Michel Puech

High-etch-rate anisotropic deep silicon plasma etching for the fabrication of microsensors

Dry plasma etching can offer many advantages in the fabrication of MEMS because of its anisotropic etching behavior, high etch rate, and its compatibility with traditional IC processing. A patented high density inductively coupled RFIC plasma source with independent source power and substrate bias control has been developed by Alcatel for deep etching of silicon. With the optimization of hardware and process parameters in a Fluorine based chemistry, processes with silicon etch rate up to 6 micrometers/min, etch uniformity better than +/- 5 percent, Si:SiO2 selectivity of more than 150:1, Si:photoresist selectivity of more than 50:1, etch depths of greater than 250 mm and profile angels of +/- 1 degree have been demonstrated. The silicon etch rate increases with increasing source power and Si:SiO2 selectivity increases with decreasing substrate bias. Substrate temperature can be maintained between -120 to +20 C during processing. The process parameters can be adjusted to give the desired performance for a particular application. Process results obtained at room temperature and at lower temperatures for different applications will be presented. The results indicate that this technology is a promising candidate for micromachining. The tool can be configured for production applications with vacuum loadlock and automated wafer handling.

High etch rate, deep anisotropic plasma etching of silicon for MEMS fabrication

MEMS fabrication faces multiple technological challenges before it can become a commercially viable technology. One key fabrication process required is the deep silicon etching for forming high aspect ratio structures. There is an increasing interest in the use of dry plasma etching for this application because of its anisotropic (i.e. independent of silicon crystal orientation) etching behavior, high etch rate, and its compatibility with traditional IC processing. Alcatel has developed a patented inductively coupled high density plasma source which delivers high etch rate, uniform, anisotropic silicon etching to depths as deep as 500 micrometers . This plasma source has been used for fabricating devices such as accelerometers, yaw rate sensors etc. Etch process performance data on some of these devices will be presented. Thus the Alcatel deep etching system provides the enabling technology requires for deep silicon micromachining of microsensors.

Device Quality SiO2 Deposited by Distributed Electron Cyclotron Resonance Plasma Enhanced Chemical Vapor Deposition without Substrate Heating

The deposition of high electrical quality SiO2 films on Si wafers has been achieved without substrate heating, (T<~100°C), using distributed electron cyclotron resonance (DECR) microwave plasmas. We have studied the effects of the reactant gas mixture composition (O2/SiH4) on the dielectric behavior of DECR SiO2. The electrical performances of both Si-SiO2 interfaces and SiO2 films in metal-oxide-semiconductor (MOS) structures were assessed by several characterization methods including critical field (Ec) evaluation, fixed charge densities (Qox) and interface traps densities (Dit) determinations. We report typical values of Ec around 6 MVcm-1, and Qox and Dit densities around 2×1010 cm-2 and 3×1010 cm-2eV-1 respectively. Thin film SOI-MOSFETs have also been fabricated to prove the DECR oxide quality.

Neutral species in inductively coupled SF6/SiCl4 plasmas

Inductively coupled SF6/SiCl4 plasmas interacting with a bulk silicon substrate and a SiO2-coated substrate have been investigated. Mass spectrometry and optical emission spectroscopy diagnostics were used to characterize the neutral population in the diffusion chamber. SiF4 molecules were detected as the dominant species, and their formation has been attributed to the high reactivity of F radicals with SiClx species. In a complementary experiment, a silicon chloride layer was deposited on the reactor walls during a SiCl4 plasma step and subsequently etched by a SF6 plasma. Time-resolved measurements of the neutral densities during the SF6 plasma step showed the importance of heterogeneous reactions between impinging F radicals and SiClx species deposited on the reactor walls. In SF6/SiCl4 plasmas, these reactions lead to a depletion in F radicals, which results in a decrease in the silicon substrate etch rate. Furthermore, this impacts on the concentration of SFx species and on the creation of new species, such as ClF, SF5Cl and S2Cl2.

SO2 passivating chemistry for silicon cryogenic deep etching

Cryogenic deep etching of silicon is investigated using SO2 for passivating the sidewalls of the etched features. The passivating efficiency of SO2 in a SF6/SO2 inductively coupled plasma is assessed comparatively with the traditional SF6/O2 chemistry by means of mass spectrometry and optical emission spectroscopy diagnostics. Emphasis is placed on the evolution of the density of various neutral species (e.g. SiF4, F, O, SOxFy, SFx). These measurements allow us to determine the SO2/SF6 and O2/SF6 gas flow ratios above which a passivation layer forms and inhibits silicon etching. Furthermore, different reaction schemes are proposed to explain the variations in relative densities measured for the two plasma chemistries. In SF6/SO2 plasmas, surface reactions involving SOF and SO2 species with F radicals are favoured, providing a greater number of SOF2 and SO2F2 molecules in the gas phase. In SF6/O2 plasmas, a higher rate of O radicals available for reacting with SFx species can account for the greater concentration in SOF4 molecules. However, these trends are significant for high passivating gas concentrations only. This is consistent with the similar etch results obtained for both chemistries when etching silicon at cryogenic temperatures with a low percentage of passivating gas.

Alternating SiCl4/O2 passivation steps with SF6 etch steps for silicon deep etching

Deep etching of silicon has been investigated in an inductively coupled plasma etch reactor using short SiCl4/O2 plasma steps to passivate the sidewalls of the etched structures. A study was first carried out to define the appropriate parameters to create, at a substrate temperature of −20 °C, a passivation layer by SiCl4/O2 plasma that resists lateral chemical etching in SF6 plasma. The most efficient passivation layer was obtained for a SiCl4/O2 gas flow ratio of 2:1, a pressure of 1 Pa and a source power of 1000 W. Ex situ analyses on a film deposited with these parameters show that it is very rich in oxygen. Silicon etching processes that alternate SF6 plasma etch steps with SiCl4/O2 plasma passivation steps were then developed. Preliminary tests in pulsed-mode conditions have enabled etch rates greater than 2 µm min−1 with selectivities higher than 220. These results show that it is possible to develop a silicon deep etching process at substrate temperatures around −20 °C that uses low SiCl4 and O2 gas flows instead of conventional fluorocarbon gases for sidewall protection.