structure having a layer of metal sandwiched between two layers of cured adhesive 11 and 13, as depicted in Figures 2(c) and 3. In other embodiments the composite material may have a multilayer structure comprising a plurality of layers of metal or metal-containing material and a plurality of layers of polymeric material resultant from cured adhesive. In such multilayer structures, each of the metal or metal- containing layers may be sandwiched between layers of polymeric material or there may be one or two outward-facing layers of metal that are not flanked on both faces by layers of polymeric material. Also in such multilayer structures, each of the layers of polymeric material may be sandwiched between metal or metal-containing layers or there may be one or two outward-facing layers of polymeric material that are not flanked on both faces by layers of metal.

By way of example, and with reference to Figures 2(a) to 2(c) and 3, the fabrication of a package of the invention is now described. In overview, an adhesive material (not shown) is coated onto the surface of a first substrate 3, followed by patterning the curable adhesive to produce a series of pattern areas 11a and 13a (which once cured provide the polymeric material 11 and 13 depicted in Fig. 3) and non-pattern areas 19 and 20 with a specific design, the patterned areas 11a and 13a comprising curable adhesive, and depositing a layer of metal 9 within the non-pattern areas 19 of the design and bonding a second substrate 5 to produce package as shown in Figure 3.

In more detail the adhesive is first deposited (not shown) on the first substrate 3. A sufficient thickness of the adhesive material is deposited to achieve an adhesive bond. The adhesive material will be of sufficient thickness so as to separate the two substrates 3 and 5 and form a gap between them. Appropriate thicknesses may be readily determined but are typically at least 1 μm and less than 1000 μm, more typically less than 500 μm. A useful thickness of the adhesive layer is of between 1 and 150 μm.

A non-patterened area 19 on the substrate 3, non patterned by adhesive material that is, may then be generated either by selective deposition of the adhesive (not shown) on the substrate 3 as a single layer (e.g. by spin-coating) or by using a known technique such as inkjet printing, stencil printing, photoresist etching, transfer printing, photolithography, dry etching or other convenient method. Using one of these methods a non-patterned region 19 between two patterned (adhesive) regions 11a and 13a is provided on the first substrate 3. Whilst the outer (11 ) and inner (13) loops of cured adhesive shown in

Figure 3 are circular whereby to define a circular non-patterned loop disposed therebetween, it will be appreciated that the loops of adhesive may each independently be of any convenient shape or pattern, and dimension, whereby to provide the microcavity within the inner loop of curable adhesive with a shape, and dimension, suitable for the microdevice to be housed in it.

Typically the inner and outer loops of curable adhesive 11a and 13a will be of the same shape, e.g. circular, oval, square, rectangular etc. Thus the regions 11a and 13a of deposited adhesive material shown in Figure 2(a) are represented as rings but any other designs may be chosen. The outer (11a) and inner (13a) loops of adhesive depicted in Figure 2(a) may be of the same material or may be of different materials. Typically they will be of the same material. Analogously, the dimensions of the inner and outer layers of Figure 2(a) may be the same or they may be different; and the thickness of the non- patterned area 19 between the inner and outer rings of Figure 2(a) may be equal or may not be equal to thicknesses (widths) of loops 11a and 13a.

As is known in the art, a degree of crosslinking of "wet" (i.e. liquid when uncured, as opposed to the dry film class of adhesives exemplified by the epoxy-based PerMX materials described above) adhesives is advantageous in allowing patterning to be achieved. Typically, such adhesives will thus be soft-cured (i.e. not fully cured but cured sufficiently (typically about 50% cross-linking) to allow patterning by dry-etching or other suitable method

Fig. 2(b) shows section X-X of Fig. 2(a) showing outer (11a) and inner (13a) rings of curable adhesive, non-patterned area 19 in between, and substrate 3.

The non-patterned area depicted in Figure 2(a) is filled with metal 9 as shown in Figure 3. This may be achieved either by depositing metal directly into the channel 23 (shown in Fig. 2(b) defined by the non-patterned volume between the inner and outer loops of curable adhesive 11a and 13a or by applying a layer of metal 9 onto the second substrate 5 such that the metal layer 9 on the second substrate is brought into alignment with the non-patterned area 19 of the first substrate 3 when the cavity 15 is formed.

Typically the metal 9 is deposited directly into the channel 23 defined by the non- patterned volume between the inner 13a and outer 11a loops. Deposition of metal 9 may be achieved by any convenient method, such as, but not limited to, vacuum deposition, electrocatalytic deposition, autocatalytic deposition or photocatalytic deposition, electroplating or electroless plating.

Where deposition is by electroplating, at least the surface of the substrate upon which the layer of metal is deposited will be modified (not shown) so as to allow electroplating to occur. This may be achieved, as is known in the art, by deposition of a thin layer of titanium or other metal, for example, whereby to allow electroplating. As described in a exemplary embodiment below, we describe use of a triple layer comprising a layer of copper sandwiched between two titanium layers. Alternative surface treatments preparatory to electroplating or other method of metal deposition will be evident to those of skill in the art and are not described in detail here.

Typically, a sandwich-type composite material of the type depicted in Figure 3 is achieved by photolithography or dry etching of a deposited region of adhesive material followed by deposition into the non-patterned region of metal, typically by electroplating. After bonding of the substrates, the resultant metal loop within the composite material is in close contact with the surfaces of the substrates to act as a barrier to assist in stopping ingress of moisture and diffusion of other gas species into the cavity. Therefore a tightly hermetic package can be obtained for MEMS, sensors and medical (drug) delivery devices.

It will be appreciated that many embodiments of the invention and variations to the embodiment immediately hereinbefore described will be evident to those skilled in the art. Some of these embodiments and variations are depicted schematically in Figures 4 to 10. In each of these embodiments, the polymer-metal composite material precursor can be, for example, fabricated using polymer surface micromachining by photolithography or dry etching and electroplating or electroless process for metal deposition, as described herein.

Figures 4 and 5 show cross-sectional views of two substrates 3 and 5 bonded together using a composite material 7 comprising a single layer of polymeric material 11 with either an inner or an outer metal barrier 9. It will be appreciated that these embodiments may be constructed by either depositing the metal barrier 9 onto the same substrate 3 on which the curable adhesive is initially provided or depositing the metal 9 onto the other substrate 5 and forming the composite material 7 (by curing the curable adhesive) after the substrates 3 and 5 have been brought together with the metal and adhesive layers aligned, whereby to form the composite material precursor.

Figures 6 and 7 show, respectively, a fabricated composite material precursor 7a with metal barrier 9 on the cover 3, or on the base 5.

Figures 8 and 9 depict embodiments of the invention in which the composite material precursor 7a is fabricated after initial provision of concentric loops 11a and 13a of curable adhesive and metallic component 9 on different substrates 3 and 5.

Figure 10 shows a fabricated composite material precursor 7a on a first substrate 3 with the metal layer 9 slightly proud of the flanking curable adhesive layers 11a and 13a. A thin layer of liquid curable polymer 25 may be applied to the edge 27 of the polymer- metal structure for example by contact printing (not shown), as described by J. Oberhammer and G. Stemme (BCB contact printing for patterned adhesive full wafer bonded 0-level packages, J. Microelectromechanical Systems, 14(2), 419-425, 2005; Sealing of adhesive bonded devices on wafer level, Sensors and Actuators A, 110(2), 419-425, 2005). The liquid polymer 25 assists in providing a strong bond between the remainder of the composite material precursor and the second substrate once the package 1 is formed (not shown). As described by Oberhammer and Stemme the liquid polymer can flow easily to allow the surface of the metal ring in contact with the surface of the second substrate after bonding.

It will be appreciated that multilayer composite materials may be provided by, for example, selective etching of a deposited region of adhesive to provide a first non- patterned regions. This may be filled with one layer of metal. A second non-patterned region can be prepared either at the same or a different time to the first and filled with a second layer of metal. It will be appreciated that the layers of metal introduced may be the same, or different, e.g. if by sequential etching is carried out. Provision of multiple metal rings can be used to improve further the hermeticity of the resultant microcavity within the package. Different metals may be useful where one layer of metal is serving as a barrier layer and another may also or alternatively function as an electrical connection to the MEMS or other device housed within the cavity

The polymer-metal composite seal design can be used both for the well-understood chip scale (bonding a single cap or cover to a chip or wafer at a time) and wafer level bonding (bonding two wafers together). The latter method is more efficient as the devices are encapsulated simultaneously at wafer level.

Advantageously, electrical interconnects to the encapsulated MEMS or other device can be made using bumps that connect to the electric contacts of the device on the surface of the device substrate to the bottom surface of a metal connection achieved by providing the curable adhesive with metal interconnects additional the metal layers provided in the composite material precursor.

Figure 11(a) shows a plan view of what may be considered to be a composite material 7 of this invention (it could equally be considered to be a composite material precursor 7a, in which the layers of polymeric material 11 , 13, 29 described immediately supra are layers 11a, 13a, 29a of curable adhesive) provided with two layers of metal 9 and 99. Within the inner layer of polymeric material 13 are provided four electrical interconnects 31 that can be provided within the polymeric material after, for example, suitable patterning of the adhesive material 11a, 13a, 29a applied to an underlying substrate 3 (not shown). It will appreciated that the electrical interconnects 31 could equally be positioned within layers of polymeric material 13 or 29, or indeed in a mixture of the three within layers of polymeric material 11 , 13 and 29.

As shown in the packages 1 depicted in cross-section in Figures 11 (b) and 11 (c), by aligning the electrical interconnects 31 with metal-filled holes, or vias, 32 present in substrate 3, through-substrate connections may be achieved, to either a conductor line 33 or an interconnect bump 35, which permits electrical interconnections to be achieved between stacked packaged devices by feeding electrical connections through the capping substrate for next level packaging. Bumps 35 can be fabricated at the same or a different time as the metal rings 9 and 99 and can be bonded onto the corresponding electrical contacts on the capping substrate by thermo-compression bonding.

An example of a fabrication of a package 1 depicted in cross-section in Figure 11(c), having through-substrate interconnectors 31 , metal-filled vias 32 and bumps 35, is shown in Figure 12(a) to (d). Such packages are of use in flip-chip packaging as depicted in Figure 12 (e). The manufacture of packaged microdevices in this way is well known to those in the art and is referred to as flip-chip packaging technology (see, for example "Flip chip technologies", ed. John H Lau, McGraw-Hill, 1995, ISBN 0-07- 036609-8). Figure 11(d) shows in cross section how the conductor lines 33 present on the outward- facing surface of substrate 3 in Figure 11(b) can be used to achieve through-substrate electrical interconnection to interconnect bumps 35 (i.e. as opposed to direct alignment of the bumps 35 with the electrical interconnects 31 as depicted in Figure 11(c)). In this way substrate 3 can serve to effect electrode (pad) redistribution (i.e. wherein the bump is not in alignment, or registration, with the electrical interconnects 31 ). The term in the art for the redistribution of the electrode pad effected in this way is that the substrate 3 is acting as an interposer. This is a well understood term of the art - see for example the article on silicon interposers at httpV/www.imicronews.com/analvsis/Silicon-lnterposers- Wait-Application.1740.html.

Analogously to Figure 11(d), Figure 11(e) shows in cross section how the conductor lines 33 present on the outward-facing surface of substrate 3 in figure 11(b) can be present on the inward- , or cavity-facing surface of substrate 3 so that the substrate 3 can likewise act as an interposer by achieving achieve through-substrate electrical interconnection to interconnect bumps 35 (i.e. as opposed to direct alignment of the bumps 35 with the electrical interconnects 31 as depicted in Figure 11(c)).

In general terms, therefore, it will be understood that in all those embodiments of the invention in which a substrate, e.g. a first substrate 3, is provided with one or more metal filled vias (32), the substrates may act as interposers, for example through the provision of conductor lines 33 as described herein.

Thus Figure 12(a) shows a cross-section of a substrate 3 having present within it unfilled vias 32. These holes may be produced, for example, by laser-drilling, chemical etching or other method known in the art. The unfilled vias are then filled whereby to afford metal-filled vias 32 as depicted in Figure 12(b). Filling or embedding of metal within the vias may be accomplished by electroplating, e.g. of copper, gold, or nickel, or by filling with conductive paste. The composite material precursor 7a that may be understood to be depicted in cross-section in Figure 11 (a) is then formed, as described elsewhere herein, with the electrical interconnects 31 aligned with the metal-filled vias 32. This is shown in Figure 12(c).

It will be appreciated that, whilst the electrical interconnects 31 depicted in Figures 11(b) & (c) and Figures 12 (c) to (e) are shown as upstanding pillars, that is to say having a generally perpendicular disposition with respect to the plane of the substrate 3, they, and also metal layers 9 and 99 in this and all other embodiments of the invention may be sloping or of other orientation if desired.

Figure 12(d) shows the second substrate 5 (e.g. a cover wafer) bonded to the components depicted in Figure 12(c) whereby to afford a package 1 of the invention. The encapsulated MEMS or other microdevice 17 is not shown, nor are additional electrical connections permitting electrical connection to the encapsulated MEMS or other microdevice 17 though electrical interconnections 31 , metal-filled vias 32 and bumps 35.

Providing the package 1 depicted in Figure 12(d) with bumps 35 affords the package 1 depicted in Figure 11(c). These bumps may be introduced, for example, by a solder ball based bumping method, a gold stud bumping method or use of electroplated copper bumps. It will be appreciated that whilst the provision of metal bumps, whereby to provide the package 1 shown in Figure 11 (c), affords a package particularly useful in flip-chip packaging, as depicted in Figure 12(d), the package 1 shown in Figure 11(c) may also be provided with conductor lines 33 to afford a package as depicted in Figure 11(b).

Whilst the bumps 35 (or conductor lines 33) are described in the discussion of Figure 12 as being introduced in the last step, i.e. after formation of the package 1 depicted in Figure 12(d), they may also be introduced after the fabrication of the through-substrate vias 32, i.e. to the structure depicted in Figure 12(d). However, metal rings 9, 99 and the electrical interconnects 31 are preferably fabricated at the same time. Fabrication of bumps 35 on the metal-filled vias 32 can be made by stencil printing, electroplating or jetting of solder materials, printing of conductive paste, gold stud bumping or other suitable bumping method typically used in flip-chip packaging of IC (integrated circuits) chips. The bonding of the ends of the electrical interconnects 31 to the electrical contacts on the MEMS substrate can be made using thermocompression, thermosonic or solder reflow bonding. It will be appreciated that direct butting for electrical connections is also possible removing the need for metal to metal joining or bonding.

The use of the package of Figure 11(c) in flip-chip manufacturing is shown in Figure 12(e) whereby electrical interconnection to a conductor line 37 on a board 39 may be achieved. This may be, for example, by reflow of solder bumps or thermocompression or thermosonic bonding, or other method known in the art.

It will be appreciated that, because as just described the manner in which the packages of this invention may be made permits the provision of electrical connection to the MEMS device to be achieved by so-called "through substrate" interconnections, as opposed to the use of interconnections present on the substrate's surface, the resultant packages are of a different structure to those described, for example in WO 2004/025727, enabled by the provision of a composite material of the present invention that allows the engineering of through substrate interconnections. This, it will be understood, represents a still further advantage of the invention although it will also be understood that the invention may also be practised in the absence of such through- substrate electrical connections and/or with electrical connections providing by way of (substrate) surface-mounted or embedded electrical connections, as described, for example in WO 2004/068665.

After the substrates 3 and 5 are brought together, the composite material 7 may be formed by curing the polymeric adhesive 11 , 13, 27 in the composite material precursor 7a. As described hereinabove, a degree of curing may necessarily or advantageously have been achieved preparatory to patterning. This may be achieved under ambient pressure, on a hotplate or in an oven, in an atmosphere of inert gas such as nitrogen or under reduced pressure, for example within a vacuum chamber.

Any appropriate heating technique may be used, whereby to effect bonding of the substrates to each other through the resultant composite material. A mechanical load may be applied to the package of the invention during its formation upon curing the curable adhesive.

Appropriate heating techniques for bonding known in the art may be used, including the use of laser bonding in accordance with the teachings of WO 2006/126015, the contents of which, and all other documents referred to herein, are hereby incorporated by reference. Accordingly bonding may be achieved in this way by directing a laser beam onto an area of at least one of the substrates to heat selectively a region of its surface in thermal contact with the curable adhesive whereby to cause the curable adhesive to bond to the surface. As described in WO 2006/126015 the laser beam may be focussed onto the area of the surface using a lens or a transmission mask, for example, whereby to produce a patterned beam that is configured in registration with the curable adhesive. A transmission mask may be used in conjunction with a patterned beam resultant from the use of a lens.

An array of laser sources may be used, and if so, the array may be configured to allow a plurality of the laser sources in the array to be activated to form a predetermined pattern of illumination. Optionally, the array of laser sources is arranged in a set pattern, e.g. in alignment with the desired loop of composite material.

Typically, the laser has an output beam with a substantially uniform power distribution or exposure incident upon the surface, the substantially uniform power distribution allowing substantially uniform heating of the area of the surface.

Typically, the laser has an output power of more than 1W and a maximum power output of up to 1kW. Often the laser has an output wavelength in the infra-red region of the electromagnetic spectrum.

The use of a laser to effect bonding allows localised laser heating using a laser beam and/or a mask to selectively direct the laser irradiation to the area(s) to be bonded. Therefore, the laser directing means may be a feature of the manner in which a laser beam is processed or created to pattern the beam or move the beam across a surface for example, and/or the use of a mask. The laser directing means functions to allow the laser to selectively heat the curable adhesive to lessen or minimise heat transfer to the surrounding substrates.

Therefore, and as described in WO 2006/126015, the temperature increase at the location of a microdevice can be, advantageously, lower than at the bonding/sealing area. The lower temperature at the device position is very desirable for devices with low temperature budget such as RF MEMS switch structures and the like. Excessive temperature rise can cause failure of switches due to stress change in the membrane films. Because laser heating is localised, rise and fall of temperature can be much faster compared with oven and hotplate based heating methods therefore reducing exposure of MEMS to prolonged heating. An example of the use of laser bonding is depicted in Figure 13 which shows bonding of the first and second substrates 3 and 5 being achieved through the application of a laser beam 41 focussed through a lens 43 and a mask 45 onto the substrates 3 and 5 having a composite material precursor 7a disposed between them.

Localised laser heating is particularly useful for chip-to-chip and chip-to-wafer packaging, allowing attachment of caps individually to encapsulate MEMS devices with minimal thermal load to the other uncapped and capped devices.

This invention may be further understood with reference to the following non-limiting example.

Example

A polymer sealing ring with embedded nickel barrier was used to produce a microcavity between a glass substrate and a silicon substrate. A photosensitive BCB polymer (Cyclotene 4026-46, Dow Chemical) and nickel were used to construct the polymer- metal composite seal using photolithography and electroplating processes. The polymer-metal seal was fabricated on the glass substrate and bonded to the silicon substrate using a hot plate based curing method for the BCB polymer.

A tri-layer of metal film was firstly deposited on a 10 cm diameter glass wafer of about 500 μm of thickness using electron beam deposition. The metal film consists of a copper layer between two titanium layers; the thicknesses of the metal layers are about 100 nm, 400 nm and 100 nm respectively. The first titanium layer is used as an adhesion layer and the second titanium layer is used to provide a surface for electroplating of nickel. The copper layer is used to improve current distribution in the plating process to obtain a uniform nickel structure. The metal layers were patterned using photolithography and wet chemical etching methods to produce interconnected rings for fabrication of nickel barrier in the subsequent processes. The outer dimensions of the square rings are 5.2mm x 5.2mm. The track width of the resultant metal network is 130 μm.

After patterning of the metal film, a layer of the BCB polymer was deposited on the glass wafer using spin coating method. After pre-baking on a hotplate the polymer film was patterned by photolithography. A photomask was used to produce two concentric square rings aligned to the metal ring and exposing a metal track width of 100 μm.

Nickel was deposited into the trench between the BCB rings to fabricate the metal barrier for use to construct a microcavity between the glass substrate and another substrate with improved hermeticity. The parameters for deposition and patterning of the BCB polymer film are given in Table 1.

Table 1. Parameters for deposition and patterning of BCB film.

The composition of the electrolyte for nickel deposition is given in Table 2.

Table 2. Formulation of the DC electroplating bath.

The DC plating current and time were 25 mA/cm² and 15 minutes respectively for producing a nickel barrier track of ~ 11 μm of depth. After nickel plating the glass wafer was diced to obtain chips each with a polymer ring with embedded nickel barrier. Figure 14 shows an optical picture 46 of a fabricated BCB ring 7a with a nickel wall 9 in the middle of the BCB track 11a and 13a of width 100 μm. Electrical leads 47 used in the electroplating are shown.

Figure 15 shows a schematic representation of the hotplate bonding setup 48. The bonding assembly comprising fabricated BCB ring 7a (comprising metal layer 9 and curable adhesive layers 11a and 13a) and substrates 3 and 5 was set up on a hot plate 49 at room temperature and then a glass wafer 51 was placed on the assembly. A metal load 53 of 1.2 kg was applied to the glass wafer 51 for force application. The bonding assembly is then heated to a temperature of 150⁰C and maintained at this temperature for 5 minutes. The temperature is then elevated to the bonding/curing temperature of 250⁰C and maintained at this level for 15 minutes. Finally the hotplate is switched off and the bonded sample is removed after the temperature of the hotplate has dropped to near the room temperature. Figure 16 shows an optical picture 54 of the glass substrate bonded to the silicon substrate. Electrical leads 47 used in the electroplating are shown. The polymer bond is free of defects. The shear strength of the bond is as high as 30 kg. A standard helium-based through-hole leak test (see Military Standard (MIL-STD-883; method 1014.9) 1996 Test Methods and Procedures for Microelectronics) showed that the hermeticity of the microcavity produced by a polymer-metal composite seal is better than the sensitivity of ~ 1x10⁹ mbar l/s of the helium detector.

Claims

1. A cavity-containing package (1 ) for a MEMS device (17) comprising a first substrate (3) having a surface (4) facing the cavity (15), a second substrate (5) having a surface (6) facing the cavity (15) and a loop of composite material (7), the loop having a surface (8) facing the cavity (15), a surface (10) facing away from the cavity (15) and two substrate-abutting surfaces (12, 14) and comprising a layer of polymeric material (11 , 13, 29) and a layer of metal (9, 99) each of which layers' edges face the cavity- facing surfaces (4, 6) of the substrates, said cavity-facing surfaces (4, 6) of the substrates (3, 5) being connected to each other by said loop of composite material (7), wherein the substrate-abutting surfaces (12, 14) abut the cavity-facing surfaces (4, 6) continuously along the length of said loop (7), whereby said cavity-facing surfaces (4, 6) of the substrates (3, 5) and the loop (7) define the cavity (15).

2. The package (1 ) of claim 1 wherein the layer of metal (9) comprises one or more metals.

3. The package (1 ) of claim 1 or claim 2 wherein the layer of metal (9) comprises nickel, gold, silver, copper, tin, lead, titanium or an alloy thereof.

4. The package (1 ) of claim 3 wherein the layer of metal (9) comprises nickel.

5. The package (1 ) of claim 3 or claim 4 wherein the layer of metal (9) consists of nickel.

6. The package (1 ) of any one preceding claim wherein the polymeric material (11 , 13, 29) is an adhesive or a cured adhesive.

7. The package (1 ) of claim 6 wherein the polymeric material (11 , 13, 29) is a thermoplastic polymer, a thermosetting polymer or a photopolymer.

8. The package (1 ) of claim 6 or claim 7 wherein the adhesive is benzocyclobutene photopolymer or SU-8 photopolymer.

9. The package (1 ) of any one preceding claim wherein the layers (11 , 13, 9, 29, 99) of the composite material (7) are of the same or of different thicknesses.

10. The package (1) of any one preceding claim wherein the first and second substrates (3, 5) independently comprise glass or silicon.

11. The package (1) of claim 10 wherein the one substrate (3, 5) is glass and the other substrate (3, 5) is silicon.

12. The package (1) of any one preceding claim wherein either or both of the substrates' cavity-facing surfaces (4, 6) are coated with one or more layers of metal.

13. The package (1 ) of any one preceding claim wherein the first substrate (3) is provided with one or more metal-filled vias (32) in electrical communication with one or more electrical interconnects (31 ) provided within the polymeric material (11 , 13, 29).

14. The package (1) of claim 13 further comprising one or more metal bumps (35) and/or one or more conductor lines (33) in electrical communication with the one or more metal-filled vias (32).

15. The package (1 ) of claim 13 further comprising one or more metal bumps (35) or conductor lines (33) in electrical communication with the one or more metal-filled vias (32).

16. The package (1 ) of any one preceding claim wherein the composite material (7) comprises two layers of polymeric material (11 , 13) that sandwich the layer of metal (9).

17. The package (1 ) of any one preceding claim wherein the composite material (7) comprises additional layers of adhesive (29) and metal (99).

18. A cavity-containing package (1 ) as defined in any one of claims 1 to 17 comprising a MEMS device (17) in said cavity (15).

19. A method for forming a cavity-containing package (1 ) as defined in any one of claims 1 to 12 comprising:

(i) providing a first substrate (3) having a first surface (4) and providing a second substrate (5) having a second surface (6); (ii) providing a loop of curable adhesive (11a, 13a, 29a) on the first surface

(4) and a loop of metal (9, 99) onto the first surface (4) or the second surface (6); and

(iii) bonding the first surface (4) of the first substrate (3) to the second surface (6) of the second substrate (5) through said loops (9, 11a, 13a, 29a, 99) whereby to form said package (1) in which the loops of metal (9, 99) and cured adhesive (11 , 13, 29) form the composite material (7).

20. The method of claim 19 further comprising applying liquid curable adhesive (25) onto the exposed edge (27) of the loop of curable adhesive, after it is applied to the first substrate (3) and prior to said connecting.

21. The method of claim 19 or claim 20 wherein said loops of metal (9, 99) and curable adhesive (11a, 13a, 29a) are each applied to the first surface (4).

22. The method of any one of claims 19 to 21 wherein two or more concentric loops of curable adhesive (11a, 13a. 29a) are provided on the first surface (4) with a gap (23) between each loop.

23. The method of claim 22 comprising applying a single loop of curable adhesive and thereafter dry-etching or photolithographing the single loop whereby to generate the gap (23) and the concentric loops (11 , 13, 29).

24. The method of claim 22 or claim 23 further comprising applying liquid curable adhesive (25) onto the exposed edges (27) of the concentric loops of curable adhesive (11a, 13a, 29a), after these are generated on the first substrate (3) and prior to said connecting.

25. The method of any one of claims 22 to 24 wherein the metal layer (9, 99) is sandwiched in said gap (23).

26. The method of any one of claims 22 to 25 wherein the metal layer (9, 99) is applied onto said first surface (4) before said connecting.

27. The method of any one of claims 19 to 26 wherein said metal (9, 99) is provided by electroplating or electroless plating.

28. The method of any one of claims 19 to 26 wherein said metal (9, 99) is provided by vacuum deposition, electrocatalytic deposition, autocatalytic deposition or photocatalytic deposition.

29. The method of any one of claims 19 to 28 wherein said first substrate (3) comprises silicon and said second substrate (5) comprises glass.

30. The method of any one of claims 19 to 29 wherein the first substrate (1 ) is provided with metal-filled vias (32) and the method further comprises providing one or more electrical interconnects (31 ) within the loop of curable adhesive (11a, 13a, 29a) such that the electrical interconnects (31 ) are in electrical communication with the metal- filled vias (32).

31. The method of claim 30 further comprising providing one or more metal bumps (35) and/or one or more or conductor lines (33) in electrical communication with the one or more metal-filled vias (32).

32. The method of claim 30 further comprising providing one or more metal bumps (35) or conductor lines (33) in electrical communication with the one or more metal-filled vias (32).

33. The method of any one of claims 19 to 32 wherein the composite material (7) comprises additional layers of adhesive (11 , 13, 29) and metal (9, 99).

34. The method of any one of claims 19 to 33 wherein a MEMS device (17) is encapsulated in said cavity (15).

35. The method of any one of claims 19 to 34 wherein said connecting is achieved by directing a laser beam (41 ) onto an area of one or both substrates (3, 5) in thermal contact with said adhesive (11a, 13a, 29a) whereby to effect its curing.