In this letter, we have investigated hydrogen degradation of InP HEMT's with Ti/Pt/Au gates. We have found that V/sub T/ shifts negative after exposure to hydrogen, and exhibits an L/sub G/ and orientation dependence. We postulate that /spl Delta/V/sub T/ is at least in part due to the piezoelectric effect. Hydrogen exposure leads to the formation of TiH/sub x/, producing compressive stress in the gate. This stress induces a piezoelectric charge distribution in the semiconductor that shifts the threshold voltage. We have independently confirmed TiH/sub x/ formation under our experimental conditions through Auger measurements. Separate radius-of-curvature measurements have also independently confirmed that Ti/Pt films become compressively stressed relative to their initial state after H/sub 2/ exposure

Adams, Stephen B.

Alamo, Jesús A. del

Blanchard, Roxann R.

Chao, P. C.

Cornet i Calveras, Albert

IEEE Electron Device Letters

R.R. Blanchard

J.A. del Alamo

S.B. Adams

P.C. Chao

A. Cornet

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Hydrogen-induced piezoelectric effects in InP HEMT's

Diposit Digital de la Universitat de Barcelona

IEEE ELECTRON DEVICE LETTERS, VOL. 20, NO. 8, AUGUST 1999 393Hydrogen-Induced PiezoelectricEffects in InP HEMT’sRoxann R. Blanchard, Jesu´s A. del Alamo, Stephen B. Adams, P. C. Chao, and Albert CornetAbstract—In this letter, we have investigated hydrogen degra-dation of InP HEMT’s with Ti/Pt/Au gates. We have found thatV T shifts negative after exposure to hydrogen, and exhibits anLG and orientation dependence. We postulate that V T is atleast in part due to the piezoelectric effect. Hydrogen exposureleads to the formation of TiHx, producing compressive stress inthe gate. This stress induces a piezoelectric charge distributionin the semiconductor that shifts the threshold voltage. We haveindependently confirmed TiHx formation under our experimentalconditions through Auger measurements. Separate radius-of-curvature measurements have also independently confirmed thatTi/Pt films become compressively stressed relative to their initialstate after H2 exposure.Index Terms— Hydrogen, InP HEMT’s, piezoelectric effect,stress, Ti/Pt/Au gates.I. INTRODUCTIONHYDROGEN degradation in III–V FET’s is a serious andwell documented reliability concern [1]–[4]. Exposureoccurs when hydrogen outgasses from packaging material andgets trapped inside hermetically sealed packages. Over time,hydrogen causes changes in device characteristics which canultimately lead to parametric module failures. Compared withthe more extensive studies of H degradation of GaAs MES-FET’s and PHEMT’s, only limited data on the H sensitivity ofInP HEMT’s is available. is generally reported to decreasein InP HEMT’s, although exposure times for published data areunder 20 min [2]. To our knowledge, a device-level solutionto this problem has not been reported for either InP or GaAstechnologies.While the detailed mechanism by which H affects the op-eration of III–V FET’s is not understood, previous researchershave traced the degradation to the presence of Pt in the gatestack [1]. Pt is known to be a catalyst for H , breaking it downinto 2H, which then diffuses through the gate. However, Chao[4] showed that degradation also occurred in GaAs PHEMT’sfabricated with Ti-only gates. This led them to speculate on theformation of TiH and a subsequent change in the Schottkybarrier height of the gate.Manuscript received December 24, 1998; revised April 5, 1999. This workwas supported in part by Sanders Lockheed-Martin. This work made use ofMRSEC Shared Facilities supported by the National Science Foundation underAward Number DMR-9400334.R. R. Blanchard and J. A. del Alamo are with the Massachusetts Instituteof Technology, Cambridge, MA 02139 USA.S. B. Adams and P. C. Chao are with Sanders Lockheed Martin, Nashua,NH 03060 USA.A. Cornet is with the Universitat de Barcelona, Barcelona, Spain.Publisher Item Identifier S 0741-3106(99)06461-7.In this letter, we present evidence that the formation ofTiH leads to the introduction of stress in the gate and in theunderlying semiconductor heterostructure. Since all compoundsemiconductors are piezoelectric, this stress induces a volumecharge distribution in the heterostructure [5]. This affects thethreshold voltage, and in turn the device characteristics. It islikely that the mechanism proposed here contributes to thehydrogen sensitivity of GaAs MESFET’s and PHEMT’s aswell.II. EXPERIMENTALThe InP HEMT’s used for this study were fabricated atMIT. The device heterostructure consists of semi-insulatingInP substrate, 2500 A˚ In Al As, bottom -doping, 50 A˚In Al As spacer, 200 A˚ In Ga As channel, 30 A˚In Al As spacer, top -doping, 270 A˚ In Al Asinsulator, and a 70 A˚ undoped In Ga As cap. The fabri-cation process features a selective cap recess, sidewall recessisolation, dielectric-assisted lift-off, Ni/AuGe/Ni alloyed con-tacts and ECR-enhanced, low-temperature Si N passivation.The gate metal is 250 A˚ Ti/250 A˚ Pt/3000 A˚ Au, with gatelengths of 0.6–10 m. On a (100) substrate, devices withgates oriented along the [011], [010], and [01 ] direction werecharacterized.Hydrogen exposure and characterization measurementswere made in a temperature-controlled wafer probe stationequipped with a sealed chamber allowing the introduction ofN or forming-gas (5% H in N ). All devices underwenta thermal burn-in at 230 C in N until no further changein threshold voltage, , was measured. The devices werethen annealed unbiased at 200 C for 3 h in forming-gas.For reference, selected burned-in devices were annealed inN under identical conditions. A detailed room-temperaturecharacterization was performed preanneal and post-anneal. Tomonitor degradation in the intrinsic portion of the device,was measured with V in order to samplenear the center of the gate.III. RESULTS AND DISCUSSIONUnder forming-gas, we found that shifted negative forall devices, as shown in Fig. 1. This is consistent with previousstudies on 0.1- m InP HEMT’s [2]. While the measuredshifts are small, they are statistically significant whencompared to the N control. Fig. 1 also shows thatexhibits a distinct gate length dependence.Fig. 2 shows the orientation dependence of , plottedas a function of . Devices with gates orientated along the0741–3106/99$10.00  1999 IEEEAuthorized licensed use limited to: Universitat de Barcelona. Downloaded on February 12, 2009 at 07:36 from IEEE Xplore.  Restrictions apply.394 IEEE ELECTRON DEVICE LETTERS, VOL. 20, NO. 8, AUGUST 1999Fig. 1. VT verses LG after annealing in forming-gas at 200 C for 3 h.Open symbols are control samples annealed in N2 at 200 C. Error barson control samples annealed for longer times indicate standard deviation inmeasurement over time. VT defined at ID = 4 mA/mm. Measurements atroom temperature.Fig. 2. Orientation dependence ofVT as a function of gate length, showinggreater negative VT for [011] devices. Forming-gas anneal for 3 h at200 C. Measurements at room temperature.[011] direction shifted more negative than those with the gatealong the [01 ] direction.The and orientation dependencies of are keysignatures of the piezoelectric effect. Similar dependencies re-ported for GaAs MESFET’s with stressed dielectric overlayershave also been attributed to the piezoelectric effect [6], [7].Hydrogen-induced degradation in GaAs PHEMT’s with Ti-only gates lead Chao [4] to speculate on the formation of TiHand a change in the Schottky barrier height. This explanationshould not result in an or orientation dependence. Instead,since all phases of TiH have a larger lattice constant than Ti[8], the formation of TiH produces compressive stress in thegate. This stress affects by inducing a piezoelectric chargedistribution in the semiconductor [5]. In order to rule out theSi N passivating overlayer as a source of stress, we alsoperformed forming-gas anneals on unpassivated devices. Theunpassivated devices showed nearly identical behaviorto passivated devices, showing that the passivation is not asource of stress.To examine this hypothesis further, in Fig. 3 we compare themeasured data with the predicted from gate stressof dyn/cm (compressive), following the method of[9]. The and orientation dependencies of agree wellwith calculations once we account for a rigid 8 mV offset. Thisadditional is affected by annealing under bias, and mayarise from H penetration into the semiconductor. While thisrigid shift is currently under study, we note that it is limited toFig. 3. Comparison of measured VT versus LG data (left axis) withthe VT determined from calculations (right axis). The calculated VT isdue to piezoelectric charges induced by a compressive stress of 1:5  109dyn/cm2 in the gate. In this calculation, the one-dimensional piezoelectriccharge distribution in the semiconductor at the center of the gate is calculated,following the method of [9]. The LG and orientation dependencies areconsistent with the piezoelectric effect. Devices annealed in forming-gas for3 h at 200 C. Measurements at room temperature.8 mV and does not exhibit any or orientation dependence.Therefore, unlike the piezoelectric component of whichincreases significantly for short channel devices, we do notexpect this second mechanism to contribute substantially todegradation in state-of-the-art 0.1- m HEMT’s.To confirm the formation of titanium hydride, we have per-formed Auger analysis on test samples of 250 A˚ Ti/250 A˚ Ptdeposited on Si wafers coated with LPCVD Si N . Thesesamples were annealed in either forming-gas or pure N at200 C for 1 h. The sample annealed in forming-gas showeda 1 eV shift in the low-energy (26 eV) peak of Ti, and theemergence of a second peak at 5 eV below the main peak.These changes are the characteristic signature of TiH [10].In conditions similar to ours, with a low atomic percent ofhydrogen ( 10%), the formation of hydride precipitates withan FCT structure has been reported in monocrystalline titanium[8]. These precipitates have a 15% volume increase withrespect to the Ti matrix. In our polycrystalline titanium, theprecipitates are expected to form at grain boundaries, and canbe affected by localized stress and dislocations [8]. Assuminga 15% for the TiH precipitates, with an overall Hconcentration of 5%, the predicted net volume expansion isabout 0.7%. A simple calculation of the film stress due to thisnet volume expansion predicts a film stress of aboutdyne/cm . The order of magnitude of the stress predictedby this simple model is the same as the stress used in ourcalculations of .In further support of the piezoelectric hypothesis, radius-of-curvature measurements have also independently confirmedthat Ti/Pt films undergo a volume expansion after H exposure.The test samples were identical to those used in the Augermeasurements. The measurement unit has a heated chuckand gas inlets for introducing either forming-gas or pure N .After reaching thermal equilibrium, the radius-of-curvaturewas measured in situ as a function of time. The results ofthis experiment are shown in Fig. 4. The Ti/Pt films annealedin forming-gas exhibits a volume expansion, as indicatedby the increase in wafer bowing, after only a few secondsof exposure. This volume expansion represents compressiveAuthorized licensed use limited to: Universitat de Barcelona. Downloaded on February 12, 2009 at 07:36 from IEEE Xplore.  Restrictions apply.BLANCHARD et al.: PIEZOELECTRIC EFFECTS IN InP HEMT’S 395Fig. 4. Change in wafer bow obtained from radius-of-curvature measure-ments as a function of time for Ti/Pt films deposited on 4-in silicon waferscoated with Si3N4. The increase in curvature indicates that the film undergoesvolume expansion due to H2 exposure, consistent with compressive stressrelative to its initial state. Anneals performed at 200 C. Measurements takenin situ at 200 C.stress in the film, relative to its initial state. In comparison, thewafer curvature of the N control sample remains relativelyunchanged throughout the anneal, indicating no change in thestress state of the Ti/Pt film.IV. CONCLUSIONSIn conclusion, we find that after exposure to H ,shifts negative for InP HEMT’s with Ti/Pt/Au gates. Theexhibits an and orientation dependence, and can beattributed in part to the piezoelectric effect. Stress developsin the gate due to the formation of TiH after exposure toH . This stress creates a piezoelectric charge distribution inthe semiconductor, which changes the threshold voltage. Wehave independently confirmed through Auger measurementsthat TiH is formed under our experimental conditions. In ad-dition, radius-of-curvature measurements have independentlyconfirmed that Ti/Pt films undergo volume expansion after ex-posure to H . The physical understanding obtained in this workshould be instrumental in identifying a permanent solution tothis problem.ACKNOWLEDGMENTThe device fabrication was carried out at the Microsys-tems Technology Laboratory of Massachussetts Institute ofTechnology.REFERENCES[1] W. O. Camp, Jr., R. Lasater, V. Genova, and R. Hume, “Hydrogeneffects on reliability of GaAs MMIC’s,” in IEEE GaAs IC Symp., 1989,p. 203.[2] P. C. Chao, M. Y. Kao, K. Nordheden, and A. W. Swanson, “HEMTdegradation in hydrogen gas,” IEEE Electron Device Lett., vol. 15, p.151, May 1994.[3] W. W. Hu, E. P. Parks, T. H. Yu, P. C. Chao, and A. W. Swanson,“Reliability of GaAs PHEMT under hydrogen containing atmosphere,”in IEEE GaAa IC Symp., 1994, p. 247.[4] P. C. Chao, W. Hu, H. DeOrio, A. W. Swanson, W. Hoffmann, and W.Taft, “Ti-gate metal induced PHEMT degradation in hydrogen,” IEEEElectron Device Lett., vol. 18, p. 441, Sept. 1997.[5] S. Adams, 1998, private communication.[6] C. P. Lee, R. Zucca, and B. M. Welch, “Orientation effect on planarGaAs Schottky barrier field effect transistors,” Appl. Phys. Lett., vol.37, no. 3, p. 311, 1980.[7] N. Yokoyama, H. Onodera, T. Ohnichi, and A. Shibatomi, “Orientationeffect of self-aligned source/drain planar GaAs Schottky barrier field-effect transistors,” Appl. Phys. Lett., vol. 42, no. 3, p. 270, 1983.[8] H. Numakura and M. Kiowa, “Hydride precipitation in titanium,” ActaMetall., vol. 32, no. 10, p. 1799, 1984.[9] P. M. Asbeck, C.-P. Lee, and M.-C. F. Chang, “Piezoelectric effectsin GaAs FET’s and their role in orientations-dependent device char-acteristics,” IEEE Trans. Electron Devices, vol. ED-31, p. 1377, Oct.1984.[10] P. Bracconi and R. La¨sser, “Investigation of titanium and titaniumhydride by AES and EELS,” Appl. Surf. Sci., vol. 28, p. 204, 1987.Authorized licensed use limited to: Universitat de Barcelona. Downloaded on February 12, 2009 at 07:36 from IEEE Xplore.  Restrictions apply.

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Hydrogen-induced piezoelectric effects in InP HEMT's

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