Patent Publication Number: US-2011061884-A1

Title: Laser assisted system and method for bonding of surfaces; microcavity for packaging mems devices

Description:
The present invention relates to laser assisted bonding of surfaces and in particular to the use of lasers to assist in the process of bonding surfaces to produce packaging within which microelectromechanical systems (MEMS) devices, micro-devices, micro-systems or the like may be contained. 
     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. 
     One problem associated with the packaging of MEMS devices and with the semiconductor industry in general, is the ability to properly bond surfaces together without causing damage or minimising the damage caused to the surface and the MEMS devices during the bonding process. 
     A number of bonding techniques are known, for example, anodic bonding, low temperature silicon direct bonding, soldering and thermo-compression bonding. 
     In addition, some use of laser radiation has been used to melt and reform metal to provide a connection. 
     U.S. Pat. No. 6,394,158B1 discloses the use of a laser to melt raised metal contacts in order to connect together a transparent substrate with another substrate. The laser transmits energy through the transparent substrate to heat touching pairs of metal contacts in sequence to form a secure connection between the substrates. 
     U.S. Pat. No. 6,713,714B1 discloses a method for thermally connecting the terminal areas of a carrier substrate by heating a chip to allow contacts to melt and re-solidify, thereby forming a connection. 
     It is an object of the present invention to provide an alternative system and method suitable for bonding surfaces that will allow bonding to occur between, for example, a pair of silicon substrates, a silicon substrate and a glass cover or the like. 
     It is a further object of the present invention to provide a cavity suitable for containing a MEMS device, a micro-device or a micro-system. 
     In accordance with a first aspect of the invention there is provided a system for bonding surfaces, the system comprising: 
     a laser; and
 
laser directing means for selectively directing a laser beam onto an area of a surface to selectively heat the area of the surface in thermal contact with a curable adhesive to cause the curable adhesive to bond to the surface.
 
     Preferably, the laser directing means comprises a lens for focussing the laser beam onto the area of the surface. 
     Preferably, the laser is adapted to produce a patterned beam which is configured to illuminate the position of the curable adhesive on the substrate. 
     Preferably, the laser directing means comprises a transmission mask. 
     A transmission mask may be used in conjunction with the patterned beam. 
     Preferably, the laser directing means comprises scanning means for scanning the laser across the surface in a predetermined pattern. 
     Preferably, the curable adhesive is arranged in a predetermined pattern. 
     Preferably, the laser directing means is configured to direct the laser onto the area of the surface in contact with the curable adhesive. 
     Preferably, the laser directing means comprises an array of laser sources. 
     Preferably, the array is 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. 
     Preferably, the transmission mask is aligned with the curable adhesive predetermined pattern. 
     Preferably, the curable adhesive is a polymer. 
     Preferably the curable adhesive is a thermoplastic polymer. 
     Optionally, the curable adhesive is a thermosetting polymer. 
     Preferably, the curable adhesive is a photopolymer. 
     Preferably, the curable adhesive is benzocyclobutene. 
     The choice of curable adhesive is informed by the desire to use a substance that exhibited 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 of low cost. 
     Preferably, curable adhesive is positioned between two surfaces and is of sufficient thickness so as to separate the surfaces and form a gap between the surfaces. 
     Preferably, the curable adhesive has a thickness of at least 1 micron. 
     Preferably, the curable adhesive has a thickness of between 1 and 150 microns. 
     Preferably, the curable adhesive layer thickness may be set to provide a predetermined cavity size. 
     Preferably, the curable adhesive is patterned such that a cavity is formed between bonded surfaces, the cavity being bounded, at least in part by the curable adhesive. 
     Accordingly, a microcavity may be formed that is suitable for containing a MEMS device. 
     Preferably the curable adhesive is patterned in a ring shape. 
     Preferably, the laser has an output beam with a substantially uniform power distribution or exposure incident upon the surface. 
     The substantially uniform power distribution allows substantially uniform heating of the area of the surface. 
     Preferably, the laser has a maximum power output of up to 1 kW. 
     Preferably, the output power of the laser is more than 1 W. 
     Preferably, a mechanical load is applied to the surface during bonding of the curable adhesive. 
     Preferably, the laser has an output wavelength in the infra-red region of the electromagnetic spectrum. 
     Preferably the system further comprises a laser transmission layer that extends across the substrate. 
     Preferably, the system further comprises a laser energy absorber layer. 
     In accordance with a second aspect of the invention there is provided a method for bonding surfaces, the method comprising the steps of: 
     depositing a curable adhesive onto the surface or between two surfaces;
 
directing a laser beam onto the surface to selectively heat the area of the surface in thermal contact with the curable adhesive to cause the curable adhesive to bond to the surface.
 
     Preferably, the laser beam is directed using a lens for focussing the laser beam onto an area of the surface. 
     Preferably, the laser beam cross sectional area is larger than the sample size. 
     Preferably, the laser is adapted to produce a patterned beam which is configured to illuminate the position of the curable adhesive on the substrate. 
     Preferably, the laser beam is directed using a transmission mask. 
     A transmission mask may be used in conjunction with the patterned beam. 
     Preferably, the curable adhesive is arranged in a predetermined pattern. 
     Preferably, the transmission mask is aligned with the curable adhesive predetermined pattern. 
     Preferably, the laser beam is directed onto the area of the surface in contact with the curable adhesive. 
     Preferably, the laser is scanned across the surface in a predetermined pattern. 
     Preferably, the laser directing means comprises an array of laser sources. 
     Preferably, the array is 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. 
     Preferably, the curable adhesive is a polymer. 
     Preferably the curable adhesive is a thermoplastic polymer. 
     Preferably, the curable adhesive is a thermosetting polymer. 
     Preferably, the curable adhesive is a photopolymer. 
     Preferably, the curable adhesive is benzocyclobutene. 
     Preferably, curable adhesive is positioned between two surfaces and is of sufficient thickness so as to separate the surfaces and form a gap between the surfaces. 
     Preferably, the curable adhesive has a thickness of at least 1 micron. 
     Preferably, the curable adhesive has a thickness of between 1 and 150 microns. 
     The curable adhesive layer thickness may be set to provide a predetermined cavity size. 
     Preferably, the curable adhesive is patterned such that a cavity is formed, the cavity being bounded, at least in part by the curable adhesive. 
     Accordingly, a microcavity may be formed that is suitable for containing a MEMS device. 
     Preferably the curable adhesive is patterned in a ring shape. 
     Other suitable shapes may be used. 
     Preferably, the laser has a maximum power output of up to 1 kW. 
     Preferably, the output power of the laser is more than 1 W. 
     Preferably, a mechanical load is applied to the surface during bonding of the curable adhesive. 
     Preferably, the laser has an output wavelength in the infra-red region of the electromagnetic spectrum. 
     Preferably the system further comprises a laser transmission layer that extends across the substrate. 
     Preferably, the system further comprises a laser energy absorber layer. 
     Preferably, the method further comprises the step of, prior to directing a laser beam onto the surface, pre-bonding the surface with the curable adhesive. 
     Preferably, the pre-bonding step comprises curing the adhesive at a temperature lower than that required to fully bond the adhesive using laser heating. 
     Preferably, the pre-bonding step occurs under vacuum conditions, below standard atmospheric pressure. 
     Preferably, the pre-bonding step occurs in a vacuum chamber. 
     In accordance with a third aspect of the invention there is provided a microcavity for packaging MEMS devices, the microcavity comprising: 
     a first boundary surface;
 
a second boundary surface bonded to the first boundary surface by means of a curable adhesive; and
 
wherein the curable adhesive is selectively deposited between the boundary surfaces to form a cavity separating the first and second boundary surfaces.
 
     Preferably, the curable adhesive has a thickness of at least 1 micron. 
     Preferably, the curable adhesive has a thickness of between 1 and 150 microns. 
     The curable adhesive layer thickness may be set to provide a predetermined cavity size. 
     Preferably, the curable adhesive is patterned such that a cavity is formed, the cavity being bounded, at least in part by the curable adhesive. 
     Accordingly, a microcavity may be formed that is suitable for containing a MEMS device. 
     Preferably the curable adhesive is patterned in a ring shape. 
     The system and method of the present invention 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 substrate(s). 
     Therefore the temperature increase at the device location can be 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. 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. 
     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. 
    
    
     
       The present invention will now be described by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  illustrates laser curing of an adhesive onto a substrate surface in accordance with the present invention; 
         FIG. 2  is a graph showing the effect of different conditions upon the curing of the adhesive; 
         FIG. 3  illustrates two surfaces separated by a layer of curable adhesive; 
         FIG. 4 . illustrates an example of the process of laser curing in accordance with the present invention; 
         FIG. 5  illustrates a system in accordance with the present invention in which a curable adhesive is bonded to two surfaces; 
         FIG. 6  is a graph showing the temperature variation over time during laser curing in accordance with the present invention; 
         FIG. 7  illustrates system in accordance with the present invention in which a curable adhesive is bonded to two surfaces and a transmission mask is used to allow laser transmission to the surface and adhesive; 
         FIG. 8   a  to  FIG. 8   c  show plan views of a number of microcavities in accordance with the invention; 
         FIG. 9  illustrates localised laser heating on an absorbing layer; 
         FIG. 10   a  to  FIG. 10   f  illustrates the effect of the focal position of a laser beam on heating of a silicon wafer surface; 
         FIG. 11  is a graph showing measured temperature of a silicon surface as a function of laser power for different positions relative to the laser focus; 
         FIG. 12   a  to  FIG. 12   c  are photographs of an exemplary uncured BCB ring; 
         FIG. 13  is a schematic view of a further embodiment of the present invention in which a heat absorbing layer is included; 
         FIG. 14  is a schematic view of a further embodiment of the present invention in which a patterned beam is used; 
         FIG. 15  is a schematic view of a further embodiment of the present invention in which a patterned beam and optical mask is used; 
         FIG. 16  is a schematic view of a diode laser source for use with the system of the present invention; 
         FIG. 17  is a schematic view of a fibre optic laser source for use with the system of the present invention; 
         FIG. 18  is a schematic figure that shows a feature of an embodiment of the present invention in which a pre-bonding step is performed under atmospheric pressure; 
         FIG. 19  is a schematic figure that shows a feature of an embodiment of the present invention in which a pre-bonding step is performed in a vacuum chamber; and 
         FIG. 20  is a schematic figure that shows the laser heating step used in a method having a pre-bonding step. 
     
    
    
       FIG. 1  shows a system  1  in accordance with the present invention in which a silicon substrate  3  mounted on a graphite platform  5  is selectively coated or patterned with a curable adhesive film  4  and cured using laser beam  7  which selectively heats the area of the substrate to which the adhesive is applied. Suitably, the adhesive film  4  is a blank/uniform BCB film on a 10 mm square silicon substrate. 
     The response of the surface to temperature changes can be characterised by monitoring the surface temperature of the BCB film  4  using a pyrometer. The operating current of the laser system can be used to control the exposure of BCB film  4  to laser irradiation  7  and the exposure time can be controlled by switching the laser on and off. 
     Fourier Transform Infra-red (FTIR) measurements on samples created using the system of the present invention show that the sample cured with a laser operating at 68 W was undercured  11 , the sample cured with a laser operating at 159 W was overcured and the sample cured with a laser operating at 97 W was fully cured. The undercured sample shows an absorption peak at the wavenumber of 1474 cm −1 . This absorption peak is the characteristic of undercured BCB film. The results are presented in the graph  9  of  FIG. 2 . 
     The fully cured sample was not damaged, as it shows no absorption peak at 1474 cm −1  but a large absorption peak at 1550 cm −1 . The overcured sample was damaged by the laser curing and does not show either absorption peak. 
     Conveniently, a large beam area is used to obtain better uniformity of exposure of the BCB samples. As the laser beam intensity distribution is not uniform across the beam, the intensity variation across a sample is less if the beam size is much larger than the sample size. The result can be a substantially uniform exposure. Though the laser has maximum output of 1 kW, less than 200 W was sufficient for the laser bonding work to be presented. Where this type of beam is used a mask is provided to ensure that the laser radiation is incident on the curable material. 
       FIG. 3  shows a microcavity in accordance with the present invention. The microcavity  167  is bounded by a glass cover  19 , a silicon substrate  21  and a polymer ring  23  (in this example made from benzocyclobutene) which is of sufficient thickness to provide a cavity of suitable depth to accommodate a MEMS device. 
       FIG. 4  shows an example of part of the method of the present invention. Firstly near IR laser radiation is absorbed by the silicon substrate  25 ; localised heating causes a temperature increase in the BCB layer  27  which causes thermal curing of the BCB layer  29 . 
       FIG. 5  shows a system  31  in accordance with the present invention. The system comprises a laser  33  transmitted through a converging lens  35  and transmitted through a glass plate  37  and glass cover  41  to selectively heat an area containing the BCB ring  43  and the silicon substrate  45 . The silicon substrate is supported on a second glass plate  46  and a graphite platform  47 . This glass plate  46  was used to achieve reproducible contact between the silicon substrate  45  and glass plate  46 . 
     The size of the silicon substrates  45  with square BCB rings  43  is suitably 4 mm×4 mm. The glass cover  41  (8 mm×8 mm) to be bonded to the BCB ring  43  is placed on top of the ring  43  with another glass plate  37  being placed on top of the glass cover  41  for applying mechanical load to the glass cover for bonding. Metal bars  39  are used as the mechanical load. The temperature of the top surface of the load glass plate  37  can be monitored with a pyrometer during the bonding process. 
       FIG. 6  is a graph  49  which shows the profile of temperature change of the top surface of the load glass plate during and after the exposure process, as monitored with a pyrometer for an exposure time was 6 seconds and laser power 131 W. The corresponding temperature within the BCB layer is higher than that shown by the graph, because of the temperature gradient in the glass cover and the load glass plate. The dynamic temperature change during the bonding process can be monitored using the pyrometer. Thermal modelling allows the determination of the actual temperature change as time in the BCB layer. 
     A range of bonding parameters allow uniform bonding. The bonding time can range from 6 seconds to minutes by choosing appropriate power levels. In general, a shorter bonding time requires higher laser power, and a higher curing temperature results in shorter curing time. It is possible to achieve good bonding at exposure times shorter than 6 seconds. Mechanical loads of 360 g and 580 g have been used to achieve uniform bonding. 
     Laser bonding of the glass cap to silicon chip using a BCB sealing ring can be achieved using exposure times of 6 seconds, more than 100 times faster than the times achieved using conventional “global” heating methods. The fast laser bonding process enables efficient manufacture of micro &amp; nano systems with uniform bonding across the BCB sealing rings and minimises the heat stress in the device. 
       FIG. 7  shows a further example of the system of the present invention. The system  51  comprises a laser beam  53  a converging lens  55 , a transmission mask  57 , a glass cover  59  a BCB ring  61  and a substrate  63 . In this example, the invention is used to produce microcavities for encapsulation and packaging micro-devices and micro-systems. In this example, the polymer (BCB) is suitably between 2 and 100 microns thick and provides an intermediate joining layer that separates the glass cover  59  and the substrate  63  to form the cavity  64 . 
     As shown in the  FIG. 7 , two substrates are bonded by laser curing the polymer ring  61  through a transmission mask  57 . Alternatively, this can be achieved by scanning a focussed beam along the polymer ring  61 . The thick polymer layer bonds and joins two surfaces of the polymer ring  61  and the inner surfaces of the two substrates  59  and  63 . This is different from laser welding of plastic parts in which the joining is between the surfaces of the two joining parts. Although in some cases a very thin absorbing layer of polymer is used in laser welding of plastics the actual joining is still between the surfaces the joining parts. 
       FIG. 8   a  to  FIG. 8   c  show plan views of a number of microcavities  65 , 73 , 81  containing MEMS devices  71 , 79 , 89 .  FIG. 8   a  illustrates a MEMS device  71  encapsulated in the microcavity  65  formed by the polymer ring  69  on the substrate  67 . Similarly,  FIG. 8   b  illustrates a MEMS device  79  encapsulated in a more irregularly shaped microcavity  73  formed by a thicker and more eccentrically shaped polymer ring  77  on the substrate  75 . Finally,  FIG. 8   c  illustrates a MEMS device  89  held within a microcavity  81  formed by two sections  85 ,  87  of a non-continuous polymer ring on the substrate  89 . 
     The substrates can be glass, silicon, ceramics, plastics and metal. The substrates can be flat or can have a recess or recesses. 
     One substrate may be transparent to the laser beam. Absorption of laser radiation for thermocuring of the polymer ring can be achieved using substrate absorption, or absorption by the polymer layer. If the polymer is transparent, absorbing agents such as carbon powder or chemical molecules can be added to produce absorption in the polymer layer. An additional absorbing layer between the adhesive layer and one or both substrates can also be used to provide absorption and hence temperature rise for curing the adhesive layer. 
       FIG. 9  illustrates schematically this process. The bonding layers comprise a transparent top layer  91  and an absorbing bottom layer  93 . A laser beam  95  is incident on the absorbing layer  93  having passed through the transparent layer  91 , and a heated zone  97  is created. An absorbing intermediate layer (not shown) may also be used. 
       FIG. 10   a  to  FIG. 10   f  illustrates the effect of the focal position on heating of a silicon wafer surface.  FIG. 10   a  is a schematic drawing of the position of the silicon wafer  99  with respect to the focal point  101  of a laser beam  103  focussed by a focussing lens  105 .  FIG. 10   b  to  10   f  show thermal imaging photographs in false colour and gradient temperature profiles of a 10×10 mm 2  silicon wafer under laser radiation for focal positions; (b)=−40 mm, (c)=−30 mm, (d)=−20 mm, (e)=−10 mm and (f)=0 mm. 
     Temperatures were measured by single point pyrometer rendering average surface temperatures of; (b)=324° C., (c)=310° C., (d)=347° C., (e)=321° C. and (f)=336° C. 
     This clearly illustrates how the temperature of the substrate can be varied without necessarily varying the laser power, and as such a feedback loop can be envisaged whereby the pyrometer provides feedback to control the position of the wafer. 
       FIG. 11  shows a graph  107  which demonstrates the measured temperature on a 10×10 mm 2  silicon surface for different positions relative to the laser focus position. 
       FIGS. 12   a  and  12   b  are photographs of an uncured BCB ring  109  before laser radiation has been applied. The BCB ring  109  has been formed on a silicon 4×4 mm 2  substrate  111 , and is around 320 μm wide.  FIG. 12   c  shows the covering glass  113  in place. Such a configuration would be used to bond the silicon substrate to the glass in accordance with the method exemplified in  FIG. 5 . 
       FIG. 13  is a schematic view of a further embodiment of the present invention in which a light absorbing layer is included. The system is shown without the laser source or laser directing means. The system  121  comprises a transparent cap  123 , an adhesion layer  125  which holds the absorbing layer  127  in place in proximity with the curable adhesive  129 , which is also positioned on a substrate  131 . 
     In this example, the absorbing layer  127  comprises a thin layer of metal or other optically absorbing material which has been deposited on the surface of the transmission cap  123  or on the substrate/wafer  131  to act as the contacting surface to the adhesive layer  129 . This additional layer  127  acts as a uniform absorber to absorb laser energy to produce the required temperature rise to cure the adhesive. Advantageously, a lower temperature rise in the MEMS device structure may be caused, thereby reducing thermal effects on the device. In these circumstances, the device substrate need not be used to absorb the laser energy. 
     A thin layer of chromium or titanium  125  may be used to improve the adhesion of the metal layer to the capping substrate  123 . The thickness of the metal film, e.g. nickel, is typically 100 nm to 10 um and is produced by vacuum deposition and/or electrodeposition or electroless deposition. The thickness of the chromium/titanium layer is typically in the region of 10 nm to 500 nm and is produced by vacuum deposition. 
       FIG. 14  is a schematic view of a further embodiment of the present invention in which a patterned beam is used The system  141  shows a laser  143  that produces a patterned beam  145  incident upon a transparent cap  147  and which heats the curable adhesive  149  on substrate  151 . The laser beam  143  with patterns produced using beam forming optics illuminates the areas to be bonded between the cap  147  and substrate  151 . 
     A laser beam with a predetermined pattern matching that of the adhesive is preferred. This improves the efficiency of the usage of the laser energy as most laser light is directed to the bonding area. The beam pattern can be generated using reflective and/or refractive optics. 
       FIG. 15  shows a system similar to that of  FIG. 14  comprising a laser  155  that produces a patterned beam  157  incident upon a transparent cap  161  and which heats the curable adhesive  163  on substrate  165 . The system of  FIG. 15  further comprises an optical mask  159 . 
     The optical mask  159  has been added in the beam path to remove peripheral light from the laser beam to ensure that only the adhesive pattern is illuminated. A laser beam with a predetermined pattern reduces the risk of damaging the optical mask by high intensity light. 
       FIG. 16  is a schematic view of a diode laser source for use with the system of the present invention. The diode laser source comprises an array  167  of diode lasers  169  mounted in a chuck or block  173 , each of which is provided with a microlens array  171  at the end of channel  175 . 
     In this embodiment the array  167  of diode lasers  169  mounted on a block  173  is pointed to the joining interface with or without lenses and near field illumination is used to heat up the substrate and the joining material to join two substrates together. 
     In this example, diode laser bars each containing a linear or two dimensional array of lasers, may be arranged in a pattern matching that of the adhesive for bonding two substrates together. In this case the lasers are mounted on a bonding chuck  173  together with the force applicator. This will result in a compact, low cost bonding head that can be fitted onto existing commercial device bonders with minimal modification. 
     The diode array can also be selectively turned on and off to change the output beam pattern as required for a particular application. Microlens arrays can be integrated with the laser bars to shape the laser beam to produce a beam pattern to match that of the adhesive. 
       FIG. 17  is a schematic view of a fibre optic laser source for use with the system of the present invention. The figure shows an array of optical fibres  177 , a chuck  179 , a channel  181  in which the fibres are mounted and a Microlens  183 . The array of optical fibres  177  mounted on a block or chuck  179  is pointed at the joining interface with or without lenses to heat up the substrate and the joining material to join two substrates together. 
     In this embodiment, the required beam pattern is generated using an array of optical fibres  177  each delivering a single laser beam. The optical fibres  177  are mounted in a block by inserting the fibres in the holes produced on the block made of metal, ceramic or glass material. This approach does not require active cooling of the block as may be the case is may be required if the lasers are directly mounted on the block. 
     The block itself can be part of the bonding chuck with a force applicator. The Microlens arrays  183  can be integrated with the fibre array  177  to shape the laser beam to produce a beam pattern to match that of the adhesive. 
       FIGS. 18 to 20  illustrate an example of a method in accordance with the present invention in which a pre-bonding step using conventional bonding is used in conjunction with a final bonding step that uses laser heating. 
       FIG. 18  shows an example in which the pre-bonding step occurs in an ambient environment. In this case, the system  191  comprises a chuck  193 , wafers  195  and  197 , and an adhesive  199 . 
       FIG. 19  shows the features of  FIG. 18  enclosed in a vacuum chamber  201  having a gas inlet  203  and a gas outlet  205 . 
     The laser heat bonding step is shown in  FIG. 20 . This step completes the curing process.  FIG. 20  shows a laser  207  that generates a patterned beam  209 , wafers  195  and  197  along with the curable adhesive  199 . 
     In this two step laser bonding process, the substrates/wafers  195 ,  197  are aligned and pre-bonded using a conventional wafer bonder to allow partial curing of the adhesive ( FIGS. 18 and 19 ). Then localised laser heating is used to complete the full curing process at a higher temperature. For example for BCB adhesive, the pre-curing may be realised below 170° C. degrees and the full curing above 250° C. using localised laser heating. 
     Advantageously, this removes the requirement of coupling laser beam into a vacuum chamber for vacuum packaging. This method also allows decoupling of force application and laser bonding. In wafer bonding a large bonding force is necessary to achieve good contact between the two wafers. But the chuck for force application is often not transparent to the laser beam, therefore it is difficult to combine laser heating and force application at the same time. It is a great advantage to remove the force requirement for laser assisted bonding. 
     Laser bonding can be achieved by projecting the laser onto the adhesive patterns to cure it in a stepped manner or by scanning a laser beam. Masks may be used to expose only the adhesive patterns to the laser beam. The scanning beam can be in any chosen cross-sectional distribution by using beam shaping optics. 
     The polymer material can be a thermosetting material such as benzecyclobutene (BCB) or a thermoplastic material. Polymers that can be patterned by photolithography are preferred. Polymer rings may also be produced by stencil printing and inkjet printing. 
     The photopatterning capability is an important advantage of the polymer curable adhesive such as BCB when used as a sealing/bonding material for MEMS encapsulation. 
     Additional films (organic, inorganic or metal) may be deposited on the substrates to improve the adhesion strength of the adhesive bonding layer and to provide a tightly sealed cavity. 
     Improvements and modifications may be incorporated herein without deviating from the scope of the invention.