Adhesive bonding (also referred to as gluing or glue bonding) describes a wafer bonding technique with applying an intermediate layer to connect substrates of different types of materials. Those connections produced can be soluble or insoluble.
The commercially available adhesive can be organic or inorganic and is deposited on one or both substrate surfaces. Adhesives, especially SU-8, benzocyclobutene (BCB) and Parylene are specialized for production of MEMS and electronic components.
The procedure enables bonding temperatures from 1000 °C down to room temperature.
Adhesive bonding has the advantage of relatively low bonding temperature as well as the absence of electric voltage and current. 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. Further, the possibility of corrosion due to out-gassed products, thermal instability and penetration of moisture limits the reliability of the bonding process. Another disadvantage is the missing possibility of hermetically sealed encapsulation due to higher permeability of gas and water molecules while using organic adhesives.
More recently, Parylene-mediated wafer bonding has been demonstrated to alleviate complications of out-gassing, as well as hermiticity, moisture penetration, corrosion and thermal stability within human living conditions, while requiring slightly higher bond temperatures (260 to 450 °C). This development enables long-term bonding solutions and extends the range of possible applications.
Overview 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. This cross-link reaction forms BCB and SU-8 to a solid polymer layer.
The intermediate layer is applied by spin-on, spray-on, screen-printing, embossing, dispensing or block printing on one or two substrate surfaces. The adhesive layer thickness depends on the viscosity, rotational speed and the applied tool pressure. The procedural steps of adhesive bonding are divided into the following: Cleaning and pre-treatment of substrates surfaces Application of adhesive, solvent or other intermediate layers Contacting substrates Hardening intermediate layer
Parylene layers for adhesive bonding are produced using CVD with the substrate at room temperature. It polymerizes in-situ during the so-called Gorham process. There is also no further substrate treatment required before or after deposition, making the procedure highly compliant with most materials.
The most established adhesives are polymers that enable connections of different materials at temperatures ≤ 200 °C. Due to these low process temperature metal electrodes, electronics and various micro-structures can be integrated on the wafer. The structuring of polymers as well as the realization of cavities over movable elements are possible using photolithography or dry etching.
The hardening conditions depend on the materials used. Hardening of the adhesives are possible: at room temperature through heating cycles using UV light by applying pressure
Process parameters The most important process parameters for achieving a high bonding strength are: adhesive material coating thickness bonding temperature processing time chamber pressure tool pressure
Surface preparation of plastics There are three major requirements of creating a desirable surface for adhesive bonding of plastics: the weak boundary layer of the given material must be removed or chemically modified to create a strong boundary layer; the surface energy of the adherend should be higher than the surface energy of the adhesive for good wetting; and the surface profile can be improved to provide mechanical interlocking. Meeting one of these major requirements will improve bonding; however, the most desirable surface will incorporate all three requirements. Numerous techniques are available to help produce a desirable surface for adhesive bonding.
Degreasing When preparing a surface for adhesive bonding, all oil and grease contamination must be removed in order to form a strong bond. Although the surface may appear to be clean, it is important to still use the degreasing process. 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.
Vapor degreasing One method of degreasing is vapor degreasing, where the adherend is dipped in a solvent. When removed from the solvent, the vapors condense on the surface of the adherend and dissolve any contaminants that had existed. These contaminants then drip off the adherend with the condensed vapors.
Instead of vapor degreasing 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. It is important that all residue that had been left behind from the solvents be removed, so that there is no detrimental effects to the adhesive bonding.
Following degreasing process After degreasing, a good test to determine cleanliness of the surface is to use a drop of water. If the drop spreads on the surface, a low contact angle and good wettability has been achieved, which indicates the surface is clean and ready for application of the adhesive. If the drop beads up or retains its shape, the degreasing process should be repeated.
Abrasion 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. To prepare the adherend for bonding, the surface can be sanded or grit blasted with an abrasive material to roughen the surface and remove any loose material. Rough surfaces produce stronger bonds because they have an increased surface area for the adhesive to bond to as compared to a relatively smooth surface. In addition, roughening the surface will also increase mechanical interlocking. 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. After this process is complete, the adhesive can be applied.
Peel ply For a peel ply, a thin, woven piece of material is applied to the adherend during fabrication. Because the material is woven, it will leave a torturous surface when removed, which will improve bonding by mechanical interlocking. Prior to adhesive bonding, the woven material acts to protect the surface of the adherend from contaminates. When an adhesive is ready to be applied, the material can be peeled off, leaving a rough and clean surface for bonding.
Corona discharge treatment Corona discharge treatment (CDT) is typically used to improve adhesion of ink or coatings on plastic films. In the CDT, an electrode is connected to a high voltage source. The film travels on a roller that is covered with a dielectric layer and is grounded. When a voltage is applied, the electrical discharge causes ionization of air, and a plasma is formed.Chan CM. (1999) Surface treatment of polypropylene by corona discharge and flame. In: Karger-Kocsis J. (eds) Polypropylene. Polymer Science and Technology Series, vol 2. Springer, Dordrecht In doing so, the surface of the film is oxidized, thus improving wetting and adhesion. Additionally, the discharge reacts with molecules of the adherend to form free radicals, which react with oxygen and eventually form polar groups that increase the surface energy of the adherend. Another way CDT improves bonding is that it roughens the adherend by removing the amorphous regions of the surface, which increases the surface area and improves adhesive bonding. Depending on the type of adherend being treated with CDT, the treatment times may differ. Some adherends may require longer treating times to achieve the same surface energy.
Flame treatment In flame treatment, a mixture of gas and air is used to produce a flame that is run over the surface of the adherend. The flame that is produced must be oxidizing in order to produce an effective treatment. This means that the flame is blue in color. Flame treatment can be performed by using a setup similar to the CDT in which plastic film travels across a roller while the flame contacts it. In addition to more sophisticated methods, flame treatment can also be done by hand with the use of a torch. However, even and steady treatment of the surface is more difficult to obtain. 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. Control during the flame treatment is critical. Too much of the treatment will degrade the plastic, which will lead to poor adhesion. Too little of a treatment will not modify the surface enough and will also lead to poor adhesion. An additional aspect of flame treatment that must be considered is possible deformation to the adherend. Precise control of the flame will prevent this from occurring.
Plasma treatment Plasma is a gas excited by electrical energy, and contains approximately an equal density of positive- and negative-charged ions. The interaction of the electrons and ions in the plasma with the surface oxidizes the surface and forms free radicals. The oxidation of the surface removes unwanted contaminants and improves adhesion. In addition to removing contaminates, the plasma treatment also introduces polar groups that increase the surface energy of the adherend. Plasma treatment can produce adhesive bonds up to four times stronger than compared to chemically or mechanically treated adherends. In general, plasma treatment is not used often in industry because it is required to be performed below atmospheric pressure. This creates an expensive and less cost effective process.
Chemical treatment 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. In addition to this, they increase the chance of other bonding forces to occur, such as hydrogen, dipole, and van der Waals bonding between the adherend and the adhesive. Chemical solutions can be applied to the surface of an adherend to either clean or alter the surface of the adherend, depending on the chemical used. Solvents are used to simply clean the surfaces of any contaminates or debris. They do not increase the surface energy of the adherend.
To modify the surface of the adherend, acid solutions can be used to etch and oxidize the surface. These solutions must be carefully prepared in order to ensure good bonding strength is developed. These treatments can be made more effective by increasing the time and temperature of the application. However, too long of time can lead to excess reaction products that form and can hinder the bonding performance between the adhesive and adherend. As with other surface preparation methods, a good test to assure a good chemical treatment is to put a drop of water on the surface of the adherend. If the drop flattens or spreads out, it means the surface of the adherend has good wettability and should allow for good bonding. A final consideration when using chemical treatments is that of safety. The chemicals used in the treatments can be hazardous to human health and before using any, the material safety data sheet for the particular chemical should be referenced.
Ultraviolet radiation treatment