Pattern transfer by plasma etch has become an indispensable tool to fabricate semiconductor devices whose critical feature dimensions are less than 3.0 μm.Traditional wet chemistry techniques do not maintain linewidth fidelity (isotropic profile) and lose applicability for features whose aspect ratio (width:thickness) is below 3:1.Plasma processes must keep pace with the rapid introduction of innovative device designs whose packing density is continually increasing.Hence many etch process developments are based on empirical, rather than theoretical, wisdom.Often the desired etch results of anisotropy, selectivity, uniformity and cleanliness become competing requirements.

Many of today's circuit designs have structures, topography and feature dimensions so interrelated that difficulty is usually encountered during the etch process development.Many of the single layer materials that were used in yesterday's devices have become multiple material films in the current advanced designs, increasing the complexity of the etch process and confusing the interpretation of the process results.An increasingly important tool that can be used for successful etch development is the statistical fractional factorial experimental design.This paper will discuss a few examples of plasma process development that involve a rank ordering of the importance of competing etch results.

The etching of 1.25×3.5 micron trenches in silicon for use in bipolar device isolation using chemistries based on CC12F2 (freon-12) and 02 has been investigated.It was found that competitive reactions in the trench favor deposition of a Si02-like material on the sidewalls and promote lateral etching of the N+ subcollector under conditions necessary for high selectivity.In addition thermally activated processes were investigated by direct measurement of the sample surface temperature during etching.The temperature threshold for the initiation of subcollector undercut was determined to be 70C and the vertical etch rate exhibited an inverse activation indicating adsorption-limited etching.Results are discussed in the context of a qualitative model, based on the etchant-unsaturate model, which describes the contribution of several phenomena to the trench shape.These include sidewall passivation, species transport, deposition, N+ doping enhancement, Si loading, and ion scattering.

Planar plasma etching of silicon for fabrication of deep trenches has been studied using the method of factorial design.A highly anisotropic etching profile with a slight positive slope is obtained using SF6, C2C1F5, and CH4 in the low energy plasma etch mode.The anisotropy is attributed to the polymerization of CH4 molecules with the active species in the plasma.The polymer is removed preferentially in the vertical direction by energetic ions from the plasma, leaving the polymer layer on the side wall to inhibit lateral etching of silicon.Good uniformity across the wafer and smooth trench surfaces are obtained with this process.Trench depths of greater than 3μm with less than 0.24μm of undercut are observed.Etch rate measurements were made and found to be greater than 1000 angstroms/minute.Selectivity over the oxide mask is greater than 15:1.

By using factorial design to statistically characterize this process, the effects of interactions between various process parameters are studied.Optimal etching conditions are obtained from the interaction terms for the process parameters.The ability to etch deep trenches with smooth surfaces, low damage, low linewidth degredation, and high selectivity make this process useful for many applications in VLSI devices.

A totally anisotropic, highly selective dry poly etch process has been developed that is capable of etching sub-2.0 micron linewidths.Doped poly etch rates of 10,000 A/min.are obtained using C12-only chemistry.Standard novolac or bilevel photoresist is used, depending on the lithography requirements.Anisotropy is achieved without the use of carbon-containing gases; as a result, minimal proximity effects are observed between dense and stand alone etched lines.Wafer maps of etched linewidths on 4-inch wafers are presented, showing mask to final bias and uniformity results.

A commercially available triode dry etching system was used for the work.The self-induced dc bias voltage can be selected regardless of the applied rf power.The effect of variable self-induced dc bias versus applied rf power is presented for selectivity, upper and lower electrode interactions, and etch rate uniformity.Characterization by the use of spectroscopy is also presented, showing changes due to varying the self-induced dc bias at a constant rf power.

The implementation of plasma deposition and reactive ion etching into a semiconductor VLSI manufacturing process is rarely trivial.In some cases isolated process modules may appear to function well until integrated into a lengthy product flow, where interactions occur with prior or subsequent processing steps.In addition, dry processes have some unique and sometimes undesirable characteristics which need to be considered.Examples of both of these kinds of problems will be shown.Opportunities still exist for research to lead to a better understanding of mechanisms and for development work to improve the equipment and specific processes.

A process for reactive ion etching of 1μm and sub-micron WSi0.6 gates for GaAs MESFET integrated circuits has been developed.Using a CF4/O2 plasma, a 200am film of WSi0.6 could be etched in ≃ 5 minutes.This is sufficiently long for accurate process monitoring.A vertical edge profile has been obtained which is desirable for a self-aligned MESFET process.To achieve the vertical edge, careful control of the degree of over etch was necessary.This was accomplished by monitoring the intensity profile of a laser beam reflected from the wafer being etched.The widths of lines etched in WSi0.6 closely matched those of the resist pattern.Both 1μm and sub-micron FET's were fabricated.with excellent linewidth control and electrical characteristics

A method of forming a uniform magnetic field in a modified hexode type etch system is described.Experimental results of etching Si, SiO2 and photoresist with NF3 are presented.The magnetic field effectively increases the ion flux and decreases the energy of the ions bombarding the cathode.It was found that adding a magnetic field increases the etchrate of Si for the conditions studied, but for SiO2 and photoresist, the decreased ion bombardment energy leads to lower etchrates under certain conditions.The resulting effect on selectivities is discussed.

During the etching of aluminum film, if the film is deposited on a surface which contains sharp topological steps, and if the etching proceeds mainly along the vertical direction, it becomes very difficult to remove the remaining residue.Experimental results are discussed in terms of effects of native oxide film and polymer film formation.A simple model is proposed to reveal the residue formation process and to explain the problem encountered during the removal of the etched residue.

A mechanistic study of Mo etching in a CF4/O2 plasma has been performed using optical emission spectroscopy, mass spectrometry and x-ray photoelectron spectroscopy.Etching characteristics of Mo for a wide range of conditions relevant to plasma processing are also reported.Addition of small amount of O2 to the CF4 plasma dramatically increases the Mo etch rates, as well as concentrations of 0 and F in plasma.However, further addition of O2 above 50 % leads to decrease in etch rates.Two types of neutral molybdenum oxifluorides and a trace amount of molybdenum hexafluoride were observed in the effluent gas.Comparing the etch rates with the concentration changes of F and 0 with and without Mo present in plasma, it is suggested that Mo is chemically etched by F and 0.Atomic oxygen enhances the etch rate by increasing the F concentration in the plasma as well as by removing a carbon layer forming desorbed CO on the surface.XPS and AES analysis results for the etched surface inferred that chemisorbed fluorocarbon radicals dissociate into carbon and fluorine atoms, which in turn form a passivating graphite-like layer and a volatile molybdenum oxifluoride and molybdenum hexafluoride layer.

The etch behaviour of Al2O3 was studied in Ar, CHF3/Ar, CF4/O2 and Cl2 low pressure RIE plasmas.The influence of dc self-bias voltage, wafer temperature, gas flow and pressure on the Al2O3 etch behaviour was investigated.This was compared with the etch behaviour of SiO2, Mo, Au and Si under the same conditions.It was found that even though aluminum does not form volatile fluorides addition of CHF3 to an Ar plasma resulted in a ninefold increase in the A12O3 etch rate.This compares to a fivefold increase in the SiO2 etch rate and a 20% decrease in Au etch rate.

An A12O3 etch mechanism for fluorine based plasmas is proposed, comprising of the formation of AlFx and its subsequent removal under influence of high energy ion bombardment.The latter appears to be the rate limiting step.

A system for plasma assisted oxidation of Si at temperatures below 800°C is described.Oxides grown in this system have an unusual thickness versus time relationship.The first 110 nm of oxide exhibit a t1/2 dependence.Any additional growth shows a t1/3 dependence.Pressure, oxygen concentration, and rf power are all important parameters in determining the growth rate and uniformity of the oxide layers.Breakdown measurements on these oxide layers show that post oxidation annealing is necessary if these oxides are to compete with thermal oxides.The type of annealing is very important in determining the breakdown voltage.The results of various annealing procedures are described.The effects of heating the wafers independent of the heat generated by the plasma, and the frequency dependence of the growth are discussed.

The surface wave (SW) plasma technology is used to investigate possible frequency effects in the deposition kinetics of plasma polymers over the range 100–915 MHz.Hydrocarbon and fluorocarbon monomers are excited by a SW produced argon plasma, at a total pressure of 50 mTorr, under various monomer flows.Using Yasuda's normalization procedure, we have so far been unable to distinguish frequency dependent deposition in this high frequency (HF) regime.However, the present data indicate substantially (an order of magnitude) higher deposition rates than those reported by Gazicki and Yasuda for low frequencies.

Optical diagnostic probes have greatly increased our understanding of low pressure discharges used in etching and deposition processes for microelectronics fabrication.Optical emission induced by energetic electron impact excitation provides a qualitative determination of concentrations of a number of important atoms and small radicals.Rare-gas actinometry, combined with high resolution line-shape measurements can convert emission data into quantitative relative number densities, and provide information on mechanism of excited state formation.Emission from products of etching reactions gives insights into mechanisms and serves as an end-point indicator for many commercial processes.Laser induced fluorescence is a sensitive probe which is most useful for small radicals.Quantitative, relative ground-state number densities, internal energy distributions, translational energies and electric fields can be determined by this technique.When optical emission or laser induced fluorescence measurements are performed with spatial resolution and in-phase with respect to the applied field, additional insights are obtained on the dynamics of the discharge processes.Finally, infrared and visible-ultraviolet absorption spectroscopy can be used to measure absolute number densities.This technique also can provide spatial and temporal resolution.

This paper reviews and compares the various optical techniques used in plasma diagnostics with particular emphasis on studies of low pressure radio frequency discharges used in etching and deposition of thin films.

The energy distributions of the positive ions bombarding the ground electrode in a parallel plate plasma etcher have been measured by means of an electrostatic analyzer after extraction through a 50 μm orifice.Three gases were studied; Ar, CF4 and SF6, with pressures varying from 5 mtorr to 200 mtorr as a function of the discharge frequency.In the low frequency regime (25–125 kHz) the maximum energy of the ions is close to the applied potential (i.e., 500–600 eV).A secondary ion peak is observed and attributed to the secondary electron ionization.SF6 plasma exhibits a behaviour different to that of Ar and CF4 plasmas because of the high electron attachment of the molecule.

Infrared absorption spectroscopy has been used to measure atomic chlorine concentrations over a range of plasma conditions in both Cl2 and CF3Cl discharges.These measurements were made utilizing the spin-orbit transitions in the ground state of atomic chlorine near 882 cm−1.The concentration studies were performed by passing light from a diode laser through a multi-pass (White) cell set in two opposed windows of a parallel plate plasma etching reactor.The plasma work was preceded by a laboratory measurement of the infrared absorption line strengths of the 2P1/2 ← 2P3/2 transition.This measurement was done in a known concentration of atomic chlorine produced in a low pressure discharge flow system by the reaction of Cl2 or HCl with excess fluorine atoms.These measurements resulted in an integrated line strength of 4.14 (±0.89) × 10−21 cm2-molecule−1-cm−1 for the strongest hyperfine component of the transition at 882.3626 cm−1.

Measured atomic chlorine concentrations in Cl2 discharges varied between 0.2 and 8.0 × 1014 atoms/cm3, representing atomic chlorine fractions on the order of a few percent.The measured atomic chlorine concentrations increased approximately linearly with increasing power and pressure, and increased with increasing frequency above approximately 1 MHz.Below 1 MHz, the atomic chlorine concentration was relatively independent of frequency.

We discuss new methods for measuring particle number densities, masses and size distributions by laser techniques.These methods do not require assumptions of particle optical properties or instrumental calibration by known particles.We summarize the theoretical basis of these techniques and discuss the light scattering fluctuation analysis, which can yield number density and size information.Experimental data for rf discharges of silane/argon mixtures are briefly presented to illustrate some of the theoretical predictions.

A review is presented of a negative glow model of low frequency, low pressure plasma reactors and the salient features are described using silane mixtures as the example.The input requirements for this model and similar models are also discussed in order to establish the sensitivity of the homogeneous plasma reactions to operational conditions.Calculations are presented to demonstrate the sensitivity to mixture ratios, buffer gas type and power loading.

Spectroscopic diagnostics are used to map key parameters of the cathode fall of a He glow discharge.Optogalvanic spectroscopy on Rydberg atoms is used to perform spatially resolved electric field measurements.The resulting E/N maps provide the ion and electron current densities, which are used to determine the local ionization rates.Local excitation rates are mapped by observing the spectral emission from the discharge.Optogalvanic spectroscopy is also used to observe electron avalanches starting from a well defined position in the cathode fall.These detailed experimental maps will be essential in studies of the nonhydrodynamic electron distribution function.Various theoretical models of the cathode fall will be compared to the experimental results.

Using optical emission spectroscopy (OES), mass spectrometry (MS), and laser-induced fluorescence (LIF), we are investigating a number of glow-discharge reactions as a function of RF power, flow rates, partial pressures, and H2 dilution under realistic thin-film deposition conditions.In this paper we report on the preliminary results of two studies:

1. 1) The formation of radical and polymeric species in SiH4 and Si2H6 plasmas, and

2. 2) The characterization of SiF4 + H2 plasmas and the detection of HSiCl and HSiF in the plasmas of SiH2Cl2 and SiH2F2, respectively.

Semiconducting thin films are produced in plasma and CVD reactors and are commercially useful materials because of a ubiquitous presence of hydrogen, a low density of defects, and electro-optical properties which vary systematically with composition.However, little is known about how variations in fundamental growth parameters produce concomitant changes in film properties, particularly hydrogen incorporation.The reason for this gap in knowledge derives partly from the lack of adequate structural probes of disordered materials.In recent years nuclear magnetic resonance spectroscopy (NMR) has emerged as a powerful probe of the microstructure of amorphous semiconductors.I will discuss recent developments in the application of NMR to studies of amorphous hydrogenated semiconductors, and interpret these results in terms of the chemistry and physics of the growth process.