Plastic Deformation of Thin Si Membranes in Si-Si Direct Bonding
Authors: Poppe Erik , Jensen Geir Uri , Moe Sigurd Teodor , Wang Dag 
Published: February 2020

High temperature annealed silicon direct bonding is a widely used process, enabling the fabrication of robust, long-term stable devices with 3D structures. However, during the high temperature anneal that is required to obtain a sufficient bond strength, thin membranes suspending sealed cavities may become permanently deformed. We have investigated the cause of such deformations and we have established how these deformations can be avoided by adjusting the cavity pressure in the bonded device prior to the high temperature anneal step.

Test structures were fabricated on 8 pcs 100 mm wafer pairs. SOI wafers with 43 µm device layer, 500 nm BOX and 380 µm handle wafer were used, together with 400 µm thick bulk wafers. Both wafer types were p-type with (100) orientation. The bulk wafers were oxidized and the oxide was patterned to allow for etching of an inlet hole after bond anneal.

Membranes were fabricated by DRIE of the SOI wafers through the handle wafer using the BOX as an etch stop. Circular membranes with diameters ranging from 780 – 3520 µm and square membranes with side edges 1044 – 3084 µm were defined. To ensure symmetric membranes all masking oxide was stripped, and a new 190 nm thick thermal oxide was grown.

Wafer pairs were cleaned in piranha, rendered hydrophilic in RCA-1, aligned, and prebonded in an EVG510 wafer bonder at 50°C, applying a force of 1000 N for 2 min. Before bringing a wafer pair into contact, the bond chamber was taken to vacuum (<1e-3 mbar) and then purged with N2to a pressure level of 200 to 250 mbar. The bond anneal furnace works at atmospheric pressure, and bonding pressure was chosen to achieve a sufficiently low deflection at the 1050 °C anneal step. Assuming the gas inside the closed cavity follows the ideal gas law, a pressure of 247 mbar at 50 °C corresponds to 1 atm at 1050 °C. One wafer pair was bonded in vacuum as a reference. For each of the prebonded laminates, one reference membrane was measured before the laminate was annealed at 1050°C for 2 hours. After bond anneal, an inlet to each cavity was etched by DRIE, and the membranes were re-measured. A cross section of the test structure is shown in Figure 1. The membranes were also inspected by an optical microscope with a halogen lamp and a Hamamatsu C8800 IR sensitive camera. Membrane deflections were measured with a Zygo New View white light interferometer (WLI).

Figure 2 shows IR images of two square membranes after bond anneal: one bonded in vacuum, and one bonded in a 200 mbar N2 ambient. What we interpret as slip lines was clearly seen in square membranes 1764x1764 µm and larger and in circular membranes of 1760 µm diameter and larger bonded in vacuum. The WLI measurements showed deflections of the 3084x3084 µm square membranes of 12-15 µm before bond anneal for both laminate types. The membranes that were bonded in a controlled N2 ambient showed the same deflection after bond anneal as before, while the large square membranes bonded in vacuum showed an increased deflection of 50-56 µm after bond anneal. After ventilating the membranes by opening the inlet holes, the membranes bonded in a controlled N2ambient were essentially flat (+/-1µm), while the membranes bonded in vacuum showed little change in deflection. There is a one-to-one correspondence between observed slip lines in the IR microscope images and membranes with permanent deflection, and non-destructive IR microscopy was found to be a safe method to verify Si crystal integrity also for smaller membranes with less deflection.

The results show that silicon membranes can be plastically deformed during high temperature bond anneal. We believe that the stress in the silicon membrane exceeds the yield strength at this temperature, thus causing slip along crystal lines in the silicon material in order to relax the stress. The fact that permanent distortion occurred only in membranes above a certain size supports this hypothesis.

In conclusion, our work shows that flexible silicon structures like membranes can be permanently deformed during bond anneal if a stress exceeding the yield strength of silicon is present during anneal at certain membrane locations. This deformation can be avoided by reducing the differential pressure across the membrane during bond anneal. Crystal slip lines can easily be detected by IR-sensitive microscope inspection.

Figure 1