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2003 International Conference on Characterization and Metrology for ULSI Technology CHALLENGES OF ELECTRICAL MEASUREMENTS OF ADVANCED GATE DIELECTRICS IN METALOXIDE-SEMICONDUCTOR DEVICES Eric M. Vogel¶ and George A. Brown* ¶National Institute of Standards and Technology, Semiconductor Electronics Division *International SEMATECH March 27, 2003 Material in this presentation is Non-Confidential Overview • Introduction • C-V Measurement Challenges • EOT Extraction Challenges • Interface State Density Measurements • Extrinsic Defect Density • Reliability Evaluation Challenges • Summary 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 2 Introduction • Lots of effects and unknowns, only four terminals! (two on capacitors!) • We will need many well-chosen measurements to extract our required parameters. 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 3 C-V Measurement Challenges A Primer of Capacitive Impedance Measurements • Capacitance measurements with an LCR meter provide two output parameters, an energy-storage component (L or C) and an energy loss component (R or G). • These are expressed in terms of either a series or parallel equivalent circuit. Series Circuit • They are truly equivalent: Rs Cs Parallel Circuit Cs = Cp * (1 + D2) D = ωRsCs = 1/ωRpCp = 1/Q Rp Cp • D is called the Dissipation Factor • Q is called the Quality Factor • They express the magnitude and phase of the sample’s response to the applied signal. 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 4 C-V Measurement Challenges ωt = δ Applied voltage Θ applied voltage pure capacitor response lossy capacitor response pure conductance response 1 0.8 Cp δ 0.6 Θ 0.4 Time Æ 0.2 0 t=0 Gp -0.2 0 1 2 3 4 5 6 -0.4 -0.6 -0.8 -1 Gp Cp Y = Gp + jωCp D = Dissipation Factor = tan δ • Ideal resistors have a response exactly in phase with the applied voltage (like the green curve). • Ideal capacitors have a response completely out of phase with the applied voltage (like the pink curve). • Lossy capacitors, like ultra-thin oxides or some high-k dielectrics, are somewhere in between (like the blue curve). 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 5 C-V Measurement Challenges Measurement Conditions and Errors • Applied ac voltages should be close to the thermal voltage (25 mV at room temperature) to maintain the small-signal approximation. • Open and short-circuit (and perhaps calibrated load) zeroing must comprehend test fixture and cabling. Probe station resonance effects may become important at higher frequencies. • Light and ambient shielding will be important for some measurements. • Even with great care taken, errors are an inherent part of all measurements. 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 6 C-V Measurement Challenges Measurement Conditions and Errors • A calculation of relative measurement accuracy of the widely used Agilent 4284A LCR meter has been done. It depends upon the measurement frequency and the nominal capacitance and conductance of the sample under test. • The relative capacitance measurement accuracy of an LCR meter decreases with: → decreasing frequency → increasing conductance → decreasing capacitance • For some conditions, errors can truly dominate the measurement! Capacitance Error (+/- %) 105 -11 -3 C = 10 F, G = 10 S -6 -11 C = 10 F, G = 10 S -10 -6 C = 10 F, G = 10 S 104 103 102 101 100 10-1 10-2 102 103 104 105 106 Frequency (Hz) 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 7 Relationship of LCR Meter Output to Sample Parameters • In general, we can’t count on LCR meter output parameters to relate directly to our sample parameters. • Only in the simplified case shown here of a high-qualityfactor, low leakage insulator on a resistive substrate can we hope to identify Cs with Coxide, Rs with Rsubstrate in a series-mode measurement. gate Cs substrate oxide Rs 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 8 Evolution of Sample Equivalent Circuits • For some samples, capacitive and resistive impedances of the insulator may be comparable, but still greater than the series resistance of the substrate or connections. Here, the parallel equivalent circuit output mode will provide a valid description of the sample. • Eventually, however, all three may become of comparable size, and none can be ignored. We then need a 3element equivalent circuit. Gp Cp Ci Ri Rs 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 9 Shortcomings of a Two-Element Equivalent Circuit • Use of the threeelement model will sometimes yield a valid model of the structure. Specific Capacitance [F/cm2] • Capacitance roll-over is a common consequence of using the two-element Cp meter output to represent a conductive dielectric on a resistive substrate. 3.5E-06 f = 100 kHz 3.0E-06 A=5.0E-5 cm^2 inv=>acc A=5.0E-5 cm^2 acc=>inv A = 1.0E-4 cm^2 2.5E-06 2.0E-06 1.5E-06 1.0E-06 5.0E-07 0.0E+00 -2.5 Go -2 -1.5 Co -1 Cp -0.5 Voltage [V] ~ ~ 0 0.5 1 Gs2 (Gs + Go)2 1.5 * Co Gs 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 10 Shortcomings of a Three-Element Equivalent Circuit 2.0E-11 Area = 4.0x10-6 cm2 f = 100 kHz Go Co Rs Ls Capacitance [F] 1.0E-11 0.0E+00 -1.0E-11 -2.0E-11 -3.0E-11 -4.0E-11 -2.5 -2 -1.5 -1 -0.5 Voltage [V] 0 0.5 1 • A series inductance in addition to the three-element circuit is necessary to model negative capacitance or capacitance rising above Co. • The source of the inductance is not clear; it could be a measurement effect or physical phenomenon. 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 11 A Distributed Transistor Channel Equivalent Circuit • This near-ultimate transmission line equivalent circuit was developed by Ahmed, et al.[1] It was used to show that channel lengths less than about 10 µm are needed to assure proper response of devices with 2 nm gate oxides. 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 12 Modeling the C-V Measurements with an Equivalent Circuit • Since a single capacitance measurement yields only two parameters, the real and imaginary parts of the impedance, we will need multiple measurements to solve for the 3, 4, or more elements in our equivalent circuits. Several approaches have been suggested: • An iterative technique to isolate and evaluate series components [2] • Use of two measurement frequencies to give four parameters • For the three-element equivalent circuit [3] • for the four-element circuit [4] • Evaluation of transmission line parameters for the distributed equivalent circuit [1] 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 13 Determination of EOT and CET • Once an accurate capacitance value representing the dielectric is obtained from the modeling above, values of EOT and CET may be determined. • Definitions: • EOT (Equivalent Oxide Thickness): The thickness of an SiO2 film having the same specific capacitance as the dielectric film in question, without any effects from quantum confinement or polysilicon depletion in its electrodes. • CET (Capacitance Equivalent Thickness): This thickness determined by computing εSiO2*Area/Cmeas where Cmeas is the measured capacitance in inversion or accumulation at some defined voltage. 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 14 Significance of EOT, CET EOT: Because of its independence of device structural effects, this is basically a materials parameter. It is normally evaluated with the device biased in accumulation, where errors involved in its calculation are expected to be minimized. CET: The simplicity of its calculation suggests that all device-related shortcomings are included in this parameter. It does, however, govern device operating parameters such as drive current. Thus it is usually evaluated in inversion, the operating mode of the device, yielding the parameter CET(inv). 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 15 EOT Determination: A Choice of Simulators 1.50 1.25 1.00 2 C (µF/cm ) • Simulators show a difference of up to 20% in the calculated accumulation capacitance. • This will lead to significant differences in EOT from the same C-V data. • Possible reasons include the use of approximations for quantum effects vs. Schrödinger equation, wave function boundary conditions, and type of carrier statistics. • It is wise to choose one simulator, and always specify it when quoting values. 0.75 0.50 UTQuant [30] NIST Schred [32] NCSU [25] NEMO [29] Berkeley [31] Tox = 2.0 nm 18 Nsub = 10 cm -3 0.25 Npoly = 1020 cm-3 n-channel, n-poly gate 0.00 -3 -2 -1 0 1 2 3 Vg (V) 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 16 Interface State Density, Dit Measurement of interface state density has been one of the greatest challenges in advanced dielectric characterization. This has been because most standard techniques have significant shortcomings for these structures. The standard techniques are 1. Terman technique: based on high-frequency C-V stretch-out. Low sensitivity for thin films, high frequency assumption often not met. 2. High-low frequency C-V: Quasi-static (low frequency) C-V not usually measurable because of high dielectric leakage. 3. AC conductance-frequency: Very sensitive but complicated technique; also leakage-sensitive. 4. Charge pumping: Requires transistor structures, also leakage sensitive, but corrections are possible. 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 17 Charge Pumping Measurement 0.1 V Vgbl Va tr tf • Base-level charge pumping measurement: - fixed amplitude Va, base voltage stepped in 0.1V increments - rise and fall times tr and tf = 100 ns - traps fill from S/D during tr, empty into substrate during tf • Charge pumping current is given by I cp = fqAG Dit ∆E f= freq, AG= channel area, Dit= average Dit over ∆E (Ef,inv –Ef,acc) 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 18 Leakage Correction: Key to D Measurement it 2 nm SiO gate 2 6.E-10 • 1MHz 5.E-10 – “Looks” like DC relative to 1 MHz, 500 kHz, or 100 kHz data 100kHz Icp Icp [A] 4.E-10 Measure Icp at 1 kHz 1kHz 3.E-10 2.E-10 1.E-10 0.E+00 -1.8 -1.6 -1.4 -1.2 -1 -0.8 -0.6 Vgbl [V] 2.0E-10 1.5E-10 1MHz-1kHz 1.0E-10 • 100kHz-1kHz 5.0E-11 0.0E+00 -1.8 -1.6 -1.4 -1.2 Vgbl [V] -1 -0.8 -0.6 3.0E+10 2.5E+10 Dit [cm -2 eV -1 ] Subtract 1 kHz Icp data from high frequency Icp data – Effectively removes DC leakage current 2.5E-10 -2 Dit • 3.0E-10 Icp [A] Corrected Icp -2 Use small transistors to reduce leakage current relative to the charge pumping current – Example: W/L = 10/1 µm 2.0E+10 1.5E+10 Note frequency independence of Dit in the 100 kHz-1MHz range. Dit 1MHz - 1kHz 1.0E+10 Dit 100 kHz - 1 kHz 5.0E+09 0.0E+00 -2 -1.8 -1.6 -1.4 -1.2 Vgbl [V] -1 -0.8 -0.6 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 19 Charge Pumping in High-k Devices Fixed Amplitude-Variable Base Measurement nFET W/L=10/1µm 10 2 Nit [10 /cycle*cm ] TH75-2072317 #08 2.4 2.2 V = 1.2V peak 2.0 10 kHz 1.8 1.6 100 kHz 1.4 1.2 1.0 0.8 1 MHz 0.6 0.4 0.2 0.0 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 • Frequency dependence of interface state density is a characteristic of many high-k gate stacks. • It may be interpreted in terms of depth-related time constant variation [5], associated with bulk of interfacial traps. 0.2 Vbase [V] 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 20 Extrinsic Defect Density in High-k Films • Extrinsic defect distributions have been driven to relatively low levels in conventional SiO2 gate dielectrics. •High-k films, because of their structure and deposition conditions, may not have the same low defect densities we have become accustomed to in conventional devices. • In thicker SiO2 films, extrinsic defects are sensed by low oxide breakdown voltage, or early failure on constantcurrent/voltage stress tests. Such failures are often not easily sensed in high-k films, particularly in large-area devices needed for low defect density resolution. • It has been observed that defects in high-k films often result in a higher level of conductivity over their entire I-V characteristic. 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 21 Typical Full-Wafer CurrentVoltage Map Lot/Wafer 1081503-03 2 Area = 5E-3 cm , 52 units 1 71% YIELD 0.1 0.01 0.001 CURRENT [-A] 0.0001 1E-05 1E-06 1E-07 Yield Criterion: Ig(-2V) < -2.0E-6 A 1E-08 1E-09 1E-10 1E-11 1E-12 0 0.5 1 1.5 2 2.5 3 3.5 4 VOLTAGE [-V] • Yield criterion easily identified for a given wafer DIE 3,1 DIE 4,1 DIE 5,1 DIE 6,1 DIE 7,2 DIE 6,2 DIE 5,2 DIE 4,2 DIE 3,2 DIE 2,2 DIE 1,3 DIE 2,3 DIE 3,3 DIE 4,3 DIE 5,3 DIE 6,3 DIE 7,3 DIE 8,3 DIE 8,4 DIE 7,4 DIE 6,4 DIE 5,4 DIE 4,4 DIE 3,4 DIE 2,4 DIE 1,4 DIE 1,5 DIE 2,5 DIE 3,5 DIE 4,5 DIE 5,5 DIE 6,5 DIE 7,5 DIE 8,5 DIE 8,6 DIE 7,6 DIE 6,6 DIE 5,6 DIE 4,6 DIE 3,6 DIE 2,6 DIE 1,6 DIE 2,7 DIE 3,7 DIE 4,7 DIE 5,7 DIE 6,7 DIE 7,7 DIE 6,8 DIE 5,8 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 22 Extrinsic Defect Density Determination Defect Density Determination: Lot-wafer 1081503-03 2 Defect Density = 60 /cm 1 Wafer data Poisson Model 0.9 Yield Fraction 0.8 0.7 Poisson Model: 0.6 Yield = exp -(Area * Defect Density) • Assuming a good fit, defect density equals the inverse of the sample area for which the yield is 37%. 0.5 Measured Parameter: Gate Current at Vg=-2V 52 units/sample area 0.4 0.3 0.2 D0 = 1/A(0.37) 0.1 0 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 2 Sample Area [cm ] • Preliminary measurements show defect densities ranging from about 1 /cm2 up to as high as 2.7x104 /cm2 or higher. No clear process dependence has yet been established. 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 23 Reliability: Defect Distribution in High-k Dielectrics ∆Dit/Dit,0 10-1 100 ZrO2 Vg,stress = 2.75 V 10-2 3 CP, F = 10 Hz 4 CP, F = 10 Hz 5 CP, F = 10 Hz 6 CP, F = 10 Hz 10-3 10-4 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103 Time (s) ∆Dit/Dit,0 100 2.0 nm SiO2 Vg,stress = 4.3 V 10-1 4 10-2 CP, F = 10 Hz CP, F = 105 Hz CP, F = 106 Hz 10-3 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 Time (s) • Interface state density generation under constant voltage stress is compared for ZrO2 and SiO2 FET’s, using charge pumping. • Higher generation rates are observed in the bulk of the ZrO2 film than at its surface; SiO2 shows no frequency (or depth) dependence. • Device parameter drift (e.g. Dit) may be a greater reliability problem in high-k films than dielectric breakdown. 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 24 Summary • The low, complex impedance of advanced gate dielectrics forces us to be thorough in our understanding, measurement, and analysis of MIS C-V characteristics. • Multi-element equivalent circuits are required to describe these structures adequately, and parameter extraction requires sophisticated modeling. • Many of our conventional measurement techniques are not applicable to these structures, or detailed correction procedures must be invoked. • The different material properties of high-k dielectrics compared to SiO2 suggest that we may find new primary reliability issues, requiring development of new characterization techniques. 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 25 Acknowledgement The authors would like to thank NIST and International SEMATECH for financial support, and C. Richter, J. Suehle, D. Heh, C. Weintraub, P. Hung, C. Young, and J. Han for valuable discussions. Certain commercial equipment, instruments, or materials are identified in this paper in order to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology or SEMATECH Inc. d/b/a International SEMATECH, nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose. SEMATECH, the SEMATECH logo, International SEMATECH, and the International SEMATECH logo are registered service marks of SEMATECH, Inc. 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 26 References 1. K. Z. Ahmed, E. Ibok, G. C. F. Yeap, Q. Xiang, B. Ogle, J. J. Wortman, and J. R. Hauser, IEEE Trans. Elec. Dev 46, 1650-1655 (1999). 2. E. M. Vogel, W. K. Henson, C. A. Richter, and J. S. Suehle, IEEE Trans. Elec. Dev. 47, 601-608 (2000). 3. K. J. Yang, and C. M. Hu, IEEE Trans. Elec. Dev 46, 1500-1501 (1999). 4. H.-T. Lue, C. Y. Liu, and T. Y. Tseng, IEEE Elec. Dev. Letts. 23, 553555 (2002). 5. C. Weintraub, E. Vogel, J. R. Hauser, N. Yang, V. Misra, J. J. Wortman, J. Ganem, and P. Masson, IEEE Trans. Elec. Dev. 48, 27542762 (2001). 4/3/2003 11:40 AM j:\stndpres\template\ISMT.ppt - 27