J. Phys. Chem. 1981, 85,3529-3532
3529
Chemiluminescence and the Reaction of Molecular Fluorine with Silicon J. A. Mucha,* V. M. Donnelly, D. L. Flamm, and L. M. Webb Bell Laboratories, Murray Hiii, New Jersey 07974 (Received: December 16, 1980; In Flnal Form: July 17, 1981)
Molecular fluorine etches silicon with a rate = [(3.94 f 0.65) X 10-12]T1/2nFle-0.397eVlkT A/min, a process that is accompanied by gas-phase chemiluminescence which exhibits the same activation energy as the etch process. The observed temperature and pressure dependencies of these phenomena are consistent with a mechanism in which SiFz is an etch product that is involved in a chemiluminescent gas-phase reaction with Fz. The results extrendsimilar studies of silicon etching by atomic fluorine. There also is evidence of desorption products other than SiFz. The reaction between Fzand SiOzis measurable at elevated temperatures and pressure (-500 torr, 100 “C) and the Si:SiOz etch ratio is greater than 1001.
Introduction The reaction between molecular fluorine and singlecrystal silicon has been studied previously in two laboratories. Kuriakose and Margravel measured an etch rate of 0.042 mg/(cm2 min torr) (-2000 A/(min torr)) at 373 K and an Arrhenius activation energy of 0.52 eV (12 kcal/mol). However, Chen et a1.2 obtained etch rates of 160 A/(min torr) at this temperature and an activation energy of 0.35 eV (8 kcal/mol). The magnitude of the etch rate reported by Kuriakose and Margrave is large enough to make a measurable contribution in our fluorine-atom studies3and in some fluorine-containingplasmas presently employed in the processing of silicon electronic devices, especially at higher temperatures. We have therefore reexamined F2etching of silicon and its oxide. The results reported here do not support either of the discordant previous works but do compare more favorably with those of Chen et. al. indicating that the F2contribution to etching in fluorine plasmas is negligible. We also report the observation of a broad, visible chemiluminescence accompanying the etching of silicon by F2,which has a spectrum nearly identical with that observed during F-atom e t ~ h i n g . ~Etch rates and the intensity of the chemiluminescence were measured as a function of temperature and fluorine pressure. The origin of the luminescence and the mechanism of the etch process are consistent with the interpretation of the F-atom results; however, in the present study, there was evidence for the formation of other desorption products in addition to SiFP Experimental Section The experimental a p p a r a t u ~ , ~sample , ~ ~ , ~preparation, and handling procedures3 have been detailed previously. (1) A. K. Kuriakose and J. L. Margrave, J . Phys. Chem., 68, 2671 (1964). (2) M. Chen, V. J. Minkiewicz, and K. Lee, J . Electrochem. SOC.,26, 1946 (1979). (3) D. L. Flamm, V. M. Donnelly, and J. A. Mucha, J. Appl. Phys., 52, 3633 (1981). (4) (a) V. M. Donnelly and D. L. Flamm, J. Appl. Phys., 51, 5274 (1980); (b) V. M. Donnelly, D. L. Flamm, and J. A. Mucha, “Optical Emission from Transient Species in Halocarbon and Fluorosilicon Plasmas”, Extended Abstracts, 157th Meeting of the Electrochemical Society, St. Louis, MO, May 1980, Vol. 80-1, p 323; (c) V. M. Donnelly, D. L. Flamm, and J. A. Mucha, “Studies of Chemiluminescence Accompanying Silicon Etching by F Atoms”, Proceedings of the 88th National Meeting of the American Institute of Chemical Engineers, paper 47C, Philadelphia, PA, June, 1980; (d) C. I. M. Beenakker, J. H. J. van Dommelen, and J. Dieleman, “Origin of the Luminescence Produced by the Reaction of Fluorine Atoms with Silicon”, Extended Abstracts, 157th Meeting of the Electrochemical Society, St. Louis, MO, May 1980, Vol. 80-1, p 330. (5) D. L. Flamm, C. J. Mogab, and E. R. Sklaver, J.Appl. Phys., 50, 624 (1979). 0022-365418112085-3529$01.25/0
Briefly, single-crystal silicon (100) samples were patterned with steam-grown thermal oxide, bonded to the end of a temperature-controlled aluminum rod, and positioned inline with the wall of an insulated aluminum reaction cell. The F2 (Air Products, Technical Grade) was passed through a sodium bifluoride scrubber to remove any traces of HF. Silicon etch depths were measured by using a Sloan Technology Model 90050 Dektak stylus thickness monitor after dissolution of the oxide mask in HF. Oxide thickness was measured with a Nanospec AFT Model 174 microspectrophotometer. Chemiluminescence, originating in the gas phase above the Si(100) samples, was monitored through a 1-in. diameter sapphire window in the reaction-cell body. A cooled photomultiplier tube (RCA C31034) equipped with a Corning CS 2-61 long-pass red filter was used to measure the emission intensity. Spectra were obtained by using the same photomultiplier tube and a 0.3-mscanning monochromator (Heath Model EU-700). Optical collection efficiency was improved by using a pair of fused quartz lenses (f/1.7) to collimate the emission and focus it onto the entrance slits (2 mm) of the monochromator. A 450-Hz chopper between the two lenses and synchronous detection with a lock-in amplifier (Ithaco Model 39730) were employed to minimize interference from background radiation. Because of the extremely weak emission levels, high pressures (5-15 torr) and elevated temperatures (>373 K) were required to obtain useful spectra. The spectra were digitized and filtered by using a statistical procedure described by Cleveland.6 Atomic fluorine, generated in a radio-frequency discharge upstream of the reaction cell,3was used to produce a reference spectrum48for comparison with the chemiluminescence during F2 etching. Fluorine atoms were also used in selected experiments to clean the silicon sample surface and thereby test for possible effects of surface contamination.
Results and Discussion Chemiluminescence Spectra. Figure 1 shows spectra (uncorrected for spectrometer response) of the chemiluminescence emanating from the gas phase above an unmasked silicon sample during etching with F atoms and F, at 473 K. Since emission levels are extremely low with the Fzetchant (a factor of 10-3-104 of that observed with F atoms), a higher pressure (10.8 torr) was necessary to increase the intensity to a level permitting detection after dispersion.
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(6) W. S. Cleveland, J. Am. Stat. Assoc., 74, 829 (1979).
0 1981 American Chemical Society
3530
The Journal of Physical Chemistty, Vol. 85, No. 23, 1981
Mucha et al.
I
(1OOOX) showed that surface pitting increases markedly with etch temperature, exposure time, and F2 pressure, with pit dimensions becoming as large as 10 pm at 500 torr and 100 “C. The 25-pm diameter stylus, which probes the vertical profile, indicated considerable roughness but could not resolve the etch pits. This suggests that the measured etch depths may be systematically smaller than the true depth. This error can be minimized by etching for long periods of time; but, as will be seen, the etch rate is too slow to make this practical in most cases. Alternately, a plot of etch depth vs. time would reveal this type of error by curvature or an apparent induction period. However, measurements in which etch time was varied from 210 to 780 min did not display this behavior, indicating that this error is less than -30% of the observed etch depths. These experiments also demonstrated that there was no induction period due to surface contamination, which had been noted when insufficient precautions were taken when etching with atomic f l ~ o r i n e . ~ The observed etch rate for Si02was 15-25 A/min at 500 torr of F2and 373 K. Thus, an Si/Si02 etch-rate ratio of 120 is achieved with F2 as compared with a ratio of 26 obtained with fluorine atoms at this t e m p e r a t ~ r e . ~ During etch experiments, luminescent intensity was monitored continuously by using the red-pass filter and photomultipler. After an initial transient, which exhibited an exponential decay time of -3.5 min, the luminosity gradually increased to a maximum value over the next 1.5-2 h. After this increase, the intensity decayed at a rate of -6% per hour, independent of temperature. The origin of the time dependence could not be established; however, it is possible that changes in surface texture are responsible for some of these phenomena. The emission intensity also exhibited unusual behavior if pressure and flow rate were abruptly changed. When the F2 pressure alone was changed, a new equilibrium intensity level was established within a period of 1-5 min, depending on the magnitude of the pressure change. This is considerably longer than the characteristic time required for flow and pressure to stabilize (always
