Spring 2003 Yield Management Solutions1

Gate Dielectric Control

12

Overcoming the Gating FactorInline Characterization of Nitride Gate Dielectric Films, withPrediction of Threshold Voltage

James Chapman and Terry Letourneau, Micron Technologies

Kwame Eason, Torsten Kaack, Xiafang Zhang, and Michael Slessor, KLA-Tencor Corporation

Inline electrical characterization is well-suited for studying and monitoring nitride dielectric films without requiring fullwafer processing.

IntroductionThe semiconductor industry strongly relieson its ability to continuously scale devicefeature size to increase performance andreduce power consumption as well as cost.One of the many challenges in CMOS scaling is the continued increase in the gatedielectric capacitance per unit area. This isaccomplished by either reducing the gatedielectric thickness or increasing the gatedielectric constant (εr). Presently, the gatedielectric is silicon dioxide (SiO2), but inthe ultra-thin gate oxide regime, utilizationof pure SiO2 is increasingly difficult due tohigh gate leakage (Ig), oxide non-uniformity,surface roughness, and boron penetrationfrom the p+ polysilicon electrodes. Thenitridation of SiO2 has been successfullyshown to improved device performance andtool commercialization.1-7

A key device performance metric is thresh-old voltage matching for NMOS and PMOStransistors. The PMOS, long channel threshold voltage (long Pch Vt) is utilizedto characterize the effectiveness of the boron(B) penetration resistance of the dielectric;however, a significant drawback of transistorcharacterization is the need for costly andtime-consuming processing. This workdescribes the correlation of long Pch Vt toinline electric and optical parametersobtained from the KLA-Tencor Quantox™and UV-1280SE tools, respectively.

ExperimentIn this study, nitride oxides were produced on highquality p-types both after oxide on the plasma nitridegate oxide and after nitration on Si (100) wafers. Inlineelectrical measurements were performed using theKLA-Tencor Quantox and KLA-Tencor UV-1280SE.The measurement sequence within dielectric formationprocess is illustrated in Figure 1. The measurementprinciples of Corona-Oxide-Si (COS) technology arehighly analogous to MOS C-V.8 The Quantox system isbased on combining three non-contacting technologies:charged corona, vibrating Kelvin probe and a pulsedlight source, as shown in Figure 2. Charged corona ionsprovide biasing, and emulate the functions of the MOSelectrical contact. The Quantox EOT parameter(GateTox™) is determined from measured dielectriccapacitance. The capacitance is determined from dQ/dVin accumulation in the COS system.9 The capacitance isconverted to thickness using εr = 3.9. In an actualapplication, some second order corrections can beapplied to acquire data to account for semiconductor

M E T R O L O G Y

GateOxidation(Base OX)

1Si

SiON

Si

PolysiliconDeposition

Anneal

2 3

Figure 1. Steps in generating the nitrided oxide film. UV-1280SE mea-

surements taken at “1” and “3”, Quantox measurements taken at “3”.

Spring 2003 Yield Management Solutions 13

The experiment is designed to have thenitridation process as the major excur-sion mode for the purpose of evaluatingthis concept. The Pchannel Vt is chosenas the end of line monitor due to itssensitivity to B penetration resultingfrom the poly doping and source/drainformation steps. The long Pchanneltransistor is chosen because the Vt isprimary controlled by the gate, unlikethe short channel device, where the turnon characteristics are heavy influencedby the drain voltage (phenomena knownas drain induced barrier lowering10).This concept is illustrated in Figure 3.

The inline characterization parametersare affected by the physical thickness,composition, and quality (or leakage) of

the film. Table 1 highlights the impact of film charac-teristics that result in a positive shift in the Vt, and thecorresponding response of the inline parameters. Thephysical interpretations of these parameters are easilyrelated to physical thickness, dielectric quality (orresistance to gate leakage) and nitrogen content.

The predicted Vt is a model built on linear combinationsof inline parameters. This approach (the model) uses aorder Taylor Series expansion of the functional responsesof Vt to the inline parameters. The model is developedusing SAS JMP4 software and has the form of

Y = ax1 + bx2 + cx3 ...

where Y is Vt; a, b, c are coefficients; and x1, x2, and x3are inline parameters. The fit of the predicted data to the

band-bending. The tunneling voltage (Vtunnel) parameteris used to monitor the high-field leakage properties ofthe oxide. All dielectrics eventually reach a point where,as more and more charge is applied, the voltage acrossthe dielectric reaches the maximum sustainable volt-age, defined as Vtunnel. Vtunnel provides a good indicationof the oxide integrity and quality in a manner similarto more traditional soft-breakdown measurements.

The inline electrical measurements were done on moni-tor wafers, with one wafer per lot, five sites per wafer.The end of line electrical data comes from two to twelveprobed wafers per lot, nine sites per wafer. The lot aver-ages are used for correlation in this study.

Results and discussionThe equation for threshold voltage is provided inEquation 1.10 The major contributions to the Vt arefilm capacitance (Cox), bulk Si band bending (YB) andsubstrate doping (NA). The later two (YB and NA) arecontrolled primarily by near surface doping of channel.The inline measurements are not sensitive to variationsin surface doping of channel due to absence of any Vt

adjusted doping on inline samples.

M E T R O L O G Y

Corona Bias,Q

Kelvin Probe,VSurf

Surface Photovoltage,SPV

CO3-, H30+

+8kV

CoronaSource,

OXIDE

P SILICON

LIGHT

SPVVSurf

MechanicalOscillator

Kelvin ProbeElectronics

TransientDetection

1.Apply QCorona Bias

Measure each ∆Q

2.Measure VS (=VOX+ψ)

Probe vibration drivesAC current:

3.Stop vibration, flash light,

and measure SPV

I ≈ Cdψdt

4. RepeatI = dC

dtV

S - V

kp

Figure 2. Quantox COS measurement theory.

Poly Poly

Gate Gate

Long ChannelGate Controlled

Si Substrate Si Substrate

Short ChannelDrain Controlled

Figure 3. Schematic highlighting the requirement of monitoring long

Pchannel devices for B penetration resistance. Variations in nitridation

will impact the degree of B penetration resistance.

Vt = öms – + 2ΨB

+ √4åsiqNAΨB

Cox

Qeff

Cox[ ]

Equation 1. Textbook calculation of Vt for MOS transistor, from SZE10.

models range in total inline parameters, the least beingtwo and the most being six (i.e. the model is based ontwo to six inline parameters).

ConclusionsIn this paper, nitrided oxide films have been character-ized using inline non-contact electrical and opticalmeasurements. The correlation obtained between theEoL Long Pchannel Vt actual and predicted (based oninline parameters) has resulted in R2 > 0.97 for indi-vidually optimized models. The individually optimizedmodels incorporate ~6 inline parameters. A two para-meter model has been successfully developed for onedevice with R2 > 0.90 and adjusted R2 > 0.88. Theseresults support that the inline monitoring is sensitiveto process variations that impact end of line measure-ments, when nitridation is the primary excursion mode.The correlation obtained between Long Pchannel Vt andmodel-based Quantox and UV-1280SE measurementsdemonstrates that inline electrical characterization iswell suited for studying and monitoring nitrideddielectric films without requiring full wafer processing.

References1. S. Hattangady et al., SPIE Symp. Microelec. Manf.

(1998).2. D.T. Grider, et al. VLSI 1997, p. 47-8. (1997).3. Rodder-M, et al., IEDM 1998, p. 623-6. (1998).4. K. Eason et al., 198th ECS Toronto, p195-203 (2000).5. F. Cubaynes. IEEE ASMC 2002, TBP.6. K. Eason et al., AVS ICMI, p251-3 (2002).7. H.N. Al-Shareef, et al., 198th ECS Toronto, p210-213

(2000).8. J. Guan et al., ECS, MA 99-2 p1106 (1999).9. T. G. Miller, Semi. International, July (1995).10. S. M. Sze, Semiconductor Devices Physics and

Technology, 1985.

Spring 2003 Yield Management Solutions14

Causes for Physical Dielectric NitrogenPMOS V1 to Thickness Quality Contentincrease (↑) (↑) (↑)

GateTox (EOT) ↑ ↓-Vtunnel ↑ ↑ ↓ρox ↑ ↑ Dit ↑Reflectivity ↓ ↓

Table 1. Parameter response table variations in gate dielectric result-

ing in a PMOS Vt increase.

Model 1 2 3Optimize 1 0.988 0.795 0.839Adjust R2 0.971 0.549 0.645Optimize 2 0.952 0.986 0.976Adjust R2 0.875 0.970 0.948General Function 0.952 0.986 0.976Adjust R2 0.875 0.970 0.948

Device

Table 2. Table highlighting the optimization Vt models for dif ferent

devices. (Note that a “General Function” is a metric which has reasonably

good application to all devices). The general function has the same

parameters but dif ferent coefficients from device to device.

actual data is presented in Figure 4. The model isbased on 12 observations, which is the lower end ofestablishing a statistical population. The methodology isapplied to three devices, each with varying base oxidesand transistor process flows. A “super-set” of parameters

have been determined for application to all data sets.The criteria for successful model generation is R2 >0.9, with adjusted R2 > 0.85. Table 2 highlights theapplication of the “general” functional model to allthree data sets. The application of this model meansthat the inline parameters comprising the model areconstant; however, the coefficients in the models are dif-ferent for the various devices. The general model func-tional form is provided in Equation 2. The optimized

M E T R O L O G Y

-0.5

0.55

-0.6

-0.65

-0.7

-0.75-0.8 -0.7 -0.6 -0.5 -0.4

Vt, S

91 (

0058

Act

ual

Vt, Predicted P=0.0002RSq=0.99 RMSE=0.0104

Summary of FitRSquare 0.986438RSquare Adj 0.970163Root Mean Square Error 0.01045Mean of Response -0.61093Observations (or Sum Wgts) 12

Summary of FitSource DF Sum of Squares Mean Square F RatioModel 6 0.03971326 0.006619 60.6119Error 5 0.00054600 0.000109 Prob>FC. Total 11 0.04025926 0.0002

Actual by Predicted Plot

Figure 4. Predicted versus Actual Vt for one device. The R2 fit is > 0.98

and adjusted R2 > 0.97, with 12 observations.

Equation 2. Super-set of parameters used in optimized model. The

“General Function” uses six parameters.

VT = f(SPV,Dit,Tox,∆Ref,Vtun,ρox,Qtotal)

It’s time to alleviate the pain of gate stack control. Because with gate dielectrics

growing more complex and measuring less than 20 Å, you’re bound to experience

problems with film thickness variations, composition, capacitance and leakage. So

what’s the remedy? It’s a combination of optical metrology to control thickness

and composition, and electrical metrology to control capacitance and leakage.

And only KLA-Tencor has both. Giving you better gate stack control. While speeding

your way to higher yields at 90 nm. Now that’s relief.

Accelerating Yield

For solutions and strategies for gate stack control, please visit us atwww.kla-tencor.com/GateXpress

©2002 KLA-Tencor Corporation

Upset over 90 nm gate stack control? Take two and accelerate yield as needed.

Quantox XP Inline, independent electricalmeasurement of gate leak-age and capacitance, andcorrelation to end of linetransistor parametric tests.

SpectraFx 100 Ultraprecise, reliable opticalmonitoring of extremelythin gate dielectrics onproduct wafers.