Increased Hydrophilicity of Silicon Surface through Plasma Treatment with Hydrogen Peroxide Gas
Authors: Spiegelman Jeff , Alvarez Daniel , Ramos Chris , Andachi Keisuke , Tsuchibuchi Gaku , Suzuki Katsumasa 
Published: July 2021

Increased Hydrophilicity of Silicon Surface through Plasma Treatment with Hydrogen Peroxide Gas

Spiegelman, D. Alvarez, C. Ramos (RASIRC), K. Andachi, G. Tsuchibuchi, and K. Suzuki (Taiyo Nippon Sanso Corporation)

Wafer bonding is a critical step in the development of advanced semiconductor and MEMS devices.1 Wafer bonding enables the bonding of dissimilar materials as well as the union of separate manufacturing pathways where the pathway steps are dissimilar and frequently incompatible to a single wafer process. Oxide wafer bonding is a common bonding method that requires the wafer surface to have a uniformly flat surface with sufficiently dense hydroxyl (-OH) groups. The hydroxyl groups provide a pathway for chemical covalent bonding between two wafer surfaces (EG Si-O-Si). Ultimately, the wafer bonding strength is a function of the density of the hydroxyl groups populated on the wafer surface before bonding.

Several challenges are associated with functionalizing wafer surfaces. Wet Thermal oxidation is a common method for incorporating surface hydroxyl groups on silicon, however this process typically takes place at very high temperatures (>600C °) where surface hydroxyl groups may reversibly desorb, thus limiting surface functionalization. Surface oxidation by Oxygen plasma is another common method for the creation of hydrophilic surfaces,2 however, this property may be attributed to bridging oxides on the surface, where hydroxyl groups may not be present. In the latter case, residual charge or surface radical species may lead to the observed improvement in wafer bonding properties. Plasma oxidation allows for lower temperature activation.2 Oxygen or clean dry air are frequently used in the plasma process with argon.

Our investigation involves the use of hydrogen peroxide gas. In recent years this material has become available in the gas phase where:

Gas-phase H2O2/H2O mixture is delivered from an ampoule-based formulation. (RASIRC® BRUTE® Peroxide)

High concentration H2O2/H2O is vaporized by in situ concentration methods and use of a membrane vaporizer as a gas generator. (RASIRC Peroxidizer®)

Early studies by Kummel3 demonstrated that Hydrogen peroxide gas provides a denser hydroxyl surface on SiGe and Ge vs water. Here, all thermal methods were utilized at reduced temperatures.

Before our current work, the creation and use of gas-phase hydrogen peroxide plasma has not been addressed. The objective of this work is to demonstrate the generation of hydrogen peroxide plasma and compare its surface functionalization characteristics to those of water and oxygen plasma methods.

A constant gas stream of H2O2/H2O mixture in a carrier gas was introduced to a remote plasma source (MKS). (Figure 1). The use of an argon carrier gas led to stable ignition of the plasma mixture. The subsequent addition of oxygen gas gives rise to plasma instability.

In the comparative study, the H2O2 gas stream was mixed with water vapor, molecular oxygen, and argon in different ratios, subjected to a remoted plasma. Functionalization was then carried out on a Si-H terminated surface (HF last silicon wafer surface). Thermal effects without plasma were also compared. Surface hydrophilicity (relative Hydroxyl density) was determined by contact angle measurements with the use of a goniometer.

The initial wafer surface after HF clean had a water contact angle of 84.9 °. The smallest wetting angle was found for H2O2/H2O/Ar plasma at 4 °. Water alone led to a wetting angle that was 62.5% larger than the hydrogen peroxide mixture. The angle increased with increasing oxygen concentration in the H2O2 gas stream. The wetting angle with dry oxygen at 200C ° was 27.3°. Thermal only H2O2 gas at 150C° was 47.3 ° and further reduced to 11.4 ° when the temperature was increased to 300C °. (Figure 2). These results along with mechanism and ramifications for wafer bonding will be discussed.

References:

Rogers, T.; Aitken, N.; ECS Trans. 16, 507, (2008).

Howlader, M.M.R, Suga, T.; J. Micro/Nanolith. MEMS MOEMS 9(4), 1-11 (2010).

Park, S.W. et al. Surface Science 652 322–333, (2016).