M. A. Blauw

Balancing the etching and passivation in time-multiplexed deep dry etching of silicon

For the Bosch deep silicon dry etch process with SF6–C4F8 a quantitative approach is developed. Essential plasma surface interactions and the transport properties of ions and radicals in high aspect ratio structures are unravelled. Balancing the interactions during etching and passivation pulses is essential for maximal profile control. In the anisotropic regime the etch rate is aspect ratio dependent largely due to depletion of fluorine radicals and with some involvement of passivation polymer redeposition. The anisotropic process tends to stop at a limiting aspect ratio because of improper removal of polymer passivation at the trench bottom. Both higher ion flux and ion energy are found to be crucial to push the Bosch process to higher achievable aspect ratios. Practical process implications are discussed. In situ ellipsometry shows that the polymer passivation step is a complex process with an ion induced component. More efficient removal of the passivation layer at the trench bottom by adjusting the p...

Kinetics and crystal orientation dependence in high aspect ratio silicon dry etching

A quantitative study of dry etch behavior in deep silicon trenches in high density plasmas (electron cyclotron resonance, inductively coupled plasma) at low temperatures (160–210 K) is presented. The quantitative approach implies etch behavior being studied in relation to the relevant particle fluxes (atomic F and O and ions) as measured by in situ diagnostics. Two etch modes are observed. In one mode faceting shows up as due to crystallographic orientation preference, i.e., Si〈111〉 being etched slower than Si〈100〉. In the other mode the normal anisotropic ion-induced behavior is observed. Controlled switch from one mode to the other is studied under influence of process parameters like pressure, ion energy, and substrate temperature. The second part of this study deals with aspect ratio dependent etching (ARDE). Both vertical and horizontal trenches have been taken into account as to distinguish between radical and ion-induced effects. The flux of radical species into the deep trench is governed by Knuds...

High-rate plasma-deposited SiO2 films for surface passivation of crystalline silicon

SiO2 films were deposited by means of the expanding thermal plasma technique at rates in the range of 0.4–1.4μm∕min using an argon∕oxygen∕octamethylcyclotetrasiloxane (OMCTS) gas mixture. The film composition was studied by means of various optical and nuclear profiling techniques. The films deposited with a low OMCTS to oxygen ratio showed no residual carbon and a low hydrogen content of ∼2% with a refractive index close to thermal oxide. For a higher OMCTS to oxygen ratio a carbon content of ∼4% was detected in the films and the refractive index increased to 1.67. The surface passivation of the SiO2 films was tested on high quality crystalline silicon. The films yielded an excellent level of surface passivation for plasma-deposited SiO2 films with an effective surface recombination velocity of 54cm∕s on 1.3Ωcm n-type float zone crystalline silicon substrates after a 15min forming gas anneal at 600°C.

Advanced time-multiplexed plasma etching of high aspect ratio silicon structures

An advanced, time-multiplexed plasma etch process for high aspect ratio structures is presented. Compared to the two pulse Bosch process, the technique consists of a sequence of three pulses. The third pulse is tailored to improved depassivation of the trench bottom prior to each etch pulse. Several depassivation chemistries are explored: O2, CO2, and SO2. In a further extension the bias voltage is also pulsed, with the aim to balance the radical and ion-enhanced components in the passivation of the sidewalls and trench bottom. The process extensions lead to improved mask selectivity and substantial range for profile control from fully anisotropic to strongly negatively tapered. The maximum aspect ratio obtained in the Bosch process could not be improved, because the ion angular distribution probably remains the limiting factor. The role of the ions in passivation and etching has been quantified in separate experiments.

Temperature influence on etching deep holes with SF6/O2 cryogenic plasma

A cryogenic SF6/O2 plasma process has been used to investigate the etching of deep holes in silicon wafers. The influence of crystallographic and aspect ratio dependence of the etch rate on the holes profile have been explored. It was found that wafer temperature, during the etching process, played a crucial role in controlling the anisotropy and deteriorative faceting due to crystal orientation dependent etching. High anisotropy and switching of the process to crystallographic independent etching was achieved by controlling the temperature. Aspect ratio dependent etching was also identified as a serious limitation for the required homogeneity in etched depth.

A capacitive probe with shaped probe bias for ion flux measurements in depositing plasmas

The application of a pulse shaped biasing method implemented to a capacitive probe is described. This approach delivers an accurate and simple way to determine ion fluxes in diverse plasma mixtures. To prove the reliability of the method, the ion probe was used in a different configuration, namely, a planar Langmuir probe. In this configuration, the ion current was directly determined from the I-V characteristic and compared with the ion current measured with the pulse shaped ion probe. The results from both measurements are in excellent agreement. It is demonstrated that the capacitive probe is able to perform spatially resolved ion flux measurements under high deposition rate conditions (2-20 nm/s) in a remote expanding thermal plasma in Ar/NH(3)/SiH(4) mixture.

High-rate anisotropic silicon etching with the expanding thermal plasma technique

Deep anisotropic silicon etching with the expanding thermal plasma (ETP) technique using fluorine-based chemistries was demonstrated. The ETP technique makes use of a remote high-d. plasma source and it has a good control of the downstream plasma chem. Both a cryogenic etching process and a time-multiplexed etching process were developed. Etch rates up to 12 micro m min-1 and selectivities higher than 300 were obtained while feature profiles were comparable to those obtained with the widely used inductively coupled plasma reactors. Therefore, this deep anisotropic silicon etching technique is an attractive alternative for the fabrication of microstructures with high-aspect-ratio features. [on SciFinder (R)]

IC-compatible processing of inertial sensors using SF/sub 6/-O/sub 2/ cryogenic plasma process

An IC-technology compatible process lor high aspect ratio trench based microinertial devices was developed, using cryogenic SF/sub 6/-O/sub 2/ plasma etching as a single postprocessing step. Furthermore it involves either silicon to silicon dry etch BCB bonding, Si wafer thinning and a method for front-to-backside alignment using ASML PAS5000/50 waferstepper. The wafer thinning is performed using TMAH wet etching bringing the advantage of possible batch fabrication. The process flow includes a single resist mask for both the Al etching and the high aspect ratio plasma etching of the mechanical structure.