In a previous blog post, Dr. Daniel Drysdale described some of the current methods for MEMS manufacturing from an etch perspective, from wet etching and its evolution, to the current suite of dry release technologies. We looked at the pros and cons of both wet and dry etch methods, and where one method would be suited over another. Here we take a closer look at the differences between hydrofluoric (HF) acid and HF vapor.
As described previously, wet processes are suitable for a number of different applications compared to dry etching, depending on the complexity of the structure and materials used.
While HF acid etching is a viable method for wafer cleaning and for isotropically etching oxide films below another material, it does have limitations in terms of the critical dimensions (CD) of the devices to be released. If the devices to be released are significantly small enough, then the HF solution or the water used in the subsequent rinsing stages can induce the static friction effect, otherwise known as stiction.
Another factor in the utilization of the HF vapor process in contrast to the HF acid etch method is the compatibility of the HF to other materials present in the device structure.
It is well-known that aluminum can be etched in HF acid. Kirt Williams reports a 5:1 solution etching at around 11nm/min. The use of HF vapor does not show aluminum etching, but a surface modification; typically, a light fluorination of the material. Where bond pads of aluminum exist, a fluorine content of less than 1% is typically required to ensure the bond holds. Wet etching in HF acid allows for no control of the end surface product in a way that the vapor process does. The biggest difference between HF acid and HF vapor is the significant reduction in water content in the vapor phase. The HF vapor process requires a significantly smaller amount of water to initiate the etch than is used as part of an etch bath solution; as such, the impact on the material surface is greatly improved. This results in the ability to use aluminum for cantilevers, bond pads and a range of other functional devices.
Additionally, the compatibility of photoresist differs when migrating from HF acid baths to vapor phase HF etching. In HF acid etching, photoresist has been utilized as a masking material to restrict access of HF to the underlying structures. The HF acid solution consists of large molecules that can only access the sacrificial material where patterning has occurred. In the case of vapour-phase HF etching, the HF molecule is in the gas phase and is significantly reduced in size. As a result, the HF molecule becomes highly mobile, to the degree that it can diffuse readily through the resist layer and attack the oxide under the masked regions of a wafer; hence, resist or any other polymer mask is not suitable within the vapor-phase etch process.
Within wet etching methods to date, other standard CMOS materials such as copper, tungsten, chromium, titanium, germanium and titanium nitride appear to behave similarly to those observed during vapor-phase etch processes. These subtle differences between two materials of aluminum and photoresist do, however, have an impact on device manufacturing, and changes the considerations of a device to be etched in either acid or in the vapor phase. In the case of aluminum, it allows for more structural options alongside a whole range of new electrical applications, and in the case of photoresist, requires the utilization of a different masking material as part of the fabrication process. Consideration of the materials being used at an early stage in design will dictate the appropriate etch techniques applicable to each specific device.