Plastic sealing adhesive accelerator - DuPont ™ Cyclotene™
Adhesive promoting chemical agent
During the manufacturing process of AP3000 and AP8000, silane is hydrolyzed through a proprietary process. This converts part of the alkoxy silane or acetoxy silyl group into silanol (Si-OH), and partially condenses to form siloxane oligomers. Silane oligomers containing silanol groups are regarded as the active components in adhesion promoter solutions. AP3000 and AP8000 are the only adhesion promoters recommended for Cyclotene™ resin.
Application of adhesive accelerator
To prepare the surface for coating with Cyclotene™ resin, the adhesion promoter solution is applied to the substrate surface and then dried by rotation. The surface of the substrate is immediately ready for BCB coating. AP3000 and AP8000 are designed for spin coating and are not recommended for steam primer application. These adhesion promoters enhance the adhesion of BCB to most surfaces. The fundamental function of adhesion promoters is to form a compatible interface between two incompatible materials. The adhesion mechanism originates from the condensation of silanol with surface oxygen atoms or hydroxyl groups. Then the surface of the metal or inorganic materials can be organic silane coverage, organic part outwards. Make the surface more compatible with organic polymers. AP3000 usually offers better adhesion than AP8000. The presence of vinyl in AP3000 (which can react with the benzocyclobutene groups in the resin) is sometimes regarded as the reason for this improvement. However, there is a lack of specific data to support this. On some surfaces, AP8000 works well, although it lacks obvious functional groups that can react with BCB polymers. The actual interaction mechanism between BCB and adhesion promoters remains unclear.
The base material is pre-cleaned
A clean surface free of particles and organic contaminants is crucial for achieving a strong interface between the Cyclotene™ film and the underlying substrate. Depending on the available equipment, a variety of different pre-cleaning treatments were used. Examples include:
O2 plasma O2 plasma;
Water flushing O2 / CF4 plasma;
Water flushing O2 / CF4 plasma;
Acetic acid rinsing Water flushing Ar plasma;
Water flushing resist stripping agent;
Water flushing O2 plasma;
Anti-corrosion agent and stripping agent Water rinse
All these methods can be used successfully, but problems may arise if they are not operated properly. For some types of plasma tools, O2/CF4 plasma may generate oxyfluorides on the metal surface, thereby reducing adhesion. Aluminium seems to be particularly vulnerable to this influence. The anti-corrosion and stripping agent has been used, but due to incomplete rinsing, residues may be produced, causing adhesion problems. The most commonly applicable pre-cleaning procedure is O2 plasma, with or without water rinsing. The RIE (Reactive Ion Etching) type plasma tool seems to be the most effective, superior to bucket etching machines and microwave plasma tools.
Some etching chemicals may attack the interface and cause the Cyclotene™ film to peel off or delaminate from the underlying surface. Sometimes this is not an attack on the adhesion promoter, but rather an etching of the substrate; As the cured BCB film is under tensile stress, the etchant continues to attack the interface, resulting in delamination.
Adhesion test methods
A variety of different methods are used to measure the adhesion of the film. This report uses the data obtained by three different test methods: tape stripping, improved Edge Stripping (m-ELT), and bump shear. The tape peel test is based on ASTM D-3359-93. In the test, a set of parallel blades were used to scratch the film to form a 1 mm grid of 10 x 10. Rating is a semi-quantitative description of the damage caused by tape adhesion and removal. The rating range is from 0B (most film delamination) to 5B (no film loss or film damage). The film was tested after drying (without temperature/humidity stress) and pressure cooker testing (PCT). The conditions for PCT are: 121 °C, 2 atmospheres of pressure, and 100% relative humidity.
The adhesion of BCB to metals
Aluminum
Although excellent adhesion to aluminium is usually observed, this surface is particularly sensitive to surface contamination, and pre-cleaning is of great importance. If O2/CF4 pre-cleaning is used, it is necessary to be careful to ensure that the aluminum surface does not convert into a high proportion of aluminum oxyfluoride, which may reduce adhesion. Please note that if you are integrating multi-layer photosensitive Cyclotene™ films with aluminium, the first layer will require a de-gummy step using fluorinated plasma, and this step must also leave an undamaged aluminium surface for the next layer of Cyclotene™ films.
Gold
Cyclotene™ films do not adhere well to gold or other precious metals. Although BCB can completely cover the gold wire and features, when the openings on the film fall on the gold interface, it may lead to a decrease in adhesion. This is because gold does not form surface oxides, so there are no oxygen atoms on the surface available for silane bonding. To avoid this situation, it is recommended to avoid contact between BCB and the gold interface as much as possible and use an intermediate layer (such as silicon nitride) to provide a surface for BCB adhesion.
III-V group semiconductors
The adhesion of Cyclotene™ films to III-V group semiconductors (GaAs, InP) is often poor, even when using AP3000. Like gold, it is best to avoid this interface and use an intermediate layer (such as silicon nitride or Ti) to provide a surface for BCB adhesion.
The adhesion of the metal on BCB
Sputtering
The most important consideration to ensure good adhesion of sputtered metal films to Cyclotene™ resin is the need for sputtering etching. Sputtering etching (usually argon) should be carried out in situ without disrupting the vacuum. 100 A equivalent silica etching is very common and can reliably provide good adhesion. The damage to the Cyclotene™ film surface caused by oxidizing plasma will significantly weaken the bonding between the metal and BCB. Slight surface oxidation may be repaired by sputtering etching, but significant surface oxidation may lead to a decrease in adhesion.
Evaporation
Usually, metals that can be sputtered can also evaporate. However, in-situ sputtering etching is still required. This means that the metal evaporation system should be equipped with an argon ion gun or ion milling.
The adhesion of inorganic materials on BCB
The integration of PECVD silicon nitride on Cyclotene™ resin has been implemented in production by multiple customers. It is important to stay within the temperature range of BCB; otherwise, it may lead to thermal degradation of the polymer. The deposition temperature should not exceed 350 °C, and it is best to be lower. When N2 is used instead of NH3 as the nitrogen source, silicon nitride often has better adhesion to BCB. NH3 plasma may damage the surface of BCB. If SiH4/NH3 chemistry is used, the pre-deposition time of NH3 should be as short as possible. It is best to introduce N2 into the chamber first, and then introduce NH3 when the plasma is stable.
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