Aspect Ratio Trenches Research Articles

Properties of titanium and aluminum thin films deposited by collimated sputtering

A study of the properties of titanium and aluminum films deposited by collimated sputtering is presented. The effect of square grid collimators on the film coverage and film bulk properties has been studied experimentally and theoretically using a Monte Carlo vapor transport simulation, SIMSPUD. The computer simulation is able to model the effect of collimation on the angular distributions of deposited atoms and predict the film coverage and deposition rate changes for both titanium and aluminum. In good agreement with these predictions, experimental films show that bottom coverage in trenches of aspect ratio (depth:width) 1.2 can be significantly improved from 50% to more than 80% for a titanium film, and from 35% to more than 82% for an aluminum film with a collimator of aspect ratio 2. As a further consequence of collimation, the microstructure and film properties are altered in comparison with conventional films. For example, free-standing micro-beam experiments and X-ray diffraction spectra both show a substantial increase in compressive stress in Ti films deposited using collimation. In addition, collimation slightly alters the grain orientation distribution in Ti films, although pressure has a much more important role in determining microcrystallinity. Most of these collimation effects can be explained in terms of changes in the angular distributions, deposition rates and peening.

Experimental study and computer simulation of collimated sputtering of titanium thin films over topographical features

An experimental study and computer simulation of collimated sputtering of titanium (Ti) thin films onto submicrometer trenches is presented. The effect of square grid collimators with aspect ratios varying from 0.5 to 2 has been studied. Simulation of collimated sputtering involves the combination of the simulation of sputter distributions vapor transport model and the simulation by ballistic deposition film growth model. This combination is able to simulate the effect of collimation on the spatial and angular distributions of deposited atoms, and is able to predict the film coverage, deposition rate change, and microstructure of the deposited films. Both experimental and simulation results show that bottom coverage in trenches of aspect ratio 1.2 can be significantly improved from 50% to more than 80% using collimation. However, a penalty is paid by a corresponding decrease in deposition rate down to 15% of the uncollimated value. Additionally, the microstructure (grain size and orientation) is altered by collimation. The good agreement between the simulation and experimental results indicates that the model will be very useful for predicting and optimizing the properties of films deposited by collimated sputtering over topographical features.

Sequential deposition of SiO2 and poly-Si in isolation trenches

Low pressure chemical vapor deposition (LPCVD) of SiO2 from tetraethoxysilane followed by LPCVD of polycrystalline silicon (poly-Si) from silane is used to refill isolation trenches. Two important characteristics of the refilled trenches which can adversely affect subsequent processing are the position of the void in the poly-Si and the depth of the groove along the top of the trench. We use evolve, a physically based, low pressure deposition simulator, to study the effects of trench width and the details of the trench profile before refill on void position and groove depth. Simulated SiO2 and poly-Si profiles in trenches with aspect ratios which range from near 4 to 20 are in good agreement with experimental profiles. In particular, evolve’s predictions of (1) the void position as a function of the position of the edge of the nitride film on the wafer’s surface, and (2) the depth of the groove as a function of the trench aspect ratio agree well with available experimental information. The reaction rate expressions used in evolve were developed in independent experiments. We do not adjust any parameters, and our simulations are entirely physically based.

Silane reduced chemical vapor deposition tungsten as a nucleating step in blanket W

The SiH4–WF6 nucleation step of chemical vapor deposition blanket tungsten has been studied in the 10–100 Torr pressure range. The deposition rate, particle generation rate, and step coverage were studied as a function of the process parameters. The deposition rate was found to be mass transfer limited over the entire range of process conditions. Deposition rates from 300 to over 70 000 Å/min were observed. An in situ detector was used to measure gas phase particle levels. Particle generation rates were found to range from 0 to 108 particles/s over the parameter range studied. The strongest dependence seen was on the SiH4/WF6 flow ratio. At ratios of less than one, no detectable particles were produced. The particle generation rate increases rapidly at ratios above one. The particle rate also increases with increasing temperature and pressure. The step coverage was found to be ∼85% for 1000 Å films in 3:1 aspect ratio trenches. Overall, the results indicate that SiH4 nucleation in the 10–100 Torr range is an attractive alternative to the ≤1 Torr processes commonly in use today.

Etching of thin metal films using a ballistic model

A two-dimensional model for plasma etching has been developed based on simulation by ballistic deposition (SIMBAD), a two-dimensional model for film growth over topography. By incorporating the effects of shadowing, adatom mobility, and incident flux distributions, SIMBAD is able to predict microstructure and density of films deposited by physical vapor deposition methods such as evaporation and sputtering. The plasma etch has been similarly modeled using an appropriate incident angular distribution for reactive species and a fixed probability of reaction. Simulation predictions are shown to agree well with experimental results for etching of tungsten films sputtered over trenches. For a 0.9-μm-thick tungsten film on a 1.8-μm-wide 0.6 aspect ratio trench with 80° sidewalls, the etch rate on the sidewalls was found to be about twice the rate on the horizontal sections of film for a CF4/O2 plasma etch. This is in agreement with the computed results.

Nearly bird's beak-free local oxidation technology for controlled dielectric formation in deep silicon trenches

Silicon deep trench isolation technology using local oxidation is reported. Scaled, high-density trench capacitors were fabricated with varying trench aspect ratios. Nearly bird's beak-free local oxidation resulted in a controlled growth of silicon dioxide on the trench bottom surfaces and significantly improved the trench gate MOS isolation characteristics. Detailed MOS capacitance measurements were performed and wafer yield in excess of 90% was demonstrated across 4 inch diameter silicon wafers.

Profile evolution in the multi-level technique

The oxygen reactive ion etching of sub-half micron features in organic layers produces a variety of profile effects depending on both system parameters and pattern arrangements. Vertical and lateral etch rates of the bottom resist in a tri-level resist system were investigated with particular attention to the trench aspect ratio. In order to properly study the etch behaviour and to evaluate their various facets, a new version of the simulation program ADEPT (Advanced Simulation of Dry-Etching Process Technology) was developed. This now permits inclusion of physical and chemical descriptions of the etch process. Examples of profile phenomena observed in the tri-level technique are compared with corresponding output from the simulation program.

Simulation of reactive ion etching pattern transfer

This paper describes a model that simulates etching profiles and process latitudes in glow-discharge bombardment-induced reactive-etching processes. Numerical results are presented for the pattern-transfer step in trilayer lithography, but this analysis is applicable to many other pattern-transfer processes. The inputs to the interface-evolution model described here are a kinetic model for the yield per incident energetic particle and a statistical mechanical model that relates the incident-yield-weighted angular distribution to the pressure, sheath thickness, and sheath voltage drop. The kinetic model is based on experimental evidence and assumes that the yield per bombarding particle is proportional to its energy. The resulting interface-evolution equation is mathematically analogous to a free-surface evolution equation in hydrodynamics. This convective partial differential equation is reduced to a coupled set of ordinary differential equations via the method of characteristics and solved numerically. More general energy-dependent yields are easily incorporated in the present formulation, but angle-dependent yields are more difficult and are not treated here. This model describes how shadowing of the surface being etched results in proximity effects in line etching and aspect-ratio-dependent etching rates in trench etching. Simulated profiles are compared to experimental trilayer etching profiles and qualitatively describe their shape and the trends that are observed as pressure or other processing parameters are varied. Simulations showing the effect of angular distributions, line proximity, and trench aspect ratio on process latitudes in trilayer lithography are presented and discussed.

Indirect trench sidewall doping by implantation of reflected ions

Ion implantation (i/i) technology is employed for silicon trench sidewall doping. The aspect ratio of trenches for high Mbit DRAM is very large (depth/width≥10), so that very small glancing-angle i/i to sidewalls is necessary. In this case, reflected ions are large in number and are implanted to the opposite sidewall. It is very important to know the elemental depth profile in the opposite sidewall to understand the implantation mechanism. For the first time, we measured the depth profiles at several positions of the opposite trench sidewall by secondary-ion mass spectroscopy for arsenic and boron ion implantations. It is found that reflected ions are distributed near the facing region of the directly implanted region with smaller energies than the primary energy. These findings are compared with the simulations based on the marlowe program.