Adhesive bonding of semiconductor wafers
Because the wafers are not in direct contact, this procedure enables the use of different substrates, such as silicon, glass, metals or other semiconductor materials.
A drawback is that small structures become wider during patterning which hampers the production of an accurate intermediate layer with tight dimension control.
[3] Further, the possibility of corrosion due to out-gassed products, thermal instability and penetration of moisture limits the reliability of the bonding process.
[4] Another disadvantage is the missing possibility of hermetically sealed encapsulation due to higher permeability of gas and water molecules while using organic adhesives.
[5] Adhesive bonding with organic materials such as BCB or SU-8 has simple process properties and the ability to form high-aspect ratio micro structures.
The bonding procedure is based on polymerization reaction of organic molecules to form long polymer chains during annealing.
[3] The intermediate layer is applied by spin-on, spray-on, screen-printing, embossing, dispensing or block printing on one or two substrate surfaces.
[7] Prior to performing the degreasing process, the compatibility of the solvent used and the adherend must be considered to prevent irreversible damage of the surface or part.
[8] The other method of degreasing requires a cloth or rag soaked in solvent, which can be used to wipe down the surface of the adherend to remove contaminants.
[7] In general, abrasion is superior to other methods of surface preparations due to the fact that it is simple to perform, and it does not produce a significant amount of waste.
[6] Following abrasion, the adherend should always be wiped with solvent or an aqueous detergent solution to clean the surface of any oils and loose material and then dried.
[9] Corona discharge treatment (CDT) is typically used to improve adhesion of ink or coatings on plastic films.
[6] Once the flame treatment is completed, the part can be gently cleaned with water and air dried, which will ensure that an excess of oxides are not formed.
[8] In addition to removing contaminates, the plasma treatment also introduces polar groups that increase the surface energy of the adherend.
[7] Plasma treatment can produce adhesive bonds up to four times stronger than compared to chemically or mechanically treated adherends.
[6] Chemical treatments are used to change the composition and structure of the surface of the adherend and are often used in addition to degreasing and abrasion to maximize the strength of the adhesive bond.
However, too long of time can lead to excess reaction products that form and can hinder the bonding performance between the adhesive and adherend.
[6] SU-8 is a 3 component UV-sensitive negative photoresist based on epoxy resin,[11] gamma butyrolactone, and triaryl sulfonium salt.
In addition, the substrate flatness, clean room conditions and the wettability of the surface are important factors to achieve good bonding results.
Subsequently, the structuring of the photo-resist using direct UV light exposure is applied but can also be achieved through deep reactive-ion etching (DRIE).
[3] The procedural steps based on a typical example are: For non-planar wafer surfaces or free standing structures, spin-coating is not a very successful SU-8 deposition method.
[13] Ensuring void-free bonding a homogeneous layer thickness of the SU-8 over the wafer surface is important (compare to cross section photo).
This polymer ensures very strong bonds and excellent chemical resistance to numerous acids, alkalines and solvents.
[17] The adhesion promoter with a specific thickness is deposited, i.e. spin-coated or contact printed on the wafer to improve the bonding strength.
The soft cure prevents bubble formation and unbonded areas[21] as well as the distortion of the adhesive layer during compression to improve the alignment accuracy.
The vacuum prevents air trapped in the bond interface and pumps out the gases of the out-gassing residual solvents during annealing.
[15] Adhesive bonding using a BCB intermediate layer is a possible method for packaging and sealing of MEMS devices, also structured Si wafers.
Its use is specified for applications that does not require hermetic sealing, i.e. MOEMS mirror arrays, RF MEMS switches and tunable capacitors.
BCB bonding is used in the fabrication of channels for fluidic devices, for transfer protruding surface structures as well as for CMOS controller wafers and integrated SMA microactuators.
