Polysilicon Etch Rate Research Articles

Study on wet etching of dummy polysilicon in narrow pattern gap using alkaline solution

The polysilicon etching in a patterned wafer is an important process. In this process, the polysilicon must be completely removed without damaging the gaps made from SiO2 or Si3N4. In this study, a wet etching method was used to completely remove polysilicon while reducing it to SiO2. Alkaline solutions known as etching polysilicon, i.e., ammonium hydroxide (AH) and tetramethylammonium hydroxide (TMAH), were used. The etch rate was confirmed according to various factors such as temperature, chemical concentration, and rotation speed, but the etch rate was most affected by the chemical concentration. Accordingly, the concentration of hydroxide ions was the main factor for etching polysilicon. The etch rate of TMAH had the same tendency as AH for various factors, but the results indicated a difference of approximately eight times (1744 Å/min for TMAH and 217 Å/min for AH, condition: 6 wt%, 70 °C, 100 RPM). The electrical double layer (EDL) was calculated by using the pH, which was measured at each concentration of AH and TMAH solution, to etch the polysilicon in the narrow gap. The EDL was affected by the hydroxide concentration; thus, the TMAH solution etched the polysilicon in the narrow gap better than AH did. Furthermore, the experiment was conducted using an additive that could decrease the SiO2 etch rate and increase the selectivity between polysilicon and SiO2. Although the etch rate of polysilicon was reduced by 23%, the etch rate of SiO2 was also reduced by 43%.

Effect of Dissolved Oxygen and Oxygen Scavenger on the Etch Rate of Polysilicon

The fabrication process has been modified to stabilize the product due to the miniaturization of the semiconductor. It is also necessary to change the material in accordance with the change in the manufacturing process. In this study, we systematically examine the problems caused by the increase of dissolved oxygen in the etching process of polysilicon through single tool equipment. To overcome this issue, surfactant and oxygen scavenger were exploited not only to reduce the surface roughness but also to increase the etch rate.

Hbr/O2유도결합 플라즈마를 이용한 폴리실리콘 건식식각

Dry etch characteristics of polysilicon with HBr/O<TEX>$_2$</TEX> inductively coupled plasma (ICP) have been investigated. We determined etch late, uniformity, etch profiles, and selectivity with analyzing the cross-sectional scanning electron microscopy images obtained from top, center, bottom, right, and left positions. The etch rate of polysilicon was about 2500 <TEX>$\AA$</TEX>/min, which meets with the mass production for devices. The wafer level etch uniformity was within <TEX>$\pm$</TEX>5 ％. Etch profile showed 90<TEX>$^{\circ}$</TEX> slopes without notches. The selectivity over photoresist was between 2:1∼4.5:1, depending on <TEX>$O_2$</TEX> flow rate. The HBr-ICP etching showed higher PR selectivity, and sharper profile than the conventional Cl<TEX>$_2$</TEX>-RIE.

Variation of Polysilicon Etch Rate Depending on the Surface Condition of the Etching Reaction Chamber

X-ray photoelectron spectroscopy (XPS) and optical emission spectroscopy (OES) have been employed to determine the possible cause of etch rate variation in a silicon etching reactor. Samples were prepared to analyze the composition of the surface of the chamber at high and low etch rate of polysilicon. XPS analysis for these samples shows that the surface of the etching reaction chamber is richer in fluorine at lower etch rate which can be restored by the seasoning of the etching chamber with chlorine-based plasma. For the same process condition, it has been known that the etch rate is lower for the lower plasma source power applied. Through analysis of OES measurements, even though the supplied external power is the same, the effective power applied to the plasma seems to be lower when the chamber wall surface is fluorine rich.

Investigation of active Si pitting and its impact on 0.15 and 0.30 μm n-type metal–oxide–semiconductor and p-type metal–oxide–semiconductor transistors

The causes of pitting on an active silicon surface and its impact on 0.15 and 0.30 μm n-type metal–oxide semiconductor (NMOS) and p-type metal–oxide–semiconductor (PMOS) transistors have been assessed. During polysilicon gate patterning, the gate oxide at the source/drain regions of the NMOS transistors may be etched away due to insufficient selectivity of the etch chemistry, improper etch chamber configuration, pattern density, and the adoption of dual-doped gate technology. The exposed silicon substrate has poor selectivity against the polysilicon gate, this causes Si pitting to occur on the source/drain active regions of the NMOS transistors. However, there is no significant pitting observed for the PMOS transistors due to the slower etch rate of the undoped polysilicon of the PMOS transistors. Pitting on the active areas severely degrades the device parameters, such as drive current, series resistance, and transconductance. This is worse for the 0.15 μm NMOS transistors compared to the 0.30 μm NMOS transistors. The channel resistance of the 0.30 μm transistors is much larger than any increase in the external resistance caused by the pitting. As a result, Si pitting has negligible effects on the device performance of long channel devices.

Low temperature SF6/O2 electron cyclotron resonance plasma etching for polysilicon gates

The use of SF6/O2 chemistry for polysilicon gate etching at a low temperature in an electron cyclotron resonance plasma etcher is considered. Sidewall angle, polysilicon etch rate, and selectivity to gate oxide and photoresist are reported as functions of temperature in the range −125 °C to 25 °C. The addition of oxygen to the plasma allows anisotropic etching to be obtained at higher temperatures than with pure SF6. 0.3 μm long gates were formed at −100 °C using an etch process with ≈3:1 selectivity to photoresist and ≈14:1 selectivity to oxide.

In situ measurement of aspect ratio dependent etch rates of polysilicon in an inductively coupled fluorine plasma

The etch rate of polysilicon in high aspect ratio structures has been studied in an inductively coupled fluorine based plasma (pure SF6). The change of the silicon etch rate with increasing aspect ratio [aspect ratio dependent etch rate or reactive ion etching (RIE) lag] has been measured in situ by interferometry. The experimental structures as well as the process conditions were chosen such that (i) the interaction of neutral fluorine atoms with the silicon surface determines the etch rate, (ii) the ion energy is minimized, (iii) the mass transport of neutrals is in a molecular flow (Knudsen) regime, and (iv) the interaction of the etch species with the sidewalls can be neglected. Under these conditions, the experimental findings indicate that the RIE lag effect is reduced for higher pressures and lower cathode temperatures, i.e., for higher fluorine atom coverages of the silicon surface. This is in agreement with the Knudsen transport model by Coburn and Winters [W. Coburn and H. F. Winters, Appl. Phys. Lett. 55, 2730 (1989)]. Probabilities for the reaction of fluorine atoms with the silicon surface between 0.03 and 0.11 can be derived when fitting the experimental data with this model. The findings reported in this work are of relevance for the formation of deep trench storage capacitors and polysilicon plugs. In particular, we show that we can control RIE lag and etch rate independently for the given process conditions.

Characteristics and mechanism of etch process sensitivity to chamber surface condition

Process variation with chamber surface condition in high density plasma reactors can seriously affect the process performance and productivity of device manufacturing. In this work, the impact of chamber surface coating on polycrystalline-silicon (polysilicon) gate etching with Cl2- and HBr-based plasma has been studied. The dependence of process sensitivity to surface coating on various processing conditions has been characterized and the mechanism of process shift has been analyzed. Based on the experimental results, the root cause of process sensitivity has been attributed to the change of surface recombination rate of free radicals on different chamber surfaces which leads to a variation in reactive neutral density. Under a certain discharge condition, the Cl and Br densities in a clean chamber with anodized aluminum and alumina surfaces are &amp;gt;60% lower than those in a seasoned chamber with silicon oxide deposition, resulting in lower polysilicon and oxide etch rate in the former case. In general, the process sensitivity to chamber surface condition depends on process regime and is higher for those processes where etching is more chemical in nature, such as processes at higher pressure and with Cl2-based chemistry. Processes with higher oxygen concentration and higher source power are also more sensitive due to the accelerating surface oxidation of polysilicon surface in a clean chamber. Approaches to reduce the effect of process sensitivity have been discussed.

Characteristics of magnetized inductively coupled plasma source for flat panel display applications

In this study, to improve the plasma uniformity in addition to the plasma density of an inductively coupled plasma source, both permanent magnets and a Helmholtz type axial electromagnet were installed to a conventional planar inductively coupled plasma source and the effects of various magnet combinations on the polysilicon etch rates and etch uniformities were studied. The application of weak axial electromagnetic field to a planar inductively coupled plasma showed increased plasma densities, therefore, increased polysilicon etch rates compared with those by conventional inductively coupled plasmas. However, the application of the electromagnetic field degraded the etch uniformity. By using both an optimized permanent magnets and the axial electromagnet around the chamber wall, we found the improvement of polysilicon etch uniformity in addition to the increase of polysilicon etch rate due to the combination of the better features of both magnets. Among the permanent magnet configurations used with axial electromagnetic field in this experiment, a permanent magnet configuration with two poles/site showed the polysilicon etch uniformity at approximately 3.0% up to 15 G of axial electromagnetic field. The use of the magnetized inductively coupled plasma also decreased the average ion energy to the wall from 20 to 15 eV, therefore, decreased the possible contamination of the substrate caused by sputtering of the chamber wall.

A new inductively coupled plasma source design with improved azimuthalsymmetry control

The geometry of an inductively coupled plasma (ICP) source impactsthe plasma and processing uniformity. A reasonably uniform source design doesnot always guarantee a uniform plasma however, because transmission line(i.e. standing wave) effects also impact on its performance. In this work wepresent an ICP source design with a geometry that enables better control overthe field profiles' azimuthal symmetry despite transmission line effects. Thegeometry is three dimensional rather than planar and consists of two layers offull and semicircular loops with the RF current generally flowing in oppositedirections. We have measured the free space magnetic fields produced by oneimplementation of the new source in the (r,θ) plane using a B-dotprobe. The new source generated fields of higher azimuthal symmetry than theplanar coil, despite a larger current variation along the new source length. Wehave also developed a three-dimensional electromagnetic model for ICP sourcesthat accounts for current variations along the source length due to standingwave effects. The model showed good agreement with the measured fields for theplanar coil. However, it showed less agreement for the new source design sincethe interaction between the loops in the different layers were not included inthe model. Langmuir probe measurements showed that the new ICP sourcegenerated high density (1011-1012 cm-3) argon and chlorineplasmas at low pressures (1-30 mTorr) at 1 cm above the wafer surface. Spatialprofiles of electron temperature and ion density in a chlorine plasma at 1 cmbelow the dielectric window showed improved azimuthal symmetry of powerdeposition with the new ICP source. Polysilicon etch rate profiles on 150 mmwafers also showed improved azimuthal symmetry and uniformity with the new ICPsource.

Demonstration of broadband radio frequency sensing: Empirical polysilicon etch rate estimation in a Lam 9400 etch tool

The sensitivity of a novel broad frequency band (1–2.25 GHz) radio frequency sensing system to plasma etching process conditions is demonstrated. This is accomplished by using the sensing system to estimate polysilicon etch rate in a Lam 9400 etch tool. A designed experiment varying physical and chemical reactive ion etching regimes was performed with five repetitions at each experimental point. A model relating broadband sensor response to etch rate was regressed using four repetitions of the data and validated on the fifth. Two representations of the broadband data were considered separately when regressing the models, with subset selection used in each case to choose the best predictor variables. In one representation, the sensor data was considered as a vector of 402 real numbers corresponding to magnitude and phase of reflection coefficient at each of 201 frequencies, resulting in an R2 of etch rate estimate of 0.997. In the other, the broadband response was parameterized on the basis of a multimodal cavity resonance model. The inferred parameters of natural frequency, quality factor and resistance were then used as the predictor variables for regression, resulting in an R2 of 0.962.

Real-Time Monitoring and Control of Plasma Etching

Real-time etch rate of thin transparent films is determined within seconds by an in-situ two-color laser interferometer. The use of two colors improves the accuracy of the calculated rates, provides an absolute measure of film thickness for endpoint prediction, and differentiates between etching and deposition. The tool state parameters, rf power to the antenna and the wafer stage, gas pressure and flow rates, are computer controlled and monitored. Real-time etch rate characterization is obtained by monitoring the etch dependence on varying tool state parameters. The density of the etch radicals, chlorine and fluorine, is obtained with xenon and argon actinometry using optical emission and mass spectra. An etch rate model, based on the input power to the wafer stage and the relative density of the etch radicals, is used to develop a model-based real-time control algorithm. This algorithm has been used to control the etch rate of unpatterned polysilicon and SiO2.

Effects of gas distribution on polysilicon etch rate uniformity for a low pressure, high density plasma

The geometry of low pressure inductively coupled plasma sources is usually considered to be a main factor in determining both the plasma and processing uniformity over large area wafers. We demonstrate experimentally that the gas flow distribution can have a major impact on both the plasma density and etch rate uniformity at low pressures where one might expect diffusion to make gas flow distribution less important. 150 mm polysilicon on oxide wafers were etched in SF6/Ar (1:1) plasmas between 6 and 20 mTorr. Using a single gas inlet produced polysilicon etch rates that varied by 30% along the gas flow direction, but were highly uniform perpendicular to the gas flow direction. A gas distribution ring, on the other hand, produced highly uniform etch rates with variations less than 4% overall using the same source. Langmuir probe measurements of the ion saturation current spatial profiles in argon and SF6 discharges demonstrated significant gradients across the reactor for a single gas inlet with the largest current density near the inlet.

Angular dependence of the polysilicon etch rate during dry etching in SF6 and Cl2

The angular dependence of the etch rate in reactive ion etching (RIE) and inductively coupled plasma (ICP) systems for polysilicon etching with SF6 and Cl2 is determined using a recently developed direct measurement method. The latter utilizes specially patterned silicon groove structures consisting of 7–10 μm wide planar surfaces which form various angles with respect to the wafer normal. The structures are produced by highly anisotropic wet chemical etching of Si through a gratinglike mask pattern aligned along specific crystallographic orientations of the wafer which results in the development of planar surfaces of various orientations. These surfaces are then coated with the materials to be studied—polysilicon in this case. The deposited polysilicon is then etched under a variety of conditions in a RIE and an ICP reactor and the etch rates determined by interferometric measurements. Since only standard Si wafers are used and the size of the pattern is only a few μm the method is fully IC production compatible, which means that one can measure the angular dependence of the etch rate directly in production etching systems. The results for RIE of polysilicon with SF6 show that the process becomes more isotropic with increasing pressure. The angular dependence of the RIE and ICP polysilicon etch rates in Cl2 atmosphere were found to vary with the substrate bias. Specifically low substrate bias resulted in an under cosine distribution whereas bias higher than 240–250 V led to over cosine distributions.

Study of shallow silicon trench etch process using planar inductively coupled plasmas

Silicon shallow trenches applied to the shallow trench isolation of integrated circuits were etched using planar inductively coupled Cl2 and HBr/Cl2 plasmas and the effects of process parameters such as gas combination, inductive power, and bias voltage on etch rates, selectivities, and etch profiles of silicon trenches were investigated. Also, the physical damages on the trench bottom and sidewall in Cl2 and HBr/Cl2 plasmas were studied using cross-sectional high resolution transmission electron microscopy (HRTEM). The increase of inductive power and bias voltage in Cl2 and HBr/Cl2 plasmas increased polysilicon etch rates in general, but reduced the etch selectivity over nitride. In case of Cl2 plasmas, low inductive power and high bias voltage showed an anisotropic trench etch profile, and the addition of oxygen or nitrogen to chlorine also increased the etch anisotropy. The use of pure HBr plasmas showed a positively angled etch profile and the addition of Cl2 to HBr improved the etch profile more vertically. HRTEM study showed defects formed near the sidewall, bottom, and bottom edge of the silicon trenches etched in Cl2/N2 plasmas, and more dense defects in HBr/Cl2 plasmas possibly due to the penetration of hydrogen in HBr. No defects were found for the trenches etched in pure Cl2 or Cl2/O2 plasmas.

Selective dry etching of oxide films for spacer applications in a high density plasma

The use of a high density plasma to etch oxide side wall spacers was investigated. Process trends and the optimum process conditions required were determined. Oxide and polysilicon etch rates, uniformities, and the selectivity of oxide to polysilicon were all measured. The resulting etch chemistry had an oxide etch rate of 350 nm/min with a selectivity of oxide to polysilicon of 30:1.

Polysilicon gate etching in high density plasmas. I. Process optimization using a chlorine-based chemistry

High density plasma etching processes using chlorine gas have been developed for 0.25 μm polysilicon etching. Polysilicon films on SiO2-covered 200 mm silicon wafers are masked with photoresist patterns, and then etched downstream using a high density helicon source made by Lucas Labs of Sunnyvale, CA. To enhance the anisotropic etching, ion bombardment is provided by radio frequency (rf) biasing of the sample. Polysilicon trenching can be suppressed by performing a two-step etching process: the first uses a high energy ion bombardment resulting in high etch rate (250 nm/min) and anisotropy and the second a lower ion energy. The second etching step shows a high selectivity (≳80) of Si over SiO2 and therefore ensures the complete removal of the polysilicon during the overetch time. Perfect anisotropic profiles can be obtained without any trenching or other undesirable anomalies. The optimum etch rate nonuniformity is 6.5% (3σ) when operating the source at maximum rf source power (2500 W), 2 mTorr reactor pressure, and adding 30 sccm of helium to a 50 sccm chlorine discharge. Polysilicon and gate oxide etch rates have been measured using a real time in situ HeNe ellipsometer. Etch rates for polysilicon and oxide increase as a function of rf bias power but show no significant pressure or rf source power dependence. After etching, the 200 mm wafers can be transferred (under high vacuum) to an ultra high vacuum analysis chamber equipped with a Fisons ESCALAB 220i x-ray photoelectron spectrometer.

Etching 0.35 μm polysilicon gates on a high-density helicon etcher

This article describes an evaluation of a helicon high-density plasma etch tool for etching 0.35 μm polysilicon gates. The objective of the evaluation was to establish whether it is possible to etch 0.35 μm lines of both doped and undoped polysilicon (simultaneously) while maintaining straight profiles across the wafer and a very high polysilicon-to-oxide selectivity. Several high-density plasma (HDP) polysilicon etch tools for 0.35 μm technology have been tested at Texas Instruments. It is known that HDP tools operating at low pressures can obtain high polysilicon etch rates and anisotropic etch profiles. Most of the HDP sources utilize two separate radio-frequency (rf) power supplies; one to generate the high-density plasma and a second to bias the chuck. This allows relatively independent control of the ion density and the ion energy. It is possible to attain high polysilicon-to-oxide selectivity in the overetch step by using a high flux (high rf power to the plasma) of low energy (low rf power to the chuck) ions. This high selectivity is particularly important when long overetches are required with thin gate oxides.

Characterization of plasma etch processes using measurements of discharge impedance

Actual power delivered to the discharge, rf voltage, rf current, phase angle, and dc bias have been measured using commercially available impedance and power meters placed directly on the powered electrode of a parallel-plate diode etcher. The variation of these electrical quantities as discharge parameters such as power, pressure, electrode spacing, gas mixing ratio, and total flow are varied has been investigated for both electropositive and electronegative gases. It was found that the transfer efficiency of the matching network depends significantly on the process conditions and can be as low as 55% of the generator output. Polysilicon etch rate experiments were performed to determine the effect of varying the actual power delivered to the discharge and possible interaction with other process parameters such as pressure, as opposed to considering solely the rf generator output. End point was detected by measuring the change in the discharge impedance while etching through a polysilicon film. The sensitivity of this technique was compared with that of more commonly used optical emission methods.

Optimization of an electron cyclotron resonance plasma etch process for n+ polysilicon: HBr process chemistry

Designed experiments were employed to characterize a process for etching phosphorus doped polycrystalline silicon with HBr in a close-coupled electron cyclotron resonance plasma reactor configured for 200 mm wafers. A fractional factorial screening experiment was employed to determine the principal input factors and the main etch effects. Linear models of the process responses indicate rf power, O2 flow rate, and the position of the resonance zone (with respect to the wafer) as the three strongest factors influencing process performance. Response surfaces generated using data from a follow-on response surface methodology experiment predicted an optimum operating region characterized by relatively low rf power, a small O2 flow, and a resonance zone position close to the wafer. One operating point in this region demonstrated a polysilicon etch rate of 270 nm/min, an etch rate nonuniformity of 2.2% (1 std. dev.), an etch selectivity to oxide greater than 100:1, and anisotropic profiles. Particle test results for the optimized process indicated that careful selection of the O2 fraction is required to avoid residue deposition and particle formation.