2016
`
`J. Electrochem. Soc.: SOLID-STATE SCIENCE AND TECHNOLOGY
`
`August 1988
`
`110, 215 (1983).
`3. H. Nakane, M. Nakayama, T. Ayabe, T. Nishimura, and
`T. Kimura, in “Proceedings of Semiconductors and
`Integrated Circuits Technology,” Vol. 1, p. 571 (1978).
`4. M. K. Lee, C. Y. Lu, and C. T. Shih, This Journal, 130,
`2249 (1983).
`5. D. E. Clark, C. G. Pantano, Jr., and L. L. Hench, “Corro-
`
`sion of Glass,” Magazines for Industry, New York
`(1979).
`6. W. A. Pliskin and H. S. Lehman, This Journal, 112, 1013
`(1965).
`7. W. A. Pliskin, J. Vac. Sci. Technol., 14, 1064 (1977).
`8. R. J. H. Lin, J. C. Lee, and P. B. Zimmer, ALO-5300-T2,
`U.S. Department of Energy, Washington, DC (1979).
`
`Etching of Tungsten and TungstenSilicide Films by
`Chlorine Atoms
`
`D. S. Fischl,*:' G. W. Rodrigues,” and D. W. Hess**
`
`Department of Chemical Engineering, University of California, Berkeley, California 94720
`
`ABSTRACT
`
`Thin films of tungsten and tungsten silicide were etched both within and downstream from a Cl, plasma discharge at
`200 mtorr pressure and temperatures below 150°C. When samples were positioned downstream from the discharge, etch-
`ing proceededsolely by chemical reaction of the film with chlorine atoms. Without a discharge, molecular chlorine did not
`etch tungsten or tungstensilicide. Downstream and in-plasma tungsten etch rates were approximately equalat 110°C, but
`the chlorine atom etch rate dropped morerapidly than the in-plasmaetch rate as temperature decreased. The chemical re-
`action between chlorine atoms and the tungsten film was proportional to the gas phase Cl atom mole fraction. A pretreat-
`mentconsisting of either a dilute hydrofluoric acid dip or a short plasma etch cycle was necessary for atom etching of
`tungstensilicide films. The etch rates of tungstensilicide in Cl, plasmas were approximately an order of magnitude higher
`andless temperature sensitive than those in the downstream (atom) configuration.
`
`The patterning of tungsten and tungstensilicide for mi-
`croelectronic circuits has been accomplished by a variety
`of halogen-based dry processing etching techniques(1-8).
`Publications on the patterning of these materials using
`chlorine have only summarized theeffects that the process
`parameters have on the anisotropy,etch rate, and selectiv-
`ity (5). The limited results suggest that the etch rate of
`tungsten is related to the concentration of chlorine atoms,
`Cl, produced by either laser irradiation (8) or gas-phase
`electron impact dissociation (5) of molecular chlorine, Cly.
`However, the etching process was controlled empirically
`and thus is not based on a fundamental understanding of
`the etching mechanism.
`Control of thin film etching processes during microelec-
`tronic device manufactureis often difficult. In large part,
`the difficulties arise from the complexity of RF glow dis-
`charges, coupled with plasma-film interactions. One
`method of simplifying the etch chemistry of these pro-
`cesses is to utilize an upstream discharge to generate reac-
`tive atoms for etching. Since ion, electron, and photon
`bombardment are absent in this configuration, the pure
`atom etch chemistry can be studied. Comparison of down-
`stream etching with in-plasma or discharge etching then
`generates insight into the role of the plasma. Furthermore,
`such investigations yield fundamental information con-
`cerning atom reactions with materials. In this paper we
`present results on the chlorine atom etching (downstream
`or flowing afterglow reactor configuration) and Cl» dis-
`charge etching of tungsten and tungstensilicide thin films.
`
`Corporation 300W RF generator and matching network to
`the upstream discharge via two 1.0 cm wide copper bands
`that were placed aroundthe 3.8 cm diam quartz discharge
`tube. Spacing between the bands was 1.0 cm. A metal box
`enclosed the discharge area to ensure RF shielding and to
`facilitate forced air cooling of the tube. The 3.8 em diam
`Pyrex flow tube contained two 90° bends to prevent light
`from the upstream discharge from reaching the reactor
`and thus the photomultiplier tube. The flow tube was
`coated with Halocarbon Corporation 1200 wax to minimize
`recombination of Cl atoms. The tubes and reactor were
`connected using MDC Corporation glass to metal Kwik-
`Flange adapters.
`Samples were placed inside the 2.6 liter Pyrex reactor
`chamber on a temperature controlled 2.5 cm diam
`anodized aluminum rod that served as thelower electrode
`for plasma etching experiments. Rod temperature was var-
`ied from 25° to 150°C using heating tape and an Omega En-
`gineering Incorporated Model 650/660 controller. In order
`to reduce the surface area for atom recombination and
`thus increase the atom concentration at the sample sur-
`face, the upper electrode was removed during tungsten
`atom etching experiments.
`Molecular and atomic chlorine traveled approximately
`60 cm from the discharge section to the sample location.
`The chlorine flow rate was controlled by a needle valve
`and measured with a rotameter. Vacuum was provided by
`a liquid nitrogen cold trap and a Busch Corporation Lotos
`corrosion resistant mechanical pump. The pressure in the
`reactor (200 mtorr) was established bya throttling valve at
`the exit from the reactor and was monitored by a capaci-
`tance manometer.
`The gas phase chlorine atom concentration at the posi-
`tion of the sample was measuredbytitration with nitrosyl
`chloride (10-13). The NOC]titrant (Matheson) entered the
`reactor approximately 7 cm upstream from the samplelo-
`cation through a manifold to disperse gas evenly through-
`out the flow cross section. Flow rate was regulated by a
`needle valve and measured with a Tylan FM 360 mass flow-
`meter. Monel and Teflon were the materials used for the
`NOCI delivery system. A Hamamatsu R1928 photomul-
`tiplier tube with a Melles Griot 03FTV008 filter was used to
`monitor the 550 nm emission (chemiluminescence) result-
`*Electrochemical Society Student Member.
`**Electrochemical Society Active Member.
`ing from the recombination reaction of chlorine atoms (9,
`‘Present address: Air Products and Chemicals, Inc., Applied Re-
`13, 14) at the position of the sample.
`search and Development, Allentown, PA 18195.
`Tungsten films were prepared by sputter deposition and
`2Present address: Department of Chemical Engineering, Univer-
`had a thickness of 100 nm and resistivity of 500 »p0-cm.
`sity of Wisconsin, Madison, Wisconsin 53706.
`Pageads Abot-o7-25 to IP 10.220.39.221 address. Redistribution subject to ECS te rms of use (see ecsdl.org/site/terms_use) uRATBSUDReExdbibital.Obl
`
`
`Experimental
`The reactor used for plasma etching (in the discharge)
`has been described in an earlier publication (5). The flow
`reactor was modified for atom etching by connectinga dis-
`charge and flowtube upstream from the etching reactor. A
`schematic of the system can be found in Ref.(9).
`An upstream RF discharge was used to dissociate Cl,
`molecules into Cl atoms, which then passed through a
`flow tube into the reactor where they reacted chemically
`with tungsten or tungsten silicide samples without the in-
`fluence of the discharge. Power was supplied by a Tegal
`
`Page 1 of 4
`
`Samsung Exhibit 1011
`
`

`

`Fraction 0.0
`ChlorineAtomMole
`
`
`0
`
`100
`Molecular Chlorine Flow Rate (sccm)
`Fig. 1. Effect of Cl, flow rate on the atom yield in the reactor at 200
`mtorr total pressure and for two different powers to the upstream dis-
`charge.
`
`200
`
`Chemical vapor deposition was used to form the 250 nm
`thick tungstensilicide films which had a resistivity of 1000
`wO-cm, and an approximate Si/W ratio of 2.5. Both film
`types were deposited onto oxidized silicon wafers, which
`were then broken into samples of approximately 1 em? [de-
`tails of film preparation can be foundin Ref. (5)]. Etch rates
`were determined from the film thickness and the time re-
`quired for the film to visually clear from the oxide surface.
`Results
`Chlorine atoms.—Molecularchlorine gas, Cl., is partially
`dissociated into Cl atoms by the upstream plasma. These
`atoms thentravel through the flow tubeto the reactor with
`some of them recombining by third body(e.g., Cl. or wall)
`collisions. The concentration of gas-phase chlorine atoms
`in the reactor at the location of the sample is varied by
`changing the Cl, flow rate and the upstream discharge
`power. Titration results yield a linear relationship between
`the Cl atom concentration and the square root of the chem-
`iluminescence intensity (13-15). The Cl atom concentration
`is then easily determined by measurementoftheintensity.
`Sinceall experiments are performed at a constant pressure
`of 200 mtorr, results are presented in terms of the chlorine
`atom molefraction.
`The effect of chlorine flow rate on the atom yield in the
`reactor is shown in Fig. 1. At constant RF power,a fixed
`amount of Cl, is dissociated in the upstream discharge.
`The number of atoms that recombine while traveling to
`the reactor depends on the flow rate, pressure, and recom-
`bination kinetics. These data show that the chlorine atom
`mole fraction increases with an increasein flow rate at con-
`stant pressure. This behavior is consistent with a decrease
`in gas residencetimeat higherflow rates since shorterresi-
`dence times mean that more of the atoms produced in the
`discharge arrive at the reactor before recombining (16).
`However, the atom mole fraction levels off at higher flow
`
`200 mTorr
`
`100 secm
`
`0.6
`
`0.5
`
`0.4
`
`0.3
`
`0.2
`
`Fraction
`ChlorineAtomMole
`
`0
`
`50
`
`100
`
`150
`
`Vol. 135, No. 8
`
`TUNGSTEN AND TUNGSTEN SILICIDE FILMS
`
`2017
`
`Residence Time in Flow Tube (sec)
`0.17
`0.08
`0.06
`
`200 mTorr
`
`125 Watis
`
`75 Watts
`
`0.4
`
`0.3
`
`0.2
`
`0.1
`
`rates because, at constant power, the numberof atomspro-
`duced decreases when the residence time in the upstream
`discharge becomestoo short. Figure 1 also showsthat an
`increase in power to the upstream dischargeincreases the
`chlorine atom molefraction. This effect is due to increased
`electron impact dissociation of chlorine molecules.
`Figure 2 showstheeffect of increasing the RF powerto
`the upstream discharge at a constant flow rate of 100 secm
`Cl, and a pressure of 200 mtorr. As expected, an increase in
`power produces an increase in the Cl atom fraction. At
`100W, one-third of the molecular chlorine molecules that
`enter the system remain dissociated when theyarrive at
`the sample location. The data shown in Fig. 2 emphasize
`the fact that molecular dissociation is not linearly related
`to the applied discharge power.
`Tungsten etching.—The effect of temperature on the
`tungstenetch rate at a pressure of 200 mtorr and a total gas
`flow rate of 100 sccm is showninFig. 3. Solid symbols rep-
`resent data from plasma etching experiments (5), where
`the sample is positioned within the discharge and subjec-
`ted to ion bombardment. The open squares are results
`from the current study, where etching of the tungsten film
`is solely due to chemical reaction with chlorine atoms. For
`these data points, the gas-phase mole fraction of chlorine
`atoms, Cl, is 0.5. Without an upstream or in situ discharge,
`no reaction of tungsten occurs with chlorine molecules,
`Cls, at temperatures up to 150°C. However, it should be
`mentioned that thermal dissociation of chlorine molecules
`and subsequent reaction with tungsten occurs at surface
`temperatures above 600°C (8, 17). As the temperatureis re-
`duced, etch rates fall and the rate enhancement by ion
`bombardmentis greater than at higher temperatures. The
`atom and plasma etch rates are approximately equal at
`110°C. Obviously, in plasma etching, the etch rate is not
`critically dependent on thermal heating whichis a control-
`ling factor in atom etching. In the case of plasmaetching,
`ion bombardment from the discharge supplies energy to
`break bonds as well as heat the surface. However, experi-
`ments using thermally bonded samples have shown that
`plasma heating does not significantly affect the tungsten
`etch rate (5). If an Arrhenius temperature dependenceis
`assumed, the apparentactivation energies of the reactions
`are 0.1 and 0.3 eV/molecule for plasma etching and atom
`etching, respectively.
`It was shownin Fig. 2 that the atom concentration in the
`reactor can be varied by changing the upstream discharge
`power. This fact is used to quantitatively determinethe ef-
`fect of the gas-phase chlorine atom concentration on the
`tungsten etch rate. The data in Fig. 4 are from experiments
`at three different temperatures with the flow rate and the
`pressure constant at 100 sccm and 200 mtorr, respectively.
`At these temperatures, the etch rate varies linearly with
`the gas-phase chlorine atom mole fraction. The solid lines
`depict the bestfit for all data and are described by
`
`Temperature (C)
`84
`60
`
`112
`
`40
`
`100 seem
`
`plasma etching
`
`2 mTorr
`
`atom etching
`
`2.4
`
`2.6
`
`28
`
`3.0
`
`3.2
`
`3.4
`
`
`
`EtchRate(nm/min)
`
`Power (Watts)
`Fig. 2. Effect of the upstream poweron the atom yield in the reactor
`mtorr pressure and 100 sccm Cl, flow.
`
`1Ragen A otb-07-25 to IP 10.220.39.221 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC Licensein place (see abstract).
`
`1/Temp X 1000 (1/K)
`Fig. 3. Effect of temperature on the atom and in-discharge etch rate
`of tungsten at 200 mtorr pressure and 100 sccm Cl, flow. The chlorine
`atom mole fraction is 0.5 for the atom etching.
`
`Page 2 of 4
`
`

`

`J. Electrochem. Soc.: SOLID-STATE SCIENCE AND TECHNOLOGY
`
`August 1988
`
`40
`
`30
`
`20
`
`2018
`
`
`
`EtchRate(nm/min)
`
`10 0
`
`0.0
`
`0.6
`0.4
`0.2
`Chlorine Atom Mole Fraction
`
`0.8
`
`Fig. 4. Effect of chlorine atom mole fraction on the tungsten etch
`rate at 200 mtorr pressure, 100 sccm Cl, flow, and three different rod
`temperatures.
`
`Etch rate [nm/min] = 2.3 x 10° Xq exp (—3900/T)
`
`where Xc is the chlorine atom mole fraction in the gas-
`phase andT is the anodized aluminum rod temperature in
`Kelvin. The pre-exponential constant in the equation does
`not change with flow rate over the range of 20-150 sccm.
`This observation indicates that the reaction is not limited
`by a mass-transport boundary layer. At 200 mtorr pressure
`and Xq = 0.5, kinetic theory predicts that approximately
`10'° chiorine atomsstrike a square centimeter of surface
`per second. An etch rate of 40 nm/min requires that ap-
`proximately 10" tungsten atoms leave a square centimeter
`of surface per second. If the etch product is assumed to be
`WCL,, the reaction probability of Cl atomsis approximately
`10-*. This is the predominate product detected by ultra-
`high vacuum experiments with molecular and atomic
`chlorine beamsandis also the product predicted by evalu-
`ation of equilibrium vapor pressures of the possible
`tungsten chloride products at these temperatures(17).
`Tungsten silicide etching—Unlike tungsten, tungsten
`silicide that is exposed to air for longer than a few days
`does not etch spontaneously with chlorine atoms. How-
`ever, if the samples are first dipped in a 5% HFsolution or
`subjected to a Cl, plasma discharge they can be etched
`downstream by chlorine atoms. Apparently, ion bombard-
`ment is necessary to removethesilicon dioxide layer that
`forms on the surface of the samples during exposure to air
`at room temperature. Therefore, tungsten silicide is etched
`by chlorine atomsbyfirst subjecting the samples to a Cl,
`discharge for 30s (200 mtorr, 150 sccm total gas flow, 10W),
`which is the shortest time found to ensure complete re-
`moval of the native oxide layers. The amount of material
`removed during the 30s is calculated based on plasma
`etching experiments at the same temperature and there-
`
`Temperature (C)
`84
`
`112
`
`60
`
`plasma etching
`
`
`
`EtchRate(nm/min)
`
`
`
`EtchRate(nm/min)
`
`200 mTorr
`
`atom etching
`
`maining film thickness is used in the atom etch rate calcu-
`lation. Althouugh this procedure is not ideal, the error is
`small. Furthermore,the etch rates calculated in this man-
`ner are in agreement with the rates obtained by using an
`HF dip prior to the etch run. Because exposure to air re-
`sults in the growthof an oxidefilm on the WSi, surface, the
`upperelectrode used for the plasma etch remains in place
`during the atom experiments. This electrode offers addi-
`tional surface area for Cl atom recombination compared to
`the electrode configuration for the tungsten etching exper-
`iments. Chlorine atom concentrations are thus lower than
`observed for identical upstream discharge studies when
`etching tungsten (compare Fig. 4 and 6). The plasma pre-
`treatment is preferred over the HF dip because thefilm
`clears more uniformly and the endpoint is easier to ob-
`serve. Without a discharge, tungsten silicide did not etch
`in molecular chlorine under the conditions investigated.
`The effect of temperature on theetch rate of tungstensil-
`icide films in the discharge and by Cl atoms is shown in
`Fig. 5, where the data are obtained at 200 mtorr and 150
`sccm Cl, flow rate. The solid symbols are the plasma etch-
`ing results and the open symbols are the Cl atom etching
`results with a Cl mole fraction of 0.2. Clearly, tungsten sili-
`cide is etched by Cl atoms, but at a much slowerrate than
`that obtained in the discharge when ion bombardmentoc-
`curs. However,
`the relatively low atom concentrations
`could also be a major cause of the reduced etch rates in the
`downstream configuration.
`Similar to the tungsten etching results shownin Fig. 3,
`plasmaetch rates of tungsten silicide are less temperature
`dependent than atom etch rates. The apparent activation
`energies calculated from the data in Fig. 5 are 0.06 and 0.16
`eV/molecule for plasma etching and atom etching, respec-
`tively.
`The etch rate ofsilicon is known to be greatly enhanced
`by ion bombardment in Cl, discharges (18, 19). Further-
`more, preliminary investigations in our laboratory indicate
`that heavily doped (approximately 10° cm~*) n* polysili-
`con films are etched by Cl atoms (downstream configura-
`tion) at comparable rates to those observed for W and
`WSi,. Therefore, the large etch rate enhancement due to
`the discharge and the magnitudeof the atom etch rate sug-
`gest that the overall etching process of tungstensilicideis
`limited by the presenceof silicon in the film. Whether this
`observation is duc to volatility considerations, a reduction
`in electron concentration in the solid (resulting in lowered
`etchant adsorption), or the fact that the strong (1.8 eV)
`Si—Sibondis essentially eliminatedin the silicide filmsis
`not clear at this time.
`Thevariation of WSi, etch rate with the limited range of
`chlorine atom mole fractions achieved is shownin Fig. 6
`for 100°C, 200 mtorr, and 150 sccm. Again, the etch rate in-
`creases with an increase in the Cl-atom mole fraction. Un-
`fortunately, because of the limited range of chlorine atom
`concentrations that could be obtained with our apparatus,
`a general kinetic expression describing atom etching of
`the tungstensilicide films was not formulated.
`
`100 C
`
`0.0
`
`0.3
`
`200 mTorr
`150 seem
`
`150 scem
`
`24
`
`2.6
`
`30
`
`3.2
`
`0.2
`0.1
`2.8
`Chlorine Atom Mole Fraction
`1/Temp. X 1000 (1/K)
`Fig. 5. Effect of temperature on the atom and in-discharge etch rate
`Fig. 6. Effect of chlorine atom mole fraction on the tungsten silicide
`of tungstensilicide at 200 mtorr pressure and 150 sccm Cl, flow, The
`etch rate at 100°C temperature, 150 sccm Cl, flow, and 200 mtorr total
`pressure.
`chlorine atom mole fraction is 0.2 for the atom etching.
`PagecdobA07-25 to IP 10.220.39.221 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC Licensein place (see abstract).
`
`Page 3 of 4
`
`

`

`Vol. 135, No. 8
`
`TUNGSTEN AND TUNGSTENSILICIDE FILMS
`
`2019
`
`Conclusions
`Tungsten and tungsten silicide thin films have been
`etched both by Cl, plasma discharges and by Cl] atoms
`without the influence of the plasma. Molecular chlorine
`did not etch the films at the temperatures investigated.
`Both films could be etched by atomic chlorine, however, a
`native oxide layer present on tungsten silicide had to be re-
`moved before the atoms could attack the film. The results
`showthat plasma interactions greatly enhance the Cl atom
`etch rate of tungsten silicide. However for tungsten, the
`etching process proceeds mainly by chemical reaction of
`the film with Cl atoms and is less affected by the plasma.
`
`Acknowledgments
`The authors thank P. Marmillion at IBM Essex Junction,
`Vermontfor supplying the tungsten and tungstensilicide
`samples used in this investigation. The corrosion resistant
`pump was supplied by Busch Incorporated, Virginia
`Beach, Virginia and maintained by Semivac Corporation
`in Milpitas, California. This work was funded by Intel Cor-
`poration and the California State MICRO program.
`
`Manuscript submitted Oct. 9, 1987; revised manuscript
`received Jan. 18, 1988.
`
`REFERENCES
`1. S. P. Murarka, Solid State Technol., 181 (Sept. 1985);
`S.P. Murarka, “Silicides for VLSI Applications,”
`Academic Press, Inc., Orlando, FL (1983),
`2. Y. Pauleau, Solid State Technol., 61 (Feb., 1987); R. 5S.
`Blewer, ibid., 117 (Nov. 1986).
`
`ocfc0a
`
`3. T. P. Chow and A. J. Steckl, This Journal, 131, 2325
`(1984).
`4. Workshop on Tungsten and Other Refractory Metals
`for VLSI Applications, Continuing Education in En-
`gineering, University Extension, University of Cali-
`fornia, Berkeley, Palo Alto, CA, Nov. 12-14, 1986.
`. D. S. Fisch! and D. W. Hess, This Journal, 134, 2265
`(1987).
`. G. Koren, Appl. Phys. Lett., 47, 1012 (1985).
`. M. Rothschild, J. H. C. Sedlacek, and D. J. Ehrlich,
`ibid., 49, 1554 (1986).
`. M. Rothschild, J. H. C. Sedlacek, J. G. Black, and D. J.
`Ehrlich, J. Vac. Sci. Technol. B; 5, 414 (1987).
`. D. A. Danner and D. W. Hess, J. Appl. Phys., 59, 940
`(1986).
`10. M. A. A. Clyne and W. S$. Nip, in “Reactive Interme-
`diates in the Gas Phase,” by D. W. Setser, Editor,
`Chap. 1, Academic Press, Inc. (1979).
`11. M. A. A. Clyne, H. W. Cruse, and R. T. Watson, J. Chem.
`Soc., Faraday Trans. I, 68, 153 (1972).
`12. M. A. A. Clyne and H.W. Cruse, ibid., 68, 1281 (1972).
`13. M. A. A. Clyne and D. H. Stedman, Trans. Faraday
`Soc., 64, 1816 (1968).
`14. M. A. A. Clyne and D. H. Stedman,
`(1968).
`15. M. A. A. Clyne and D. J. Smith, J. Chem. Soc., Faraday
`Trans. 2,75, 704 (1979).
`16. C. G. Hill, “An Introduction to Chemical Engineering
`Kinetics and Reactor Design,” p. 262, John Wiley
`and Sons, Inc., New York (1977).
`17. M. Balooch, D. S. Fischl, D. R. Olander, and W. J. Siek-
`haus, This Journal, 135, 0000 (1988).
`18. S. C. McNevin and G. E. Berker, J. Vac. Sci. Technol. B,
`3, 485 (1985).
`19. R.H. Bruce, Solid State Technol.,, 64 (Oct. 1981).
`
`ibid., 64, 2698
`
`Phosphorous Vacancy Nearest Neighbor Hopping Induced
`Instabilities in InP Capacitors
`|. Experimental
`
`DepartmentofElectrical and Computer Engineering, Centerfor Advanced Materials Research, Oregon State University,
`Corvallis, Oregon 97331
`
`M. T. Juang, J. F. Wager, and J. A. Van Vechten”
`
`Variable temperature bias-stress measurements were performed on n-type InP MIS capacitors. Two distinct activa-
`tion energies at 40-50 meV and1.1-1.2 eV were obtained over a temperature range of 100-350 K. These energies are consist-
`ent with the instability mechanisms of thermionic tunneling into native oxide traps and phosphorous vacancy nearest
`neighbor hopping (PVNNBH). Theestimated fraction of shift in these particular samples due to PVNNHvaries both with
`stress time and with temperature from about 20%for short times at 300 K to about 80%for long times at 350 K.
`
`ABSTRACT
`
`The drain current of an InP metal insulator semicon-
`ductorfield effect transistor (MISFET)is often observed to
`decrease as a function of time after the application of a
`positive gate bias which induces an accumulation of elec-
`trons in the channel. Various models have been proposed
`for this drain current drift (DCD) phenomena[see Ref.(1)
`for a critical review of proposed DCD models}.
`In this study, we have employed variable temperature
`bias-stress measurements (see the section on Experimen-
`tal Procedure for a description of this technique) of InP
`MIS capacitors in order to determine the dominant DCD
`mechanisms from an analysis of the activation energy of
`the flatband shift. There are two advantages inherent in
`bias-stress measurements of MIS capacitors compared to
`DCD measurements of InP MISFET’s.First, fabrication of
`the MIS capacitor requires fewer processing steps so that
`the interface chemistry can be precisely controlled and the
`electrical instabilities may be correlated to the interface
`chemistry. A second advantage is that the flatband shift
`depends only on the carrier density in the channel while
`*Electrochemical Society Active Member.
`Inin + Vp* + 4e° = Vin” Inp™?
`{1]
`Rage dot04-07-25 to IP 10.220.39.221 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC Licensein place (see abstract).
`
`
`drain currentinstabilities depend on both the carrier den-
`sity and carrier mobility. Thus, interpretation of bias-
`stress measurements is more direct than that of DCD.
`Twodistinct activation energies at 40-50 meV and 1.1-1.2
`eV were obtained from variable temperature bias-stress
`measurements over a temperature range of 100-350 K. The
`40-50 meV activation energy dominates the flatband shift
`at low temperatures and is consistent with thermally acti-
`vated tunneling of electrons from the InP conduction
`band into a discrete trap in the native oxide. An activation
`energy of 1.2 eV was predicted (2) for phosphorous va-
`cancy nearest neighbor hopping (PVNNH) in which the
`channel electrons are captured by shallow acceptors that
`are created by the hopping of an In atom into a phos-
`phorous vacancy.
`PVNNH leads to DCD in the following manner. Con-
`sider an InP MISFETwhich has processing-induced P va-
`cancies in the channel region underits gate. Nearest neigh-
`bor hopping of an In atom into the P vacancy is described
`by the following defect reaction
`
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