Papers

AVS1997 Session PS+MS-MoM: Plasma Sensors

Monday, October 20, 1997 8:20 AM in Room A5/6

Monday Morning

Start	Invited?	Item
8:20 AM		PS+MS-MoM-1 Wafer Level Ion Energy Analyzer for Plasma Analysis M.G. Blain, C.A. Nichols, J.E. Stevens (Sandia National Laboratories); V. Resta (Sematech) A micron-scaled, wafer level ion energy analyzer for analysis of processing plasmas was fabricated using standard Si integrated circuit materials and processing techniques. The analyzer is based on the retarding potential technique and consists of two grids and a collector plate made of polysilicon and separated by 1 µm and 2 µm of SiO₂. Approximately 0.6 % of the 1.1 cm² analyzer area is covered with 0.75 µm diameter ion collection holes. Energy resolution is predicted to be less than 0.1 eV for 20 eV incident ions based on ion trajectory modeling. The analyzer can be operated on the wafer on which it was fabricated or can be separated into 1.6 x 1.2 cm die for use as a discrete probe. Experimental tests in ICP argon plasmas have demonstrated the basic functionality of the analyzer. Measurements show energy spreads of 3-5 eV which are consistent with the expected presheath voltage drops. Perturbations of the plasma-surface geometry near the analyzer collection surface were observed to affect the measured energy spread.¹ We also report on efforts to passivate the Si grids for operation of the analyzer in halogen plasmas. * This work was supported by the United States Department of Energy under Contract DE-AC04-94AL85000 and by Sematech. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy. ¹J.R. Woodworth and B.P. Aragon, Appl. Phys Lett. 70, 1947(1997).
8:40 AM		PS+MS-MoM-2 A Compact Floating Energy Analyzer for Measuring Energy Distributions of Ions Bombarding a rf Biased Electrode A. Perry (Lam Research Corporation); E.A. Edelberg (University of California, Santa Barbara); N. Benjamin (Lam Research Corporation); E.S. Aydil (University of California, Santa Barbara) A compact floating ion energy analyzer (CoFIEA) capable of measuring the energy distributions of ions bombarding a rf biased electrode has been developed. The CoFIEA, which can detect ion energies of up to 500eV, was used to measure the energy distribution of ions arriving at the substrate electrode (chuck) of a Lam TCP inductively-coupled plasma reactor. Ion energy distribution (IED) measurements were made for argon, neon, argon-neon and oxygen plasmas with the chuck grounded, floating and rf biased at 4MHz. The effects of the modulation of the plasma and chuck potential on the IED were investigated as a function of TCP power (13.56MHz), chamber pressure and rf bias power. As the bias power is increased (0-100W) the ion current collected at the analyzer remains constant but the total energy in the ions scales linearly with the power, indicating that the bias is not contributing to plasma production. The effect of the ratio of applied field period and ion transit time across the sheath on the energy distribution is shown by varying the ion mass. Temperature, charge density and potential measurements taken with a Langmuir probe are used to calculate modulation parameters for the sheath at the chuck. The measured IEDs and trends with operating conditions agree well with simple sheath model predictions.
9:00 AM		PS+MS-MoM-3 Preliminary Empirical Results Suggesting the Mapping of Dynamic in situ Process Signals to Real Time Wafer Attributes in a Plasma Etch Process E.A. Rietman, J.T.C. Lee (Bell Laboratories, Lucent Technologies) In situ monitoring of the wafer attributes in real-time is essentially non-existent in modern semiconductor manufacturing. For plasma etching processes several new diagnostic techniques (e.g. full-wafer imaging interferometry and ellipsometry) provide improved endpoint observation and some provide metrics for the state of the wafer at a given time. However, the methods that do provide metrics are usually quite expensive for a manufacturing environment. We propose a method whereby simple and economic endpoint methods can indicate in real-time the state of the wafer. Our method consists of finding the algorithm to map in situ wafer-state signatures (e.g. interferometry, ellipsometry) to wafer attributes and then mapping the process signatures (e.g. reflected-rf power, pressure, flow rate, OES) to wafer-state signatures. From these we then have an abstract mapping from the process signatures to the wafer attributes in real-time, thus circumventing the need for the expensive diagnostic. In this paper we demonstrate that a learning machine can perform the mapping between process signatures, as a function of time and wafer state signatures, as a function of time.
9:20 AM		PS+MS-MoM-4 Spectral Interferometry -- A Novel Process Control Diagnostic. H. Lee, F.P. Klemens, A. Kornblit, H.L. Maynard, T.C. Lee (Bell Laboratories, Lucent Technologies) The gate structure of advanced integrated circuits can have gate oxide films as thin as 1 to 2 nm. To etch these structures successfully -- without breaking through the extremely thin films and minimizing plasma induced damage -- it is becoming essential to have an in situ process control diagnostic that enables one to stop or modify the etching process before the film is completely removed. By using thin film interference signals from different portions of the UV-visible spectrum, one can simultaneously monitor the instantaneous film thickness as well improve the sensitivity of the signal to an impending endpoint. Furthermore this diagnostic is sufficiently robust to be considered for use in the factory environment. Our Spectral Interferometer uses an external Xe lamp (white light) source in addition to light emitted by the plasma. Light reflecting off of the surface of the patterned wafer is focused into a monochromator that simultaneously records the thin-film interference signal for all wavelengths in the range 200-900 nm. One can select wavelengths that are transparent to the film to monitor the etching rate and the etch depth over time. Other wavelengths that are relatively absorbed by the film can be used to detect and predict endpoint. This combination of information from various spectral regions gives one a clearer picture of the process than one would have by monitoring only one wavelength. We demonstrate the utility of spectral interferometry to predict endpoint during the etching of wafers patterned with three different multi-layer gate-stack structures: α-Si/SiO₂/Si, WSi_x/α-Si/SiO₂/Si, and TiN/α-Si/SiO₂/Si. We compare our results to those obtained by in-situ ellipsometry.
9:40 AM		PS+MS-MoM-5 Plasma Sensors for Diagnostics and Control in Semiconductor Process Development and Manufacturing M. Surendra (IBM T.J. Watson Research Center) Sensors are becoming increasingly important in plasma processing, both in development and in manufacturing. The requirements in these two environments are not always the same. In development, more elaborate sensors (e.g. full wafer monitors) are important for providing quick evaluation of process performance, such as uniformity, whereas in manufacturing there is significant emphasis on cost, simplicity, and robust operation. The types of sensors range from systems which provide direct process performance (e.g. full wafer interferometry), detailed diagnostics (optical emission spectroscopy, residual gas analysis) to process health monitors. The last category not only includes rf harmonic sensors, but also controller outputs from the plasma process system as well, such as throttle valve and tuning capacitor positions, dc bias, etc. A discussion of the various types of sensors and their uses in both detailed process diagnostics and extended process monitoring will be presented. Data reduction methods are relevant in the latter category. In addition, methods to extract more information beyond general process health from simple single point sensors will be discussed.
10:20 AM		PS+MS-MoM-7 A Deposition-Tolerant Ion Flux Probe for In-Situ Process Control J.P. Booth, G. Cunge (Université J.Fourier-Grenoble, France); N.St.J. Braithwaite (The Open University, United Kingdom) In many plasma processing applications (etching, thin film deposition, surface modification) the ion flux to the treated substrate surface is the major factor determining the processing rate. Measuring this quantity thus gives a direct monitor of the reactor performance, which can be used both for process optimisation and diagnostics and as a parameter for real-time process control via feedback loops. Ion fluxes are measured routinely using Langmuir probe techniques in "clean" reactors (i.e. having only functioned with inert gases). However, these techniques cannot be applied to many industrial plasma processes based on reactive molecular gases which rapidly deposit insulating films on Langmuir probe surfaces. Our novel ion flux monitor ¹^,², overcomes this problem by measuring alternating signals from a planar Langmuir probe. Any insulating film on the probe surface acts as a capacitor, and will allow alternating signals to pass. The probe consists of a 1 cm² electrode with a concentric 2 cm² guard ring, and forms part of the reactor wall, so that the reactor geometry is not perturbed. Short pulses of RF voltage are applied to the probe via a blocking capacitor, generating a DC bias at the probe surface. Immediately after each RF pulse the probe is biased sufficiently negative that only positive ions arrive at the surface. The current flowing to the probe is measured at this point, and is equal to the ion current to the probe surface. As the peak RF power applied to the probe (< 1 W) and the duty cycle (1/100) are small, the perturbation of the process is negligible. Results obtained in fluorocarbon gas RIE and chlorine ICP reactors will be presented. ¹N.St.J. Braithwaite and J.P. Booth, Patent application BF 95/11181, (19th September 1995, France). ²N. St.J. Braithwaite, J.P. Booth, and G. Cunge, Plasma Sources, Science and Technology, 5, 677 (1996).
11:00 AM		PS+MS-MoM-9 High-Density Plasma Diagnostics: Using Optical Emission Spectroscopy to Determine Electron Temperature, Plasma Density and Cl₂ Percent Dissociation M.V. Malyshev (Bell Laboratories, Lucent Technologies and Princeton University); V.M. Donnelly, N.A. Ciampa, A. Kornblit (Bell Laboratories, Lucent Technologies) Trace Rare Gases Optical Emission Spectroscopy (TRG-OES) was used to determine electron temperature (T_e) and density for several configurations of one commercial and two laboratory, inductively coupled plasma reactors for a broad range of pressures and powers in Cl₂, Ar, He, Ne, BCl₃/Cl₂, and HCl/Cl₂/N₂ plasmas. Emission intensity was recorded from typically twenty Paschen 2p atomic levels of rare gases, which were added in small amounts to the discharge. The observed intensities were compared with values computed from a model that assumes a Maxwellian electron energy distribution. Results are compared with Langmuir probe and microwave interferometer measurements. TRG-OES is shown to provide a better relative measurement of T_e, compared with the Langmuir probe. T_e was independent of power and decreased with increasing pressure. For Cl₂ discharges, T_e changes from 2.8 to 2.0 eV between 0.5 and 20 mTorr. These values are ~1 eV lower than Langmuir probe results, which were plagued by artifacts due to the reactor configuration. Plasma density grows almost linearly with power and behaves differently with pressure in the different reactors. The contributions of rare gas metastable states to emission from all of the levels were evaluated by pulsing the plasma on and off for 100 us periods and observing the emission rise and decay times. The percent dissociation of Cl₂ was determined as a function of pressure, power, and reactor design, by observing emission from an ion-pair state of Cl₂ (306 nm) and dividing its intensity by that from a level of Xe (834.7 nm) - which is the best energy match and relatively free of contributions from its metastable levels. At the highest power (900W, ~0.1W/cm³) dissociation increased with decreasing pressure from 50% at 10 mTorr to 90% at 0.5 mTorr.
11:20 AM		PS+MS-MoM-10 RF Electrical Measurements as Sensors for Physical Properties of Plasmas M.A. Sobolewski (National Institute of Standards & Technology) Radio frequency (rf) current and voltage measurements are an important and convenient tool for monitoring rf discharges. These measurements are compatible with commercial reactors and with the manufacturing environment. Recently, several types of rf sensors for process-relevant plasma properties have been proposed. These sensors rely on models that relate measured electrical parameters to physical properties such as the densities, energies and fluxes of electrons and ions. Unfortunately, these models often rely on untested assumptions. In particular, the sheath regions of the plasma are difficult to model without the aid of simplifying assumptions. To test these assumptions and to provide a firmer foundation for future rf sensors, electrical studies were performed in a variety of discharges, including Ar and Ar/Cl₂ discharges in a high-density inductively coupled plasma reactor and CF₄/O₂ and Ar discharges in the capacitively-coupled GEC Reference Cell. External measurements of current and voltage waveforms were combined with capacitive probe measurements of the rf plasma potential and measurements of the ion saturation current at powered electrodes. Together, these measurements provide enough information to test models of the plasma and its sheaths; to determine the ranges of plasma density and uniformity, pressure, rf voltage, and rf frequency over which common simplifying assumptions are valid; and to identify which applications for RF sensors are most promising.
11:40 AM		PS+MS-MoM-11 Real-Time Control and Modeling of Plasma Etching. M. Sarfaty, C. Baum, M. Harper, N. Hershkowitz (University of Wisconsin, Madison) To meet the challenge of real-time advanced process control, the instantaneous process state needs to be determined. The relatively high process rates in high density plasma tools as well as the shrinking thickness of the films, require a fast estimate of the process state in order to implement process control. The research objective is to determine within a second the process rates and to implement real-time etch rate control. Several in-situ, non-invasive and non-destructive sensors such as two-color laser interferometer and a full wafer interferometer are used to monitor the process state in a magnetically confined ICP tool. The gas phase state is monitored by optical emission spectroscopy and a residual gas analyzer. The equipment state, gas flow, pressure, and RF power to the antenna and the electrostatic chuck are computer controlled and monitored. The two-color laser interferometer enables us to determine in real-time, within a second, the etch rates of thin transparent films. The absolute thickness of the film is determined during the process, thus providing an end-point prediction. The advantages of two-color laser interferometry for real-time process monitoring and control will be described. Langmuir kinetics modeling of the measured etch rates of polysilicon and SiO₂ films in Cl₂ and CF₄ discharges using tool state parameters will be described. The etch rate model enabled us to develop a model-based real-time control algorithm. Results of real-time etch rate control runs of un-patterned SiO₂ and polysilicon films will be described. This work is funded by NSF grant No. EEC-8721545.