Validación (VAL)

To complete the reactor, the channels had to be covered. It had been decided to use glass plates as channel caps to allow observation o f any changes to the reactor during and/or after reaction. The temperatures which would be used limited the choice o f bonding techniques to anodic bonding and fusion bonding. O f the two, fusion bonding requires smoother surfaces for successful bonding and higher temperature. As such, anodic bonding was chosen, with fusion bonding as a possible alternative.

Pyrex 7740 glass was used as it has a similar thermal expansion coefficient with silicon. This is important to reduce the stress in the structure during and after the bonding. Because silicon is rather brittle, the glass wafer was made fairly thick (3 mm) to make the structure stronger. The added thickness would also make the reactor easier to handle and be connected to inlet and outlet tubing. However, there was the possibility that the thickness of the glass would cause difficulties in bonding.

Therefore, as with etching, exploratory work had to be performed. The commercial anodic bonders used comprised essentially o f two metal plates located within a pressure chamber. The items to be bonded would be placed between the plates and heated, while the chamber was depressurised. Once the desired temperature was reached, a high voltage would be applied across the plates to cause bonding to occur. During this investigation, changes o f equipment occurred, resulting in interruption o f work. In the end, a simple system was built at UCL to perform this function.

In each o f the following cases, the etched silicon wafer was cut up into its component reactors. The Pyrex wafers obtained were slightly larger than each reactor to provide protection against chipping the sides o f the silicon. Each Pyrex wafer had 3 holes pre­ drilled, corresponding to the location o f the inlets and outlet o f the reactor.

It must be noted that because the reactor is catalytic, a catalyst had to be deposited. Several techniques were attempted, both before and after bonding. To reduce repetition, all references to catalyst deposition in this section will be brief, with more details given in Section 3.5.

3.4.1 Initial Attempt

This was done at the CMF using an AML Anodic Bonder. The procedure used was: i) Silicon nitride was first stripped using the RLE 80. CHF3 and O2 flowrates were 70

seem and 4 seem respectively. Chamber pressure was 100 mT and 150 W power was supplied. Process time was 8 minutes.

iii) The anodic bonder was designed such that the silicon wafer would be loaded on a bottom plate that was vertically mobile, while the glass wafer would be loaded on to the bottom o f a fixed top plate which was part o f the cover for the pressure chamber. Alignment would then be performed visually using a microscope by looking through a glass window. Unfortunately, as the glass wafers used in electronics are much thinner than those used in this project (3 mm), the glass wafer could not be clipped to the top plate. As such, it had to be placed resting on top o f the silicon wafer and properly aligned before sealing the chamber.

iv) The pressure chamber was then brought to a vacuum. This is not necessary for making a microreactor, but was part o f the procedure for using the machine, which is used to package electronic components.

v) The temperature o f both plates was then ramped up gradually to 365 °C. Heating was provided by 8 heat lamps placed around each plate to provide even heating. vi) A voltage o f 1.5 kV was then applied across the plates for bonding.

vii) After 90 min, the power was switched o ff and the chamber pumped to atmospheric pressure. Following this was a lengthy wait for the chamber to cool down before the completed reactor could be removed.

When the voltage was first applied, a large current was detected. As the bonding proceeded, the current decreased until it reached zero (with corresponding increase o f voltage to 1.5 kV). Interference fringes which had formed at the surface o f the silicon and glass interface upon clamping slowly disappeared throughout the process, to be replaced by a darker grey “wetted” colour. The initial results were promising. Bonding was achieved over most o f the reactor, with only small isolated unbonded patches towards the edges.

3.4.2 Further Exploratory Work

Two further attempts were made at anodic bonding, the first with the machine used previously, and the second using a new AML anodic bonder. For the older machine, the procedure used was as before. The sample was KOH-etched, and had silver deposited in the reaction channel through evaporation (refer to Section 3.5). However, during the process o f preparing the sample for silver deposition, the bonding surface was dirtied. A cleaning attempt was made by immersing the sample in acetone in an ultrasonic bath for 5 min. Some marks were still noticeable, but it was decided to carry on with the experiment.

After the process time elapsed, the sample was cooled using the recommended cooling procedure. At first, the sample seemed to be completely bonded. However, upon cooling, the structure rapidly de-bonded at various spots. When tested, the glass and silicon wafers were relatively easy to pull apart. The poor bonding in this experiment may have been due to the following reasons:

a) dirty surface: grease or other contaminants such as silver may have been present on the silicon wafer’s bonding surface. This would have interfered with bonding b) rapid cooling: the unbonding was noticed to speed up whenever the cooling rate

was increased. This may be due to stress within the silicon and glass wafers. There is also the possibility that either wafer had been warped during processing. c) contaminants: a greasy smell emanated from the bonding chamber after the

experiment. It is possible that some oil from the diffusion pump had leaked into the chamber during the process, thus contaminating the bonding surfaces.

The next attempt was performed using a new machine. This machine was an improvement over the last one, as it used a turbo pump instead o f diffusion and backing pumps. As such, the pump down cycle was rapid. The heating arrangement was similar, as gradual heating was required to reduce stress in the wafers.

Because acetone did not seem to remove all traces o f organic material, a new cleaning procedure involving a conventional plasma de-scum process using the RIE 80 was introduced. The process is the same as that used for photoresist removal, and would, in theory, remove all traces o f organic compounds. The anodic bonding conditions were the same as before, except that a longer bonding time o f 1.5 h was used.

A new development occurred during the bonding cycle: a crackling sound was heard when the voltage was applied. Also, the current dropped from 10 mA to virtually zero in 5 s. None o f the interference fringes usually seen at the initial stages o f bonding were evident. Instead, the entire interface had taken on the ‘w etted’ appearance seen upon complete bonding. As this had taken only a fraction o f the prescribed time, it was decided to leave the sample in the chamber for the original period o f time.

More care was taken with the cooling process this time. Unfortunately, the pieces still de-bonded as cooling proceeded. Again, it was observed that each time a quicker cooling phase was entered, more unbonding took place.

3.4.3 Development Work

Because o f the samples were still de-bonding, it seemed likely that the surfaces were still not sufficiently clean. Therefore, instead o f using an oxygen plasma for cleansing purposes, a commercial organic cleaner, EKC 265, was used. The solution was first

warmed to 70 °C before the reactor was immersed in it for 30 min. Distilled water was then used to rinse the reactor. Nitrogen was first used to dry the sample, followed by baking at 100 °C for 15 min.

The following procedure was used for the bonding process: • chamber at vacuum

• platen temperature: 385 °C • voltage: 1.0 kV

• current limit: 1.4 mA • duration: 1 0 min

Additionally, stainless steel washers were placed on top o f the inlet/outlet holes on the glass. It was thought that this would help focus the electric field and therefore improve bonding as, previously, the areas most susceptible to unbonding were around the inlets/outlet. Bonding is believed to have taken place after 2-3 min, as the current was seen to drop to about zero by then. However, the experiment was left running for the full 10 min. Bonding was successful, with no noticeable large unbonded areas.

Anodic bonding was also performed at the Central Research Laboratories (CRL) following the above procedure, but instead using a piranha solution (1 : 1 H2SO4/H2O2 mixture) for cleaning and without the use o f the stainless steel washers. Bonding was also successful, suggesting that the washers were not required to aid the process. It was also decided to use the piranha solution for all future bonding experiments as well, as it removes inorganic substances as well. Another reason was that the organic solvent proved incompatible with the deposition o f silver through evaporation, as detailed in section 3.6.1.

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Figure 3.15: T-m icroreactor sealed using anodic bonding

3.4.4 Simple Anodic Bonder

In an attempt to perform as much o f the fabrication at UCL as possible, a simplified version o f the anodic bonder was set-up. Because the reactor did not need to be vacuum sealed, a pressure chamber and vacuum pumps were unnecessary. A hot plate was used as the heat source. A flat stainless steel plate was placed on top o f the hot plate as the cathode. The silicon wafer was placed on top o f the cathode, with the glass resting on top o f it. A screw, which was mounted on a clamp, would press on to the top o f the glass wafer to provide clamping pressure and serve as the anode. The main concern was that the temperature o f the glass would be far removed from that of the silicon wafer, thus causing a lot o f stress after the bonded reactor cooled.

Both silicon and glass wafers were first cleaned in piranha solution before bonding. The bonding behaviour differed from that outlined above. As the voltage increased, the current would increase slowly as well. However, past 1 kV, the current would start fluctuating wildly and sparks would start appearing. Therefore, in this set-up, the voltage was manually increased from 500 V to 1.5 kV. The process took about 2 h. The longer period was due to the fact that bonding was not complete over the entire

wafer. Once the anode contact point was to be moved to the unbonded sections, bonding occured as before. This difference in behaviour may be due to the use o f a point as the anode as opposed to a plate. While complete bonding takes a lot longer with this set-up, it does allow the process to be carried out in-house at UCL.