在氢氧化钾+羟胺溶液中高速蚀刻硅片上的穿孔

In the fabrication of complicated MEMS inertial sensors, there are many instances where metallisation cannot be done by usual methods such as lift off or conventional metal etch. In such cases, a shadow mask needs to be used. A silicon shadow mask can be realised by the fabrication of through holes in silicon. This also can be used as a cover wafer for wafer level encapsulation by fabricating a cavity in the device region. The fabrication of the shadow mask is carried out by doing double side wet anisotropic etching using potassium hydroxide (KOH) + hydroxylamine (NH₂OH) solution at low temperatures of 50–60°C. The low temperature of etching is preferred so as to reduce the etching of thermal oxide used as a mask layer. This work focuses on the etching characteristics of the KOH + NH2OH solution at low temperatures in terms of etch rate, undercutting, surface morphology, and selectivity of an oxide layer with silicon. This is very useful in through hole etching using silicon dioxide as a mask. The effect of aging on these characteristics is also studied and this is useful in continuous etching spread over few days especially during pilot production. The results are compared with those obtained using a pure KOH solution.

In the fabrication of complicated MEMS inertial sensors, there are many instances where metallisation cannot be done by usual pattern methods such as lift off or conventional metal etch. In such cases, it is necessary to fabricate a shadow mask for the fifinal metallisation of the sensor. Shadow mask fabrication in silicon (Si) using deep reactive ion etching (DRIE) is complicated as this necessitates the use of a support wafer to prevent Helium coolant leakage. The support wafer needs to be glued on to the processing wafer using cool grease or by any other bonding techniques. This further complicates the process and increases the number of steps involved in the fabrication of the shadow mask. Owing to these reasons, the wet etching route is preferred for the through hole etching in Si for shadow mask fabrication. In order to reduce etching time to half, wet etching is performed from both sides of the Si wafer after accurate back side alignment of the mask pattern. This also helps the oxide mask layer to withstand the long duration of etching. However, wet etching has its own limitations such as an arbitrary structure with vertical sidewalls that cannot be fabricated.

Potassium hydroxide (KOH) and tetramethylammonium hydroxide (TMAH) are most commonly used as wet anisotropic etchants for wet bulk micromachining in MEMS. When compared with TMAH, KOH provides a higher etch rate and higher etch anisotropy between Si{100}/Si{110} and Si{111}. This is especially useful in the fabrication of V-shaped holes for direct wire bonding with the device underneath the through hole wafer in wafer level encapsulated devices. The high etch rate of Si is desirable to reduce the etching time. Although a higher etch rate can be obtained at a higher temperature (75–80°C), it increases the etch rate of thermally grown Si dioxide (SiO₂) which is commonly used as a mask layer. In order to overcome this problem, the etching is carried out at lower temperatures in the range of 50– 60°C. However, at low temperatures, the KOH etch rate is very poor and this necessitates the addition of special additives such as hydroxylamine (NH₂OH).

The effect of different concentrations of NH₂OH on the etching characteristics of Si{100} using 20 wt% KOH and 5 wt% TMAH is studied . 15% NH₂OH is found to be the optimal concentration in 20 wt% KOH to obtain improved etching characteristics. The etch rates, etched surface morphologies, and undercutting results in 15% NH₂OH-added 20 wt% KOH are presented in the literature. Etch rate of Si{100} in 15% NH₂OH + 20 wt% KOH solution is nearly three times higher than that in pure KOH at the same temperature. The etch rate of SiO₂ also increases but not signifificantly compared to that of Si. The etch selectivity, which is defifined as the ratio of Si etch rate and SiO₂ etch rate, increases remarkably in NH₂OH + KOH solution. The addition of NH₂OH does not affect the etched surface roughness and morphology signifificantly. For through hole fabrication, etching needs to be carried out for long durations. Hence it is important to study the aging effect of KOH + NH₂OH solution as NH₂OH is an unstable alkaline solution. At lower etch temperatures, the oxide etch rate is low and hence selectivity is high.

In this work, 15% NH₂OH-added 20 wt% KOH is used to fifirst study the etchant characteristics and then their variation due to the aging effect at lower temperatures. The experiments are carried out at temperatures in the range of 50–60°C. The lower temperature is selected to achieve high etch selectivity between Si and SiO₂. The etching characteristics are then compared with etchant characteristics of 20 wt% pure KOH solution at the same temperatures. Furthermore, this study is used to fabricate a through hole wafer which will be used in the fabrication of a MEMS inertial sensor.

Fig1

The etching characteristics of Si{100} and Si{110} in 15% NH2OH-added 20 wt% KOH and pure 20 wt% KOH solutions are studied in detail at lower temperatures in the range of 50–60°C. The aging effect of the NH₂OH-added KOH solution on etching characteristics is also studied over 5 days for both the crystal orientations. These studies are implemented in practical application towards the fabrication of through hole Si wafer. The same was carried out using pure KOH solution as well to compare the results. It was found that 15% NH2OH-added 20 wt % KOH at 55°C is an optimal condition to form through a hole in Si{100} wafer using a thermally grown oxide layer as a mask. Hence this work can be directly used for wet bulk micromachining towards the production of MEMS devices.

Initially, the through hole fabrication is carried out using a pure 20 wt% KOH solution at 55°C. Fig. 12 shows the images of four inch Si{100} wafer at three different stages of though hole etching using a pure 20 wt% KOH solution at 55°C. It can be seen that the SiO₂ starts getting etched out from one side and it gets removed totally by the time through holes are formed. The total time of etching for realising through holes across the wafer is found to be 14 h. This confifirms an application where we can effectively use 15% NH2OH-added 20 wt% KOH solution at lower temperatures.