Configuring a Precision System for On-Wafer Capacitance

Transcription

1 Application Note Innovating Test Technologies Configuring a Precision System for On-Wafer Capacitance Development of new aggressively scaled MOS processes and devices depends on high accuracy CV measurements. Moore s law has led to devices with short gate lengths and critical overlap and inversion capacitance values. Ultra-thin oxides often make it necessary to conduct higher frequency CV measurements to isolate true CV gate capacitance values from the high tunneling currents. Recently, new high frequency CV instruments have been introduced; the Agilent 4294A supports higher frequency CV characterization to 110 MHz. But with typical on-wafer probing systems, capacitance measurements above MHz can be troublesome. For highly repeatable, high accuracy CV measurements and to reach the full potential of the new high frequency LCR/Z instruments, a new system approach with new accessories is needed. Cascade Microtech, the innovator and leader in high frequency probing systems, offers a very practical and complete probing solution specifically designed to support precision CV measurements to 110 MHz. CVMOS-APP-1002 Data subject to change without notice Cascade Microtech, Inc., 2430 NW 206th Avenue Beaverton, Oregon 97006, USA Toll Free: Phone: Fax: Europe: Japan: sales@cmicro.com Copyright 2002 Cascade Microtech, Inc. The Cascade Microtech logo, MicroChamber, Attoguard, and WinCal are trademarks of Cascade Microtech, Inc. All other trademarks are the property of their respective owners. All specifications subject to change. 1

2 These key system elements support advanced CV: Short-Open-Load (SOL) Standards LCR/Z meter calibration to the probe tip Aux Chuck Thermally isolated SOL standard locations Off-wafer reference capacitors quick and easy system verification SW stored off-wafer cal locations one mouse click moves to cal standard Dual-input 4 Terminal Pair coax probes eliminate phase and inductance error High Frequency Accessories up to 110 MHz Semi-automatic 300mm or 200mm wafer prober with optimized features: a. MicroChamber measurement enclosure faraday shield b. Attoguard integrated guard shield 2 Making the Connection LCR/Z Analyzer to the Wafer Dual input DCP probe supports the 4TP connection method Prober rear connection panel with triax and BNC feedthroughs BNC-to-Triaxial adapter LCR/Z Guard thru Shield removed A properly designed measurement path is critical for precision capacitance measurements that are uncompromised by probe system residuals. The following describes wafer connection methods for planar and vertical devices that have topof-wafer connection. Also a wafer chuck connection method is described for vertical devices that do not have a top-of-wafer connection available. Wafer Connection Methods For top-of-wafer connection to planar or vertical devices there are three primary connection methods: 1. BNC to SSMC Cable Four Terminal Pair (4TP) Method Cascade Microtech provides 1-meter and 2-meter BNC to SSMC coaxial cables which may be used for direct connection from the LCR/Z instrument to dualinput DCP probes. Cable diameter is small for a reduced bend radius this allows it to easily pass through the flexible membrane into the Cascade Microtech shielded MicroChamber environment. BNC connectors mate with the LCR/Z instruments BNC connection. On the probe side the SSMC connector mates with the dual-input SSMC connectors of the Cascade coaxial DCP probes. Four BNC to SSMC cables are needed for the LCR/meter connections LPOT, LCUR, HPOT and HCUR respectively. The 1-meter BNC-to-SSMC connection provides better high-frequency performance above 80 MHz. In some cases a 2-meter cable may be needed to reach the LCR/Z meter. With the 2-meter cable it may not be possible to achieve calibration above about 80 MHz. 1-or 2-meter BNC to SSMC Cable Method or SQ DCP Probe BNC to SSMC Cable can be used to connect directly with DCP dual input probes Advantages Disadvantages Simple and Direct 1 meter cable may be too Connection short to reach instrument Good HF Coax not good for DC Performance with Frequent cabling changes 1 meter cable 2. Coaxial-triaxial Four Terminal Pair Method In some cases it may be necessary to switch frequently between capacitance measurements and DC measurements to execute a test plan. Therefore it may not be convenient to use the BNC-SSMC cable method since the DC Parameter Analyzer instead requires triaxial cabling. A more convenient connection method may therefore be preferred which allows a combination of DC and CV measurements to be made without changing cabling to the wafer probes. In this case a coaxial BNC-BNC cable from the LCR/Z instrument can be connected to the precision DC triaxial cabling system at the probers rear connection panel. A special BNC-to-Triaxial adapter is used to connect to the triaxial feed thru on the panel. Standard Cascade triaxial cables and accessories are then used to complete the CV connection from the panel to the probe. The probers rear connection panel allows easy changeover from a LCR/Z meter connection to a DC Parameter Analyzer connection. A BNC-to-Triaxial adapter transfers the instrument guard to the triaxial guard (Cascade part number ). For measurements below 1 MHz, use

3 For the LCR/Z meter connection, Cascade Microtech provides a special 55-inch BNC coaxial cable which, when used with other Cascade accessories, provides 2 meters of electrical length to the wafer. This is recommended when using an Agilent 4284 with 2-meter calibration. Using Cascade Triaxial Environment To LCR/Z Meter BNC Cable Advantages Easily Switch from CV to DC Without changing cabling BNC to Triax Adapter Triax to Mini-triax Cable LC Triax- SSMC Adapter DCP Probe or Triaxial Feedthru Panel for DCM or on Prober K for EW RF Disadvantages Poor high frequency performance above 60 Mhz Agilent 42941A impedance probe For the Impedance probe (42941A) a Ground-Signal Infinity Probe is recommended 3. Impedance Probe (42941A) On high-frequency LCR/Z Analyzers like the Agilent 4294, an impedance probe is available. With the impedance probe a single-sided topside-only measurement can be made. In this case a Cascade Microtech Ground-Signal RF probe can be used. The proper pitch for pad contact would need to be specified. An SMA to SMA cable should be used for the connection between the impedance probe and the ACP probe. Special Considerations when Using a Substrate Connection via a Topside Tap When the Two-probe method is used for making measurements through a vertical wafer device from the top of the wafer down through the substrate (chuck), some special considerations must be taken into account. 1. Connect the wafer triaxial chuck force and guard together This prevents charges from accumulating between the high isolation chuck guard layers when CV bias voltage is applied. This can be implemented by connecting a BNC to triax adapter (part number SQ ) to one of the rear triaxial chuck connectors on the rear of the prober. 2. The LCR/Z stimulus side terminals (HPOT, HCUR) should be connected to the wafer substrate (probe). This keeps the measurement side probe at the wafer top and reduces measurement errors due to a number of parallel capacitive paths from the large wafer substrate (chuck) area. Chuck Connection when Topside-only probe are used Triax Coupler Force to Guard Short Wafer Force Guard to LCR/Z meter to prober BNC to triaxial adapter Recommended Triaxial Coupler for chuck connection SQ SQ Shield Layer Wafer Chuck Connection Method When making a CV measurement on a vertical device that does not have a topof-wafer tap, the wafer chuck can be used to make the backside substrate connection. In this case the LPOT, LCUR probe is used for the top-of-wafer connection. HPOT, HCUR is connected directly to the chuck via the triaxial connection at the rear of the prober. For connection to the back of the wafer via the chuck s dual triaxial inputs, the BNC to triax adapter (Cascade Part Number SQ ) should be used. 3

4 Connection when Chuck is Driven from the Prober Rear Panel BNC Coaxial Cable to LCR/Z Meter BNC to Triax Adapter Guard Removed Wafer Force Guard SQ Shield Layer Note on the Four Terminal Pair (4TP) Method The 4TP configuration reduces the effect of lead impedance and phase related error at high frequency. Both connection methods 1 and 2 above support the Four Terminal Pair 4TP connection which is recommended in the LCR/Z instrument literature for best results. With the 4TP configuration, the instruments High Side Stimulus (HCUR) and measurement (HPOT) terminals as well as the low-side current measurement (LCUR) and voltage measurement (LPOT) terminals are connected together at the probe, very close to the DUT. This eliminates unwanted residual factors associated with the measurement path DUT connection such as phase error which may be present above about 100 KHz. Probe Guard Connection Strap ( ), connects low-side guard to high-side guard DCP Probe Guard Connector The dual-input DCP probe fully supports the four terminal pair (4TP) configuration as recommended above. Further, the Cascade DCP probe has a shield (guard) which is also connected very close to the device to eliminate stray capacitance paths. A probe guard connection strap (Cascade part number ) can be used to connect the guards of the high side probe to the guards of the low side. This assures a complete circuit between the measurement paths inner conductor and the outer conductor (guard). Without the strap inaccurate measurements may result or an unbalanced condition can occur at high frequency. Always remove the connection strap when making DC measurements. Note if the BNC-to-triax adapter (Cascade part number ) is used at the rear panel, the connection strap is not needed. Calibration G-S ISS. ( ) When probing on-wafer using extension cables, LCR/Z meter compensation and calibration play a key role in reducing system related errors. Calibration eliminates parasitics related to system cabling and probes, reducing the effects of error sources between the on-wafer device and the instrument s calibration plane it defines the calibration plane at the wafer probe tips. For low-frequency calibration (<1 MHz) Open-Short only calibration may be adequate. For high-frequency autobalancing bridge-based (ABB) systems, like the Agilent 4294 with 50 Ω output impedance, rely on 50 Ω load calibration above 5 MHz. Cascade Microtech calibration standards provide precision 50 Ω calibration standards accurate to within +/-0.3%. Phase Compensation Any time a setup, cabling or probe are changed, an instrument phase compensation should be performed. Phase compensation assures that the instruments measurements side null-loop circuit is stable over frequency with all extension cables and probes in place. SOL Calibration A Short-Open-Load (SOL) calibration should be performed following phase compensation, when a new measurement sequence is being initialized or any time probe spacing is changed. 4

5 Thermally isolated, auxiliary cal substrate location Cascade supports the SOL calibration in several ways: 1. Impedance Standard Substrate (ISS) Cascade offers Impedance Standard Substrates which have Shorts and precision 50 Ω +/-.3% Loads. OPEN calibration can be performed with probes in the air. The ISS recommended for the DCP style probe is Cascade part number Off-wafer 60 ff reference capacitors In addition to calibration structures, the off-wafer impedance standards have reference capacitors which can be used to quickly verify system operation and to check for drift. 3. Thermally Isolated Auxiliary Chucks Cascade provides convenient off-wafer cal standard mounting locations. The off-wafer location supports the standards without exposure to the heating and cooling of a wafer thermal chuck. This assures consistent calibration reference values without thermally related drift in the standard values. 4. Software stored cal locations By storing SOL cal locations, the system software allows quick probe movement to ISS standards when a re-calibration is needed. Stored locations also eliminate operator probe placement uncertainty which results in more consistent calibrations. SOL calibrations using the Calibration Substrate ( ) Recommended Procedure 1. Place the ISS on the auxiliary chuck Place the ISS in the orientation that best suits the wafer probe orientation. For example, if the probes are oriented East- West, you may want to mount the ISS North-South (long end vertical). Remember to turn on the ISS vacuum to secure the ISS in place. 2. Locate and Store Cal Locations For convenience and quick re-calibration, the Short-Load Standard positions as well as an Open can be named and saved using the prober software. A. With wafer probes set at device measurement separation distance, orient the ISS in X, Y, and Z so that the probe contacts the Short location. NOTE: It is important that SOL calibration be performed with the probe tips spaced and located as they will be for probing the target device. B. In the prober s Nucleus Software, open the motion control window. C.Right-click the mouse on the lowest stored location button on the motion control window. D.Click Change Settings and enable x, y, z. E. Click again on this location and click on Set To Current Position. F. Click one more time and label this location Short. G.Repeat the above procedure to store Load and Open locations. You can now perform an SOL calibration using stored cal locations. Reference Capacitors Next to the ISS Short and Load standards are reference capacitors of a nominal value of about 60 ff. Also on the bottom of the ISS are capacitors of about 440 and 860 ff nominal values. You can use the setup procedure in the previous section to store the 60 ff reference capacitors locations on the ISS. The ISS reference capacitors are a convenient way to ensure basic system integrity and to detect system degradation or loss of calibration. These off-wafer capacitors on the thermally isolated aux chucks do not exhibit the same drift as on-wafer reference devices. For the best possible capacitance measurements, the following prober features are recommended: Nucleus Prober Control software allows you to create buttons for Cal and Reference Capacitor Locations 60 ff 440 ff 860 ff 5

6 MicroChamber measurement enclosure faraday shield This localized measurement enclosure and faraday shield surrounds the wafer and protects the probes and DUT from external influences like electrostatic and electromagnetic interference. Unlike dark box enclosures, it allows for convenient microscope viewing and probe adjustment without having to open the enclosure. Available on both 200mm and 300mm Cascade probers, it also provides a dark, compact, fastpurging measurement environment needed for CV measurements. Attoguard integrated guard shield By extending the autobalancing bridgebased guard over much of the wafer top, the patented Attoguard reduces position dependent capacitance variation, providing more accurate capacitance measurements. Low-Noise Thermal Systems It is often necessary to make capacitance measurements at hot or cold water temperatures. However, conventional thermal chucks add unwanted electrical noise which limits measurement integrity. With designed-in noise suppression features, Cascade Microtech s guarded thermal chuck systems assure low noise injection. Attoguard reduces capacitance variation with chuck movement to < 2 ff Summary Repeatable, high-accuracy CV measurements depend on appropriate selection and configuration of the entire on-wafer measurement system; probes, cables, and wafer prober. To reach the full potential of the high-frequency LCR/Z instruments, more methodical calibration is necessary. A wisely chosen and properly applied on-wafer measurement system will support both today s and future semiconductor process generations. Cascade Microtech, Inc., 2430 N.W. 206th Ave., Beaverton, OR Toll Free: Phone: Fax: Europe: Japan: sales@cmicro.com 6 Copyright 2002 Cascade Microtech, Inc. All rights reserved. No part of this manual may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from Cascade Microtech, Inc. CVMOS-APP-1102 Data subject to change without notice