A multiple beam apparatus has been constructed to facilitate the study of ion-enhanced fluorine chemistry on undoped polysilicon and silicon dioxide surfaces by allowing the fluxes of fluorine (F) atoms and argon (Ar+) ions to be independently varied over several orders of magnitude. The chemical nature of the etching surfaces has been investigated following the vacuum transfer of the sample dies to an adjoining x-ray photoelectron spectroscopy facility. The etching ‘‘enhancement’’ effect of normally incident Ar+ ions has been quantified over a wide range of ion energy through the use of Kaufman and electron cyclotron resonance-type ion sources. The increase in per ion etching yield of fluorine saturated silicon and silicon dioxide surfaces with increasing ion energy (Eion) was found to scale as (Eion1/2−Eth1/2), where Eth is the etching threshold energy for the process. Simple near-surface site occupation models have been proposed for the quantification of the ion-enhanced etching kinetics in these systems. Acceptable agreement has been found in comparison of these Ar+/F etching model predictions with similar Ar+/XeF2 studies reported in the literature, as well as with etching rate measurements made in F-based plasmas of gases such as SF6 and NF3.
Articles you may be interested inComparison of advanced plasma sources for etching applications. V. Polysilicon etching rate, uniformity, profile control, and bulk plasma properties in a helical resonator plasma source Comparison of advanced plasma sources for etching applications. II. Langmuir probe studies of a helicon and a multipole electron cyclotron resonance source Comparison of advanced plasma sources for etching applications. IV. Plasma induced damage in a helicon and a multipole electron cyclotron resonance sourceWe have studied the etching pcrformance of two commercially available low pressure, high density plasma sources and their application for the etching of 0.35 /hm features in polysilicon films. The two sources are a rf-inductively coupled helicon made by Lucas Labs of Sunnyvale, CA and a multipole electron cyclotron resonance (ECR) source made by Wavemat of Plymouth, MI. The sources are mounted on a dual chamber etching platform to remove platform dependent effects. Performance metrics consist of measuring the poly silicon etching rate, etching rate uniformity, and profile control in HBr gas-phase chemistry. The effect of applied source power, applied rf-bias power, and reactor pressure on the etching rate and uniformity is examined using a response surface experiment. Profile control is determined by examining nested and isolated lines and trenches using oxide mask/polysilicon/oxide structures. In both sources, high unifomlity and vertical profiles are obtained at low reactor pressure, high applied source power, and applied rf-bias powers between 50 and 60 W. To decrease thc lateral etching rate and increase the anisotropy of the etching process, approximately 3% of O 2 is added to the feed-gas. For the helicon, the operating point for best uniformity is at 2.0 mTorr, 2500 W applied source power, and 57 W applied rf-bias power resulting in a measured etching rate of 2340 A/min and uniformity of ±3. 3%(20"). For the ECR, the operating point for best uniformity is at 2.8 mTorr, 1370 W applied source power, and 60 W applied rf-bias power resulting in a measured etching rate of 2580 Almin and uniformity of ± 1.4%(2a). Since both sources exhibit remarkably similar performance for the etching of polysilicon films, other factors such as ease of operation, plasma stability, and plasma ignition sequence become relatively more important when deciding which source to use for a particular application.
As part of our research effort in evaluating the etching performance of high density plasma sources, we measured ion energy distribution functions near the wafer surface for a helicon and a multipole electron cyclotron resonance source (ECR). The following two salient results stand out: first is the remarkable similarity in behavior of the two sources which was also observed in previous studies comparing etching rates, profile control, and Langmuir probe diagnostics; and second is the surprising level of coupling between the applied rf bias and the bulk plasma. For both sources, the ion flux increases strongly with source power, decreases by 20%–40% as the reactor pressure increases from 2.0 to 5.0 mTorr, and is weakly modified by the applied rf bias. The mean ion energy is strongly influenced by the applied rf-bias and is relatively insensitive to source power and pressure. The ion flux exhibits high uniformity for both sources, with the helicon exhibiting slightly better uniformity. However, we note that instabilities in the ECR discharge from mode jumps caused by different operating conditions and changing reactor wall conditions, such as temperature, result in poorer uniformity. The behavior of ions with respect to applied source and rf-bias powers follows roughly the trends expected of quiescent, high density plasmas in contact with a rf-biased electrode (i.e., independent control of ion flux and mean ion energy). However, there exist subtle effects upon the ion flux, such as bimodal energy distributions, brought about by the coupling of the rf-bias power into the bulk plasma. This coupling may be an essential parameter in wafer platen design that must be addressed in order to obtain high etching rate uniformity.
The effect of plasma induced damage on device yields becomes increasingly crucial as gate oxide thicknesses approach the 50–70 Å range for quarter micron design rules. To quantify plasma induced damage for new low pressure, high density plasma sources, inductively coupled carrier lifetime and photoluminescence are measured. Also, UV-visible spectroscopic ellipsometry is used to closely examine the wafer surface both during and after plasma exposure. This study is part of an ongoing research program that compares the etching performance of two commercially available, advanced plasma sources: a Lucas Labs helicon and a Wavemat multipole electron cyclotron resonance source. For n-type and p-type, nominally 100 μs lifetime wafers, plasma exposure in both sources under optimum polysilicon overetch conditions results in a degradation of carrier lifetimes of less than 5% for unprotected versus oxide protected crystalline silicon. For wafers with an initial carrier lifetime of 400 μs, exposure to plasmas in both sources leads to a decrease in carrier lifetimes to approximately 345 μs. The decrease in carrier lifetime, although measurable, is at least an order of magnitude smaller than measured in conventional reactive ion etching (RIE) processes. To simulate an RIE etching process, rf-bias power is applied without source power so that both sources behave like highly asymmetric, parallel plate RIE reactors with the wafer platen acting as a driven electrode and the source wall acting as a grounded electrode. Under these conditions carrier lifetimes below 45 μs were measured in both sources with wafers that had initial lifetimes of 400 μs. Photoluminescence measurements indicate incorporation of hydrogen into the etched material, but no formation of interstitial defects associated with plasma induced damage. For samples exposed to a plasma at 25 and 50 W rf-bias, hydrogen produced from HBr dissociation readily incorporates into the sample. Exposure to a plasma at 200 W applied rf-bias results in a decrease in carrier lifetime and an increase in hydrogen incorporation. Under high rf-bias conditions, hydrogen incorporation is probably aided by ion surface damage. Ultraviolet-visible ellipsometry does not detect any morphological damage induced in silicon wafers, but as with carrier lifetime and photoluminescence measurements, significant damage is detected after exposure to rf-bias power without applied source power. Ellipsometry also detects a difference between real-time and post process spectra after HBr and Cl2 plasma exposure corresponding to the desorption of a film approximately 12 Å thick. This is not observed after Ar plasma exposure. Instead, the roughened surface remains stable which shows that the differences observed during and after plasma exposure for HBr and Cl2 are indeed the result of a chemically reacting layer.
Articles you may be interested inComparison of advanced plasma sources for etching applications. III. Ion energy distribution functions for a helicon and a multipole electron cyclotron resonance source Comparison of advanced plasma sources for etching applications. I. Etching rate, uniformity, and profile control in a helicon and a multiple electron cyclotron resonance source
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