Multi-layer Depositions

Whilst single layer depositions were found to be relatively straightforward with smooth, uniform contaminant free layers of up to six microns thick produced in a single stage, multiple layers proved to be more complex.

PECVD tools utilising the showerhead electrode/gas distribution system as used here are known to have the potential to cause ‘defects’ in the planar surface due to the nature of the gas flow. Each gas outlet hole has an associated flow of gas where the precursors enter the plasma. With incorrect deposition parameters it is possible for these localised high gas flow regions to cause defects in the deposited surface. This is due to the variation and non uniformity of the gas flow in the vicinity of the showerhead holes. The effect of such defects can be seen in minor cases as mild bub- bling or micro-cracking of the deposited layer (as presented earlier in figure 6.3) and in serious cases as large scale (several millimetre diameter) bubbles or delaminated layers. Such defects are clearly shown in figure 6.4. This particular case was the

Figure 6.4:Large scale bubbling and delamination of deposited layers

result of depositing three successive 6 micron thick layers in an attempt to create a 3- layer sample suitable for UV writing. After each 6 micron deposition step the wafer was transported into the machine loadlock and stored under vacuum whilst the de-

Chapter 6 Development of Silica Samples for UV Writing

position chamber was etched back using CF4. Following the etch back the wafer

was returned to the chamber for the next deposition. After all three depositions the wafer was annealed at 1100‰for 1 hour. It was clear that this process route required

significant alteration to produce high quality layers.

In addition to the gross level of layer disruption observed in figure 6.4 much smaller bubbling and delamination can occur. These are observed to be uniformly distributed over entire wafers or around the edges and not centred about the regular grid pat- tern associated with showerhead effects. Examples of this are shown in figures 6.5 to 6.7. These figures are of a two layer sample with a 6 micron thick germanium doped layer deposited on top of a 6 micron thick boron and phosphorous doped underclad an intermediate anneal at 1100‰was performed between each deposition. Such re-

sults are typical of this combination of layers if no special precautions are taken.

Figure 6.5:Wafer showing delamination and bubbling after annealing

Thus there are two issues to be overcome in developing a multi-layer deposition pro- cess. Firstly process parameters must be controlled such that gas flow and plasma density (controlled through the chamber pressure during depositions) prevent lo- calised defects. Secondly the layer recipes, deposition and anneal steps must be tailored such that the layers adhere without bubbling to the substrate and other de-

Figure 6.6: Surface bubbles after annealing. Left: Bubble still intact. Right: A burst bubble. Both bubbles have a diameter of approximately 1mm.

posited layers.

To overcome showerhead defect formation and delamination, the chamber pressure during deposition can be controlled. By increasing the chamber pressure the effec- tive density of the plasma can be increased. This has the effect of ‘dispersing’ the individual gas jets more efficiently such that gas flow and plasma uniformity at the substrate surface is improved. Care must be taken after changing chamber pressures that deposition rate and refractive index are adjusted to compensate for the slight changes that will occur as a result of the change in deposition conditions.

An alternative method to avoid showerhead patterns in deposited layers is to rotate the wafer slightly from its usual position prior to processing. Ordinarily wafers are aligned to always have the same orientation when entering the deposition chamber. Thus, each showerhead outlet is always directly above the same point on the wafer. It was found that this procedure reduced showerhead defects but the delamination still occurred. It should be noted however, that this procedure should be avoided when many wafers are to be processed. As wafers are not completely circular, rotating them results in areas of the platten building up different thickness of deposited silica. Even though the chamber is frequently etched back, this may ultimately result in a build up of deposited material causing the wafers not to sit flat. This will vary both the height and thermal contact of the wafer with the platten resulting in non uniform depositions. Therefore, showerhead effects were minimised through the control of deposition chamber pressure.

Chapter 6 Development of Silica Samples for UV Writing

Figure 6.7: Multiple closely spaced bubbles. These are typically less than 1mm in diameter. The apparent dirt on the sample is flakes of silica from delamination across the wafer.

forward and a combination of deposition parameters and annealing regimes was used to develop a suitable process. The development of these two aspects of the pro- cessing were largely concurrent but for clarity they will be separated somewhat into distinct sections.