Vertical Etch Rate Research Articles

Codeposition on diamond film surface during reactive ion etching in SF6 and O2 plasmas

The surface of polycrystalline diamond films during reactive ion etching in electron cyclotron resonance SF6 and O2 plasmas has been examined by scanning electron microscopy, Raman spectroscopy, and x-ray photoelectron spectroscopy. A SF6 plasma required a high voltage in negative substrate biasing in order to achieve an equivalent etching rate to an O2 plasma. This was ascribed to the codeposition effects and the low mean energy of ions upon impact in a SF6 plasma. The elemental composition of the amorphous layer formed at the etched film surface in the two plasmas was almost equivalent and mainly composed of carbon and oxygen, except a low concentration (⩽1%) of sulfur and fluorine in the case of a SF6 plasma. The significant codeposition on diamond surface was the most likely the result of plasma–wall interactions induced by the fragments from SF6, and highly responsible for suppressing the ratio of lateral to vertical etching rate leading to anisotropic etching in a SF6 plasma, in contrast with the little or no codeposition leading to isotropic etching in an O2 plasma. The results give the substantial implications for the etching process in practical SF6/O2 plasmas utilized in device fabrication.

The effects of thermal treatment on the anisotropic etching behavior of Cz- and Fz-silicon

We investigated the effects of stepwise thermal treatment of [100]-CZ- and FZ-silicon on the crystal defects and the etching behavior in KOH solutions. The anisotropy (the quotient of the vertical etch rate and the underetching of the mask), formed crystal defects as well as the surface roughness of the exposed {100}- and {111}-planes are examined. Thermal treatment of the silicon substrate can result in a precipitation of the interstitial oxygen. The precipitated oxygen causes elastic stress in the crystal which can be relieved by a generation of defects [1,2]. We found out that the evolution of these defects accelerates the lateral etch rate considerably. For the reference, the samples without thermal treatment the anisotropy has a value of 120 for FZ-silicon and 70 for CZ-silicon, respectively. However, with rising temperature and process time the anisotropy decreases to 30 for FZ-silicon and 15 for CZ-silicon. At the same time, the surface quality of the {100}- and {111}-planes degrades with increasing temperature. The roughness value Ra rises from 5 nm to approximately 30 nm. Along with the variation of the etching behavior during thermal treatment, we further present a suitable model for the oxygen-dependence of the etch rate.

Surface diffusion model accounting for the temperature dependence of tungsten etching characteristics in a SF6 magnetoplasma

To explain the influence of the substrate temperature Ts on the etching characteristics of tungsten in a SF6 magnetoplasma, we have extended the surface diffusion model originally developed for the etching of the W–F system at constant (ambient) temperature. It allows us to understand our experimental observations which include the influence of Ts on the anisotropy and the fact that the lateral (spontaneous) etch rate of W as a function of 1/Ts does not follow an Arrhenius law. The model is valid as long as the pressure is sufficiently low (⩽0.5 mTorr) to neglect the influence of coadsorption and passivation effects, likely related to oxygen contamination of the gas phase coming from the fused silica discharge tube interacting with fluorine atoms. Consistency of the model is well demonstrated by observing that the lateral to vertical etch rate ratio as a function of 1/Ts, under different plasma conditions, leads to a unique value of the activation energy (R=0.65 eV) for the associative desorption of WF6, the volatile reaction product of tungsten with fluorine adatoms.

Dry Development for 0.25 μm Top Surface Imaging

The dry development of 0.25 μm diffusion‐enhanced silylated resist structures produced in a LAM Research Corporation TCP™9400™ inductively coupled plasma etcher is studied. The processes consist of a low‐selectivity, ‐containing, descum step followed by an oxygen‐based main and overetch step. An extensive study, based on statistically designed experiments, shows the effect of different process parameters on anisotropy, selectivity, and the etch rates of silylated and unsilylated resist that occur when using different dry development chemistries. Solutions that independently control and enhance the selectivity and the anisotropy, while maintaining vertical profiles and acceptable etch rates, are demonstrated. Optimal processes for the different chemistries are compared. The effect that the different chemistries exert on the lithographic performance is quantified, and the strong reduction of the proximity effect, both in terms of linewidth and profiles, resulting from addition is studied. It has been demonstrated that the results also apply to sub‐0.25 μm geometries and that none of the tested chemistries interfere with subsequent etching of the polysilicon substrate.

Precise Evaluation of Pattern Distortion with Variation of the Impurity Concentration and Conductivity of Silicon Films

Many types of silicon film are used as materials for the gate electrodes of ultra large scale integration (ULSI) devices. The influence of varying the type of silicon film on local side etch depth and etch rate is investigated using an ECR plasma etching system. Resistivity of silicon films is varied using various impurity concentrations and annealing temperatures. The etch rate of silicon film is confirmed to increase as resistivity decreases in the case of phosphorus doped film. However, the etch rate is constant or decreases to a small extent in the case of boron doped silicon film. In contrast, it is found that the dependence of local side etch depth on resistivity is different to the dependence of etch rate on resistivity. The local side etch depth decreases in the case of phosphorus doped silicon and is almost constant in the case of boron doped silicon as the resistivity decreases. Thus it is found that local side etch depth does not always increase as resistivity of silicon film decreases. The depth of local side etch is influenced by the vertical etch rate, because the ratio of the just etching time to the overetching time is fixed. It is found that the local side etch rate is almost constant in this experiment. Therefore, it is concluded that the difference in local side etch depth is mostly caused by the difference in vertical etch rate.

Mechanisms for Microscopic Nonuniformity in Low-Pressure, High-Density Plasma Etching of Poly-Si in Cl2 and Cl2/O2 Mixtures

Etch anisotropy and microscopic uniformity have been investigated in low-pressure, high-density plasma etching environments. Polycrystalline Si films masked with a photoresist pattern of lines and spaces were etched in electron cyclotron resonance (ECR) Cl2 and Cl2/O2 plasmas with additional rf biasing. Experiments were performed by varying the gas pressure, substrate temperature, and percentage of O2 added. Moreover, the profile evolution during etching was simulated taking into account the transport of neutral and ionic species in microstructures and the following surface reactions: adsorption of neutral reactants, ion-stimulated desorption of reaction products, surface oxidation, and redeposition of etch products. The etched profiles obtained in ECR Cl2 and Cl2/O2 plasmas were compared with simulated results, and interpreted in terms of the deposition of etch products and the surface oxidation that occur competitively during etching. Etch products desorbed from the surface in microstructures have a sticking coefficient S p\\lesssim0.1, while etch products arriving from the plasma have a much larger coefficient S p\\gtrsim0.5. It was also shown that the competitive surface oxidation by incoming oxygen atoms plays a more important role in achieving aspect-ratio-independent etching than the deposition of etch products: the increased deposition of etch products resulted in enhanced sidewall tapering with little change in vertical etch rate; the surface oxidation resulted in inverse reactive-ion-etching lag without significant change in sidewall tapering.

Numerical study of the etch anisotropy in low-pressure, high-density plasma etching

Evolution of etched profiles has been numerically studied during low-pressure, high-density (LPHD) plasma etching of Si in Cl2. The surface etch rates were calculated using a reaction model of synergism between incoming ions and neutral reactants, including the spread of ion angular distributions due to their thermal motions and the transport of neutrals arising from the reemission on surfaces in a microstructure. Etched profiles were then simulated using a so-called two-dimensional string algorithm to examine the effects of ion temperature kTi and energy (or sheath voltage) eVs on the etch anisotropy for different neutral-to-ion flux ratios Γn/Γi toward the substrate. Numerical results indicated that in typical Cl2 LPHD plasma etching environments, where the neutral-to-ion flux ratio is Γn/Γi∼1 and the ratio of sheath voltage to ion temperature is eVs/kTi∼100, the chlorinated surface coverage is microscopically nonuniform in etched features: The coverage is very low at the bottom (α∼0.1), whereas the sidewall surface (α∼1) is almost saturated with neutrals. This microscopic nonuniformity of the coverage in etched features is the proposed mechanism responsible for the inversely tapered profiles that often occur in LPHD plasma etching. Additionally, the decrease in vertical etch rate in microstructures or the reactive-ion-etching lag due to neutral shadowing effects is also found to become significant in LPHD plasma etching. At such a low flux ratio of Γn/Γi∼1, more directional ions with a higher ratio of eVs/kTi≳500 are required for the anisotropic etching; e.g., for an ion energy (or sheath voltage) of eVs=50 eV, the ion temperature in a plasma is required to be kTi≲0.1 eV.

Oxide-cluster nucleation, growth, and saturation on Si(001)-(2 x 1) surfaces: Atomic-scale measurements and models.

Nucleation rates for oxide clusters on Si(001)-(2\ifmmode\times\else\texttimes\fi{}1) surfaces have been determined as functions of temperature (600--650 \ifmmode^\circ\else\textdegree\fi{}C), pressure (2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}8}$--1.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}7}$ torr), and dose (\ensuremath{\le}1200 L) by counting oxygen related pinning sites in scanning tunneling microscopy images. The low-dose temperature dependence indicates that the nucleation mechanism is related to thermally activated processes. At high doses, the nucleation rate was suppressed, while the vertical etch rate remained constant. Simulations of these high-dose experiments indicate good agreement with a dual-species oxidation model, but not with a single-species model. \textcopyright{} 1996 The American Physical Society.

Fabrication of Si field emitters by dry etching and mask erosion

A Cl2 plasma generated by an electron cyclotron resonance source was used to etch field emitter arrays in Si. Compared to wet etching, emitter arrays with sharper emitter tips and higher packing density can be formed by dry etching. Mask erosion was used to control the etch profile in Si. SiO2 masks with different profiles were patterned by wet and dry etching, and the effects of the initial SiO2 masks slope and etch conditions on the resultant Si profile were investigated. The lateral etch rate of SiO2 decreased from 363 to 150 nm/min as the SiO2 slope increased from 17° to 45°. As a result, the slope of the Si etch profile increased from 52° to 78° after dry etching to a depth of 1.8 μm. The ion flux and energy, controlled through coupled microwave and rf power, were used to obtain the desired etch rate and basewidth of the emitters. By increasing the pressure during etching, the lateral etch rate of SiO2 was reduced and more vertical Si profiles were developed. As pressure was increased from 0.5 to 10 mTorr, the lateral etch rate of SiO2 reduced from 414 to 144 nm/min while the vertical Si etch rate did not change significantly. This caused the slope of the Si etch profile to increase from 42° to 69°. Using the above technique, arrays of sharp emitter tips in Si with 2.2 μm basewidth and 11 μm height were fabricated and packing densities up to 1×107 tip/cm2 were achieved.

High density, low temperature dry etching in GaAs and InP device technology

Electron cyclotron resonance (ECR) plasma etching of GaAs, AlGaAs, and GaSb in Cl2/Ar or BCl3/Ar, and of InP in CH4/H2/Ar discharges is performed down to −30 °C substrate temperature and characterized in terms of rate, anisotropy, damage introduction and near-surface stoichiometry. Below −10 °C the vertical etch rates of all these semiconductors decrease due to reduced volatility of the products, and there is suppression of lateral etching with chlorine-based mixtures, but no change in anisotropy of CH4/H2/Ar etched features. Etching of typical dielectrics (SiNx,SiO2) and metals (W) used in bipolar or field effect transistors in SF6/Ar discharges was also characterized, and for substrate temperatures below −20 °C increased concentrations of fluorine-containing residues are found on the exposed semiconductor surface after removal of the dielectric or metal. These can be removed by a 500 W microwave ECR H2 plasma with low additional (−25 V) dc bias on the sample. The effect of an additional lower collimating magnet on the etch anisotropy was also examined and found to have a significant improvement at low ion energies. The uniformity of etching was improved when the lower collimating magnet was operational.

Basis of macroscopic and microscopic surface shaping and smoothing by plasma assisted chemical etching

This is a summary of the basis of macroscopic shaping (or figuring) and microscopic pattern transfer and microsmoothing (or polishing) by plasma assisted chemical etching (PACE). The ability of PACE to precisely shape surfaces without mechanical contact stems from the reliable etch rate map of a plasma assisted chemical etch tool by programmed scanning of the etch tool’s profile over the surface to be shaped. For a given shape modification and the plasma tool removal rate map this programmed tool dwell time is obtained by deconvolution using Fourier transforms. The transforms of these quantities also yield their spatial frequency characteristics and clarify PACE’s figuring behavior, but do not explain the observed polishing. Both polishing and micropattern transfer etch profiles are governed by a differential surface evolution equation, whose behavior is reviewed here. We also show why the PACE process is able to etch directionally, produces negligible subsurface damage and vertical etch rates to 100 μm/min or volume removal rates of ∼10 mm3/min for a 1 cm diam plasma due to the atypical plasma regime employed.

Low Temperature Chlorine‐Based Dry Etching of III–V Semiconductors

The dry etching characteristics of , , and in or high density discharges were examined over the substrate temperature range −30 to +60°C. Sidewall etching is suppressed at temperatures of ≤ 20°C and the vertical etch rates of all three materials decrease by factors of two or more in the range from 0 to −30°C. Condensation of etchant and product species occurs at −30°C in both and discharges, leading to rough surface morphologies. Use of a bottom collimating magnet on the electron cyclotron resonance source produces faster etch rates at room temperature, but has little effect at −30°C because of the condensation problem.

High temperature scanning tunneling microscopy studies on the interaction of O 2 with Si(111)-(7 × 7) surfaces

The interaction of O 2 with Si(111)-(7 × 7) surfaces in the pressure range between 5 × 10 −7 and 5 × 10 −4 Pa was investigated in situ, at reaction temperatures between 850 and 1130 K, by scanning tunneling microscopy. The range of experimental parameters includes the active oxidation regime (high temperature O 2 etching), the transition to passive oxidation (thermal oxide growth) and reaches into the latter regime. In the active oxidation regime the reaction proceeds via a step flow mode, where Si is effectively removed at the edges of retracting terraces or via nucleation and lateral growth of two-dimensional holes in the topmost bilayer in the inner areas of extended terraces. Examination of the step propagation velocity for different O 2 pressures, sample temperatures and local terrace widths shows that this velocity is proportional to the O 2 pressure and the terrace width and independent of temperature. The vertical etch rate r v, in contrast, depends only linearly on the O 2 pressure and is independent of the other two parameters. No O ad is present on the surface under these conditions. These findings are consistent only with a mechanism involving oxygen adsorption and SiO formation on the entire terraces and subsequent mass transport via diffusion of vacancies in the Si surface layer, which are finally incorporated at steps or in vacancy islands (vacancy diffusion model). Other models proposed or discussed earlier can be ruled out. At higher pressures, in a distinct transition regime, oxide growth begins by heterogeneous nucleation of small oxide clusters at steps and (7 × 7) domain boundaries, simultaneously with the ongoing etch process. These clusters, which are often lined up along the above defects, stabilize steps against etching and can act as a barrier for moving steps. After removal of the first Si bilayer the step flow process is effectively stopped and further reaction proceeds via slow growth of the oxide covered areas and simultaneous Si removal in between those areas. Under reaction conditions where few oxide clusters are formed, the ongoing etch process leads to an apparent three-dimensional growth of the oxide features. For conditions well in the passive oxidation regime homogeneous nucleation of small oxide clusters (20 Å width) on the terraces sets in, in addition to the other two reaction paths. It is found to be the dominant process and leads to surfaces covered by a layer of these small oxide clusters. The high density of these features points to a short diffusion length and hence to a low mobility of the O adatoms, different from the high mobility of the vacancies at these temperatures.

PSG layers for surface micromachining

The effect of both the deposition parameters and subsequent thermal processing upon LPCVD phosphosilicate glass (PSG) is experimentally investigated with an emphasis on establishing the effect of phosphorus content and PSG processing on the essential characteristics of PSG, such as etch rate, shrinkage, etch rate reduction and distribution of phosphorus after annealing. The effect of PSG upon the stress in overlying micromachined polysilicon layers is also investigated. The entire processing was carried out at a maximum temperature of 850 °C in order to avoid a significant increase in the thermal budget and to maintain fabrication compatibility with already integrated standard bipolar devices. The shrinkage of the PSG is shown to increase linearly with phosphorus content, while both the vertical-etch rate (as-deposited and after annealing) and the under-etch rate demonstrated an exponential increase with phosphorus content. The large value of the etch rate reduction after anneal (which can be as high as 70%) and its inverse proportionality with shrinkage (which is up to 8%) show that densification cannot be the only cause of the large variations in etch rates after annealing. The chemical meta-stability of the deposited PSG is identified in this research as a major cause of etch rate reduction after anneal, rather than the densification. Stabilization is obtained within 20 min after a 850 °C anneal. Increasing the phosphorus content of PSG from 2 to 6 at.% results in a reduction in sheet resistivity of the subsequently deposited and doped polysilicon from 1.4 to 0.9 kΩ/□, both with and without anneal of the PSG. The phosphorus content and the anneal of the PSG have negligible effect on strain in the overlying polysilicon.

Low temperature electron cyclotron resonance plasma etching of GaAs, AlGaAs, and GaSb in Cl2/Ar

Sidewall etching of GaAs, AlGaAs, and GaSb in electron cyclotron resonance Cl2/Ar discharges is found to be completely suppressed by cooling the semiconductor sample to −30 °C during the process. Vertical etch rates of ≳1500 Å min−1 at 1 mTorr and −50 V dc bias are obtained for all three materials under conditions where the lateral etch rates are negligible. Ex situ chemical analysis of the sidewall shows substantially increased Cl-containing residue on low temperature etched samples, which can be removed by a 5 min H2 plasma clean-up step. The exploitation of temperature to control undercutting enables use of simpler gas chemistries because there is no need to form a sidewall polymer.

Studies on dislocation etching kinetics of flux-grown YFeO 3 crystals

Results of etching (110) and (001) planes of YFeO 3 crystals are presented and discussed in detail. It is shown that boiling H 3PO 4 or H 3PO 4+HNO 3 in the ratio of 1:1 is the dislocation etchant of YFeO 3 crystals. It is established that the latter etchant is best suited for both (110) and (001) planes insofar as the definition of pit and control over etching are concerned. The kinetics of etching are investigated by making a quantitative study of the etch rate at different temperatures and at different concentrations of H 3PO 4. Variations in areal extension and depth of pits with time in H 3PO 4 or H 3PO 4 mixed with HNO 3 are investigated. It is shown that the areal and vertical etch rates increase with increase in temperature on both (110) and (001) planes. Addition of HNO 3 to H 3PO 4 brings down the minimum temperature required for etching (110) and (001) planes. The velocities of dissolution parallel and perpendicular to the surface are measured. The activation energy responsible for the dislocation sites for dissolution parallel and perpendicular to (110) and (001) planes are estimated. The Arrhenius factors for areal ( A' A ) and vertical ( A D ) dissolution are A' A =1.44×10 9 and 3.99×10 7 μm/s, and A D =8.12×10 3 and 1.31×10 4 μm/s for (110) and (001) planes, respectively.

Dry processed, through-wafer via holes for GaAs power devices

The fabrication of through-wafer via holes in GaAs substrates by plasma etching and laser drilling is reported. Using a low pressure (15–20 mTorr), low dc bias (−150 V) Cl2/BCl3 discharge with Cl2-to-BCl3 ratios ≤0.2, we are able to produce narrow (≤30 μm) via holes. This enables the use of a higher density of vias in closer proximity to the active GaAs power devices than can be achieved with normal diameter (100–150 μm) holes. Microwave enhancement of the plasma density using an electron cyclotron resonance source increases the GaAs vertical and lateral etch rates and requires use of low Cl2-to-BCl3 ratios in order to retain the anisotropic nature of the vias. Multiple pass (∼100 per hole) drilling of vias with a Q-switched Nd-YAG frequency doubled (532 nm), 30 mW laser has also successfully produced through-wafer connections. This provides a maskless, versatile method for producting vias customized for a particular wafer, but is less developed than plasma etching.

Low-Temperature Microwave Plasma Etching of Crystalline Silicon

Low-temperature microwave plasma etching of crystalline silicons is described. Vertical and lateral etch rates of Si and the selectivities of Si to photoresist are measured as a function of water temperature within a range of -150 to +30° for SF6, and (3) the selectivities become high at low temperatures (e.g., >40 at -90°C and 2.3 Pa). This etching enables highly anisotropic Si etching at a high etch rate and high selectivity with fluoride gases. Less-polymerizing-type gases can provide high etch rates. An etching model of the ion-bombarded surfaces in discussed. The model implies that separate control of the side wall reaction and the horizontal surface reaction is archived by the low-temperature etching.

Image potentials and the dry etching of submicron trenches with low-energy ions

The image potential between an ion and an etched sidewall is shown to be important in the dry etching of high-aspect ratio features. Low-energy ions are attracted to the walls of an etched trench, resulting in wall collisions which can lead to loss of etch directionality and slow vertical etch rates. Estimates are given for the depth at which one-half of the ion flux into an etched trench is lost to one of the etched walls, as a function of trench width and ion energy. The effect is particularly important in the etching of quarter-micron features and smaller using ion energies less than 200 eV.

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.