Final Report: Alternative Wafer Cleaning Using HF-H2O ProcessingEPA Grant Number: R826737
Title: Alternative Wafer Cleaning Using HF-H2O Processing
Investigators: Sawin, Herbert H.
Institution: Massachusetts Institute of Technology
EPA Project Officer: Richards, April
Project Period: January 1, 1999 through December 31, 2001 (Extended to December 31, 2002)
Project Amount: $340,000
RFA: Technology for a Sustainable Environment (1999) RFA Text | Recipients Lists
Research Category: Sustainability , Pollution Prevention/Sustainable Development
The objectives of this research project were to: (1) find the reaction mechanisms of oxide etching both hydrogen fluoride (HF)/water (H2O) and HF/alcohol processes; and (2) develop a vapor phase HF cleaning process to remove the metallic contamination and natural oxide on the silicon surface. Even though the HF vapor process has been studied intensively for several decades, the commercial application is not very successful because of the unknown nature of the process. This study focused on possible applications in the semiconductor industry where a replacement to the aqueous phase cleaning process is desirable. The ultimate purpose of this project is to provide feasibility for the HF vapor process to be vacuum compatible and clustered with the cleaning process.
The oxide etching in the HF/H2O vapor process occurs in both a gas-phase regime (submonolayer, monolayer, and multilayer adsorbed regime) and a condensed phase regime, depending on the partial pressures of HF and H2O in the gas phase and the temperature of the substrate. The condensation of HF and H2O occurred at lower partial pressures of reactant gases than predicted by vapor-liquid equilibrium data. The ternary mixture of HF, H2O, and silicon tetrafluoride (SiF4) from the oxide etching reaction caused this depression of the condensing point.
In the condensed regime, the etching rate is less sensitive to the temperature and the partial pressure of the reactants at a high pressure of HF. The etching rate in this regime is generally 1-2 orders of magnitude higher than that of the gas-phase regime. The etching rate in this regime also is affected by the mass transfer rate in the gas phase. The etching rate is proportional to a scaling factor, (QD/Lp)1/2, for the mass transport.
In the multilayer adsorption regime, the etching rate is linearly dependent on the partial pressures of reactants and is relatively low. The etching rate of oxide at a high reactant pressure can be affected by the product concentration on the surface when mass transfer resistance is present in the gas phase. The etching rate in this regime is affected greatly by the temperature of the substrate. The mass transfer rate is limiting the etch rate of oxide in the multilayer adsorption regime.
In the monolayer adsorption regime, the etching rate is expressed by Langmuir-Hinshelwood kinetics. The etching rate is governed by surface kinetics in this regime. Advantages of the monolayer etching regime are: (1) smoother etched surface; (2) low selectively to tetraethoxysilane (TEOS); (3) haze-free etched surface; (4) no metal attack; and (5) vacuum compatible process. Although the monolayer etching regime showed promising results, the etching rate in this regime is affected greatly by the surface state of the oxide layer, which often caused irreproducible etching results. The electrostatic charge on the surface and its polarities are responsible for the irregular etching results.
A positive charge enhanced the etching reaction in the submonolayer and monolayer etching regime, whereas a negative charge mainly enhanced the etching in the multilayer etching regime. Direct ionization of HF on the oxide surface is responsible for the enhancement in the monolayer regime. In the multilayer, it is believed to form a thicker adsorbed layer by negative charge on the oxide, resulting in the higher etching rate. The adsorption of reactant also is enhanced by the vapor pressure reduction of HF and water from fluorosilicate formation instead of SiF 4 under the basic condition induced by the negative charge.
The use of a clustered plasma etch, ash, plasma oxide growth, HF/H2O dry clean, and metal deposition sequence has been demonstrated successfully in collaboration with researchers in this laboratory supported by the Semiconductor Research Corporation. The process sequence consists of etching oxide in an inductively coupled reactive ion etching process and performing an in situ oxygen ash in the etching chamber, followed by in vacu transfer to an HF/H2O process. At each stage of this process, the surface composition was monitored using x-ray photoelectron spectroscopy. A novel HF/H2O process that is being developed under another U.S. Environmental Protection Agency (EPA) grant was used. It operates using process conditions that have been reported in the literature (i.e., the submonolayer reaction regime). In this operating regime, the pitting, hazing, and preferential etching at the feature bottoms are avoided. In addition, low selectivities between thermal and deposited TEOS oxides are observed.