Etch Rate Uniformity Research Articles

Characterization of a low pressure, high ion density, plasma metal etcher

As the backend device geometries shrink to near 0.5 μm, the challenges placed on plasma metal etching continue to mount. Profile control, critical dimension and etch rate microloading, selectivities to masking and substrate materials, defect densities and uniformities are being pushed to tighter limits. The typical results of the low ion density, low pressure batch systems and the low ion density, high pressure single wafer etchers cannot satisfy the demands of the 0.5 μm technologies. The very low pressure, high ion density plasma sources ostensibly offer some advantages in plasma metal etching. A transformer coupled plasma metal etcher is characterized with respect to the aforementioned process outputs. Statistically designed experimental data to optimize the selectivity to photoresist and the etch rate uniformity are presented. Additionally, in situ resist stripping in a transformer coupled plasma strip chamber is studied. Experiments designed to understand the causes of polymer residues are presented. Methods are given to eliminate polymer residues. Galvanic pitting of the AlCu layer in the vicinity of theta phase Al2Cu precipitates at grain boundaries was found to be a function of the in situ resist stripping and water rinsing conditions. The pitting was eliminated by optimizing the postetch plasma ashing and water rinsing conditions. Profile microloading and aspect ratio dependent etch effects are reported.

Etch characteristics of an amorphous refractory absorber

An ECR etch process for defining sub-0.25 μm features in a TaSiN absorber layer has been developed. A 2000 Å PECVD oxide layer served as a hard mask during the etch. The effect on feature profile has been determined as a function of both rf power and back side temperature. Etch rate uniformity is excellent, with 3σ deviations of less than 4% over the center 40 mm of the substrate. Micro-loading issues associated with the etch process have also been characterized. Minimal line edge roughness is observed, and the feasibility for defining 0.10 μm features has been demonstrated.

Large diameter microwave SF6 plasma production with ring-shaped permanent magnets

A large diameter and uniform microwave plasma (about 30 cm in diameter) over 10 cm from the source has been produced by scaling up a preliminarily proposed device with an annular slot antenna and satisfying the ECR (electron cyclotron resonance) condition in a chamber with neodymium ring-shaped permanent magnets. The spatial structure of the plasma density in both non-reactive Ar and reactive SF6 gases was obtained by a Langmuir probe. The tentative etching of silicon (Si) was performed in the SF6 microwave plasma. The spatial profile of the etch rate in the radial direction was found to be similar to that of electron density in the SF6 plasma. The uniformity of etch rate was within 2.5% at the position where electron density profile reached the best uniformity ( approximately=5%). The etching profile of the poly-Si pattern in the microwave plasma using pure SF6 gas was also observed by a scanning electron microscope (SEM).

Across wafer etch rate uniformity in a high density plasma reactor: Experiment and modeling

A study of silicon oxide etch rate uniformity in a high density, rf inductively coupled system with an rf capacitively coupled substrate electrode is presented. By introducing spatial variation of rf coupling to the substrate, etch rate uniformity across the wafer can be altered from a profile that is ∼20% higher in the center to one that is ∼10% lower in the center. The effect of spatially varying rf coupling impedance to the substrate is dependent on substrate resistance. Predicted etch rate profiles are obtained with a two-dimensional analytic model of the plasma source that is coupled to an equivalent circuit discretization of the electrode assembly, substrate, and sheath. Model results compare favorably with experimental measurements.

Performance and modeling of a permanent magnet electron cyclotron resonance plasma source

A plasma etch reactor with a permanent magnet electron cyclotron resonance (ECR) source and multipolar process chamber has been characterized with plasma parameter and etch measurements. The ECR source uses a compact permanent magnet structure, rather than current‐driven electromagnet coils, to establish the axial magnetic field required for ECR operation. The source is compact and requires no input electrical or cooling power. Plasma densities greater than 1011 cm−3 and ion current densities above 10 mA cm−2 are obtained; the plasma parameter uniformity is 1.2% (1σ) over 20 cm. The etch rate in undoped polysilicon is 3450 A/min without applied bias, and the etch rate uniformity is 1.8% (1σ) over a 14 cm wafer diameter. The radial and axial plasma parameter profiles agree with the predictions of an ambipolar diffusion model.

Development of a gate metal etch process for gallium arsenide wafers

The reactive ion etching of TiWN, which is used as a gate metal on gallium-arsenide device wafers, was studied in a parallel-plate, single-wafer plasma reactor operating at a frequency of 13.56 MHz. We discuss our experimental program designed to develop a highly uniform TiWN etch process with low linewidth loss for 100 mm GaAs wafers, using a sulfur hexafluoride, trifluoromethane, helium chemistry. The effects of different gas compositions, plasma power, inter-electrode gap, chamber pressure, and electrode temperature on the TiWN etch rate, linewidth loss, and etch uniformity were determined. The effects of adding oxygen and/or nitrogen to the above mixture were also studied. In preliminary experiments on Si wafers, standard design of experiments methods were used to narrow the ranges of parameters for further experiments to develop an optimum process for Si wafers. The results of these experiments guided us to the optimum process for GaAs wafers. The optimum conditions, for both Si and GaAs wafers, are presented.

Linewidth uniformity versus etch rate uniformity in refractory metal plasma etching

Etch rate, etch rate uniformity, linewidth uniformity, and microloading in tungsten, molybdenum, and niobium plasma etching have been studied. Electrical linewidth data with typically 100–300 measured lines/wafer have been used. Relation between linewidth uniformity and etched depth uniformity (as measured by profilometer) has been explored. Effects of tungsten film deposition processes (oxygen contamination) have been studied. Tungsten etch rate maximum in SF6/O2 is at 85/15 ratio for the oxygen contaminated films, but higher oxygen percentage in SF6/O2 is required for maximum etch rate of pure films. Oxygen impurities in the tungsten film affect the etch rate but neither linewidth nor linewidth uniformity. In molybdenum etching in Cl2/O2 plasma the etched depth uniformity gives an overly pessimistic uniformity value even though linewidth uniformity is acceptable 12% (3σ). Addition of 3%–6% of CHF3 to Cl2/O2 is shown to change microloading characteristics and to improve linewidth uniformity. On 100 mm wafers, 6%–12% (3σ) linewidth uniformities were obtained for the different metals. Linewidth differences between isolated and array lines were 4%–8%.

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

We have studied the etching performance of two commercially available low pressure, high density plasma sources and their application for the etching of 0.35 μm 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 polysilicon 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 uniformity 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 the lateral etching rate and increase the anisotropy of the etching process, approximately 3% of O2 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 Å/min and uniformity of ±3.3%(2σ). 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 Å/min and uniformity of ±1.4%(2σ). 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.

Etching Characteristics by M= 0 Helicon Wave Plasma

The each characteristics of SiO2 and Al–Si–0.5%Cu were studied using the M=0 helicon wave plasma etching apparatus. A high concentration of F radicals was observed during SiO2 etching using fluorocarbon gases. The etch rate of SiO2 was strongly dependent on the concentration of F radicals in the plasma. A method of increasing the selectivity of SiO2 to poly-Si was discussed. The Al–Si–0.5%Cu film deposited on the wafer of 200 mm diameter was anisotropically etched with high selectivity to photoresist using Cl2 and BCl3 gases. The etch rate uniformity across a wafer was also discussed.

Intelligent model-based control system employing in situ ellipsometry

An intelligent model-based control system provides a framework for the efficient application of in situ sensors to semiconductor manufacturing. This article discusses the components of such a system. The specific example presented in this article is a system utilizing in situ ellipsometry developed for a polysilicon gate etch. The ellipsometer was used for robust endpointing, process modeling, fault detection, and run-to-run supervisory control. The methodology for each sensor application is presented. The coordination of the various applications is also discussed and implementation issues are covered. The goal of the controller for polysilicon gate etch is to achieve a target mean etch rate, while maintaining a spatially uniform etch. This process suffers from machine aging, which degrades the etch rate and final film uniformity. Experimental results demonstrating that the controller compensates for process drift are shown. Implementation of this system resulted in a 36% decrease in standard deviation from the target for the mean etch rate. All of the wafers processed with feedback control met the specifications on nonuniformity, while some of the wafers processed without control did not. In addition, the data indicated that controlling the etch rate may improve the control and uniformity of the linewidth change.

Interferometric Real‐Time Measurement of Uniformity for Plasma Etching

A new technique has been developed to measure etch rate uniformity in situ using a charge coupled device (CCD) camera which views the wafer during plasma processing. The technique records the temporal modulation of plasma emission or laser illumination reflected from the wafer; this modulation is caused by interferometry as thin films are etched or deposited. The measured etching rates compared very well with those determined by helium‐neon laser interferometry. This technique is capable of measuring etch rates across 100 mm or larger wafers. It can resolve etch rate variations across a wafer or within a die. The CCD measurement technique is a valuable tool for process development and has potential use as a real‐time diagnostic for process control.

Comparison of advanced plasma sources for etching applications. III. Ion energy distribution functions for a helicon and a multipole electron cyclotron resonance source

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.

Comparison of advanced plasma sources for etching applications. II. Langmuir probe studies of a helicon and a multipole electron cyclotron resonance source

Radial profiles of electron temperature, electron density, and ion current density near the wafer surface are presented for a rf-inductively coupled helicon and a multipole electron cyclotron resonance source as a function of applied source power, applied rf-bias power, and reactor pressure. Both sources show similar trends with respect to changing process conditions, with electron densities in the mid 1011 cm−3 range and electron temperatures between 5.5 and 6.5 eV. An increase in applied source power results in an increase in both electron density and ion current density as expected. An increase in applied rf-bias power results in a 10% change in the measured electron density and ion current density; however, the electron temperature shows a much stronger dependence, indicating that ion flux and ion energy are not completely independent. Radial uniformity is similar in both sources, with the helicon exhibiting better uniformity at the conditions explored. Neither applied source power nor rf-bias power significantly affect radial plasma uniformity above the wafer in both sources, but higher reactor pressure does degrade uniformity in the helicon. We attribute this to the 2:1 geometric expansion between the upper antenna section and the lower plasma confinement section where the measurements are taken. The conditions resulting in optimum radial uniformity of electron temperature, electron density, and ion current density are similar to those for the optimum etching rate uniformity measured in previous studies; yet the uniformity obtained from the Langmuir probe diagnostic is not affected by changing process conditions to the degree that etching uniformity is affected. This can be a result of the difference in chemistry between the two studies since hydrogen bromide was used for etching and argon was used for Langmuir probe measurements. It may also indicate that the correlation between plasma uniformity and etching uniformity is relatively weak or second order compared to other variables affecting uniformity such as rf bias of the wafer.

Experiments with back side gas cooling using an electrostatic wafer holder in an electron cyclotron resonance etching tool

Wafer temperature, etch rate, and etch uniformity measurements of SiO2 wafers were made to characterize the use of back side helium cooling with an electrostatic wafer holder in an electron cyclotron resonance etching tool. The etch rate was found to be independent of the wafer temperature in the range between 20 and 110 °C. A 7% increase in etch nonuniformity (3σ) at higher backside pressures was attributed to helium, which leaked around the edge of the wafer, displacing the etchant gas. A back side pressure of 2–3 Torr provides a balance between wafer temperature control and helium leak rates.

Diffusion-limited etching of p-GaAs in NaOHNa 2EDTA solutions

Mass-transport-limited etching of p-type GaAs using sodium hydroxide (NaOH) and disodium ethylenediamine tetraacetate (Na 2EDTA) has been studied. Isotropic etching of (100)-oriented p-type GaAs was obtained for pH values between 8 and 13, and uniform etch rates were obtained over 0.05–0.25 μm/min with an unstirred solution. Etching cycles consisted of anodic oxidation for 20–40 s followed by oxide removal when the potential was set just negative of the rest potential for 5–10 s. The diffusion-limited currents were found to be relatively insensitive both to changes in oxidizing voltage and to variations in the hole concentration in the GaAs. Etching in the mass-transport limited regime was found to be a convenient and useful approach when profiling the GaAs hole concentration with electrochemical capacitance-voltage measurements.

Improvement of dry development of photoresist in a RIPE source for 0.4 to 0.35 μm technologies

Abstract We present the study of a dry development of silylated resist in a Resonance Inductive Plasma Etcher (RIPE) LUCAS source. An optimised process point has been demonstrated on a device using 0.4 μm I-line CMOS technology. Top Surface Imaging (T.S.I.) is a technique which can push the ultimate resolution of I-line exposure system waiting to a defined DUV lithography process. One severe limitation is roughness of the edge of the pattern. The RIPE source works at low pressure and uses triode type plasma generation. The combination of low pressure and controlled bias of the wafer has been demonstrated to be the key point for reducing the pattern roughness. Compared to Magnetron Reactive Ion Etching (MRIE) source, the etch rate uniformity is widely improved. A process point has been selected from a parametric study of the RIPE-LUCAS source and used for the gate patterning of a 0.4 μm CMOS device.

Dependence of etch characteristics on charge particles as measured by Langmuir probe in a multipolar electron cyclotron resonance source

A multipolar electron cyclotron resonance (ECR) plasma source was characterized by Langmuir probe measurements and the charged particle energy and density are related to photoresist etch rate. Ion density was found to increase with microwave power but decrease with source distance, and is independent of rf power and flow rate. Among the different gases investigated, Ar plasma was found to have higher ion density compared to O2, N2, and Cl2 discharge. Ion density peaked at 5 mTorr for Ar and 2 mTorr for O2 plasma. For a N2 plasma, the maximum ion density was found to occur at 10 mTorr at 3 cm and 2 mTorr at 23 cm below the ECR source. Electron temperature is ∼3 eV for pressure ranging from 1 to 5 mTorr but decreases to 2 eV at 20 mTorr. Photoresist etch rate follows the same trend as ion density. It increases with microwave power and decreases with source distance. On the other hand, photoresist etch rate increases with rf power and flow rate, even though the ion density remains constant. This suggests that photoresist etching also depends on ion energy and concentrations of neutral species. Ion density uniformity of ±1% for a N2 plasma was measured across the central 24 cm along the stage at 23 cm below the source. For a source distance between 3 and 23 cm, uniformity of ±3% was obtained over a 12 cm diam region on the stage. In an O2 plasma, photoresist etch rate uniformity of ±2% was measured for a 10 cm diam wafer and the corresponding ion density uniformity was ±1%.

Fabrication of micromachined silicon tip transducer for tactile sensing

The tunneling vacuum diode was employed to construct a novel displacement transducer for tactile sensing. High sensitivity of the emission current to variations of cathode–anode separation is the most important feature. The device was fabricated on a silicon wafer. The device components are a single tip or an array of tips made by wet etching, and a metal anode diaphragm. Nitric acid-ammonium fluoride etchant is used at room temperature to etch the silicon tips, with an easily controlled etch rate and excellent uniformity. A unique sacrificial layer technique creates and controls the spacing between cathode tip and anode diaphragm. Highly sensitive responses of the device current to different force loads on the anode diaphragm are presented.

Radio-frequency reactive sputter etching in magnetron fields

The etch rates of materials in rf discharges can be enhanced at low target voltages and low discharge pressures by the application of magnetic fields. In a previous article magnetic enhancement in diode and hollow cathode systems was experimentally studied using an axial magnetic field, applied perpendicular to the target. The field in the present article is applied parallel to the target surface, in a magnetron configuration. Experimental results are given here for etch rate, uniformity, and power input. Etch rates of SiO2 were studied as a function of CF4 pressure, magnetron field, and mechanical discharge confinement. Various electrode shapes were used to confine the discharge and hence increase discharge ionization. Etch rate uniformity at low pressure was measured as a function of electrode confinement and magnetic field, and is discussed in terms of both the motion of glow electrons and the measured sheath (or dark space) heights. Etch rates increase with increasing gas pressure, electrode confinement, or magnetron field. At 2 Pa CF4 pressure and 975 Vpp at 13.56 MHz in a diode machine, etch rates increased by a factor of 10 using either electrode confinement or a 208 G magnetron field. Both methods improve electron trapping, thereby increasing ionization and etch rates. With applied voltage and gas pressure held constant, magnetron fields increase etch rate by a factor of 9 for a diode, 5 for an unconfined hollow cathode, and 1.7 for a target confined hollow cathode. Magnetron fields result in very poor etch rate uniformity, which can however be improved by adding electrode confinement to improve electron trapping. Etch power efficiency varied by only 33% over the magnetic field range studied here, indicating little magnetic field-induced depletion of chemically reactive species even in our most intense discharges. However, compared to the diode, power efficiency was up to two times greater in the electrode-confined hollow cathodes due to their more efficient utilization of these chemically reactive species.

Al Etching Characteristics Employing Helicon Wave Plasma

Characteristics of helicon wave plasma employing Cl2 gas and its application to Al etching have been studied. Production of Cl ions dominates that of Cl atoms at 1200W RF power at 2 mTorr. Liberated oxygen from a quartz tube sputtered with Cl ions oxidizes Al and the probe surface. This is overcome by adding 10% BCl3 to Cl2. Electron density, temperature and ion current in Cl2 plasma at 2 mTorr are 5×1010 cm-3, 5.5 eV and 15 mA/cm2, respectively. Formation of negative chlorine is dominant at higher pressure. Although the Al etch rate is 5000 Å/min, selectivity to resist and SiO2 is poor due to high substrate bias voltage. Initiation time depends strongly on bias voltage, while net etching time is not changed. This implies that Al etching follows a chemical reaction even at 2 mTorr. Considerably uniform etch rate was achieved in zero magnetic field in a reactor. Slight variation which may result in the helicon wave plasma was observed.