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IMPROVE ACCURACY OF ON-WAFER TESTS VIA LRM. CALIBRATION The line-reflect-match (LRM) calibration method combines the advantages of other techniques. T WO techniques are commonly utilized to calibrate network analyzers and wafer-probing systems. The short-open-load-through (SOLT) method requires three impedance standards and a throughline. The through-reflect-line (TRL) technique requires only a throughline and one or more offset-length transmission lines as references. A new calibration technique, the line-reflect-match (LRM) method, combines some of the advantages of SOLT and TRL calibration without their disadvantages. Each type of calibration determines a twelveterm error model that quantifies systematic errors such as signal leakage, impedance mismatches, and frequency response. However, the calibration standards used. by any method determine its relative STEPHEN LAUTZENHISER. Product Marketing Engineer, Hewlett-Packard Co., Santa Rosa, CA, (707) 577-3140; and ANDREW DAVIDSON, Develop ment Engineer, and KEITH JONES, Product Development Manager. Cascade Microtech, Inc., Beaverton, OR, (503) 601-1000. Reprinted with permission from MICROWAVES & RF - January 1990 Copyright 1990 Penton Publishing, Inc. D E stations. A variety of impedance and transmission-line standards can be used to calibrate on-wafer measurement systems. These electrical references are typically fabricated on a substrate employed specifically for calibration. Because the SOLT and LRM techniques require only one transmission-line standard, they can be used with fixed probes. The TRL method requires transmission-line standards of different lengths and cannot be used with fixed probes. Calibration with the SOLT method requires four known references, all of which must be accurately modeled. Open standards are usually provided by open-circuited probes. However, open coplanar probes exhibit capacitance that must be empirically determined for each probe tip.’ On-wafer Short references have an inductance that must also be measured. As frequency increases, accurate modeling of Short and Open standards becomes more complex due to the increasing effects of parasitic capacitances and inductances. Broadband loads and transmission lines are usually less difficult to model. The TRL technique requires only a through-line (Through) and a transmission-line offset (Line) as references. The TRL method also employs highly reflecting impedances (Reflects) as standards, but their exact electrical characteristics are not required.’ However, Reflect standards must be accurately repeated at each test port. A limitation of the TRL technique is the limited bandwidth of Line standards. Most Line standards can only be used over an 8:l frequency range. For broadband measurements, several Line standards may be required. At low frequencies, Line standards can become inconveniently long. The SOLT calibration technique is often preferred when S-parameters are measured with respect to an ideal characteristic impedance such as S I G N F E A T able for obtaining S-parameters with respect to the impedance of onwafer transmission lines. When accurate on-wafer Line standards are available, the TRL method usually offers better accuracy than the SOLT technique. The LRM calibration method is similar to the TRL technique.3 However, a single transmission line is used as the Through standard, and broadband Loads provide impedance references. The LRM technique takes advantage of the highquality coplanar transmission lines and loads that can be fabricated on many microwave substrates.? By using a pair of coplanar Loads instead of offset transmission lines, the LRM method avoids the low-frequency limitations of the TRL technique. Additionally, the LRM method can be used with fixed probes, and accurate Short and Open refer- U R E ences are not required. To compare the accuracy of the SOLT, TRL, and LRM calibration techniques, each method was used to calibrate a Cascade Microtech probe station and an HP 8510B network analyzer from Hewlett-Packard Co. The measurement system was calibrated from 45 MHz to 40 GHz using a coplanar. impedance standard substrate. COMPARING ACCURACY For the ‘SOLT calibration, ‘Load, Short, and Through standards were provided by an impedance standard substrate. Open-circuited probe tips served as Open. references. The same transmission line used for the Through standard in the SOLT calibration was employed as the Line standard in the LRM calibration. Additionally, the LRM calibration utilized the same loads as those used for the SOLT calibration. The TRL calibration employed two Line standards with delays of 8 and 40 ps to cover 1.5 to 40 GHz. For measurements at lower frequencies, SOLT calibration data was used. The Open standard in the SOLT calibration was modeled with a capacitance of 15 fF, while the Short was assumed to have zero inductance. The SOLT Load had 7-pH parasitic inductance and the Through had l-ps delay. Measurements of a 15-ps transmission line indicated the relative accuracy of the calibration. techniques (Fig. 1). Results from-the LRM and TRL calibrations are similar, but data obtained using the SOLT method has relatively poor accuracy.’ The errors resulting from the SOLT calibration are mainly due to inaccurate definitions of the Open and Short standards. The same transmission line was measured as an open as skin-effect and radiation losses increase. Both the using the TRL calibration technique are low by about 0.1 dB at 2 GHz. This is probably the result of losses in the Line standard. The TRL method assumes that the characteristic impedance of Line standards are real values (resistive). However, signal loss produces a complex value for the characteristic impedance of a transmission line. Dispersion also affects calibration accuracy. To illustrate this, a 50-R resistor was measured (Fig. 3). Although an on-wafer resistor is essentially a series resistor-inductor circuit at low frequencies,” the data obtained using TRL calibration does not follow such a model. Dispersion and losses in the Line standards can 6. The overall quality of an LRM calibration Is illustrated by measuring an open stub and displaying the results on a Smith chart. n to verify that the correct amount of overlap is used standard. Excessive overlap on the Load standard durthe Open standard. Too little overlap has the opposite effect. Using probes with pitch, overlap during calibration was varied from an ideal value of 25 Measurements of the Open standard indicate the effects of varying overlap during calibration (Fig. 4). When a 40-ps offset Open was measured, calibration errors were more pronounced (Fig. 5). The overall quality of an LRM calibration is illustrated by a broadband measurement of a 15-ps open stub (Fig. 6). A smooth spiral indicates an accurate calibration. The accuracy of the LRM calibration method demonstrates its superiority over the SOLT and TRL techniques. Requiring fewer standards and offering wider frequency coverage, LRM will likely become the preferred method of calibrating on-wafer microwave measurements. Cascade Microtech, Inc., 206 NW 206th Avenue, Beaverton, Oregon 97006, USA Tel: (503) 601-1000 Fax: (503) 601-1002 E-mail: [email protected] Japan: (03) 5478-6100; Europe: +44 (0) 1295-812828 WWW site: www.cascademicrotech.com E-mail: [email protected] IMPLEMENTING THE LINE-REFLECT-MATCH (LRM) CALIBRATION TECHNIQUE FOR ON-WAFER MEASUREMENTS This supplement details how to define each of the LRM standards within the HP 8510B TRL feature set. It also offers some practical guidelines for making the highest quality on-wafer calibrations. DEFINING THE LRM CALIBRATION STANDARDS The LRM calibration is implemented by changing the definitions of the HP 85108 TRL standards. Before calibrating, the calibration standards must be manufactured and then be defined in a calibration kit. An HP 8510B and good onwafer through-lines, shorts, and fixed loads amount to all the hardware required to make LRM possible. The Standard Definitions Table and the Standard Class Assignments Table below show how each LRM standard is defined within and assigned to its corresponding TRL class. The LRM tine, LRM Reflect, and LRM Match standards are respectively assigned to the TRL Thru, TRL Reflect, and TRL Line standard classes. The standard number of the LRM Match must, for example, be assigned to the TRL tine class of standards. The standard numbers selected are only one example; other numbers may be used. Cascade Microtech’s latest Impedance Standard Substrate (P/N 101-190) and accompanying cal kit tape, probe 1.2 (P/N 019-010), are specifically arranged to help users perform LRM calibrations quickly and accurately. Quick contact of the tine, Reflect, and Match standards is possible since each set of standards are equidistant. These ground-signal-ground coplanar standards are defined collectively as cal kit PROBE.LRM and are specified below in Table 1 and Table 2. Why a Load can be defined in the TRL Line class The zero-length definition of the LRM tine standard allows the substitution of fixed loads as transmission lines in the TRL Line class of standards. In measuring the Match as a TRL tine standard, the HP 8510B measures all 4 S-parameters of probes terminated in loads. However, only the Match’s reflection parameters are used in the error correction algorithm. The Match’s transmission parameters are ignored since all the required transmission information has already been taken in measuring a zero length LRM line. Impedance Reference The measured impedance of the load at each probe tip establishes the reference impedance. If the load’s impedance is not 50 ohms, another value may be specified when defining it in the cal kit. If the Match’s defined impedance is 50 ohms in a 50 ohm environment, then the Match is as- COMMENTS: LRM LINE The LRM tine must currently be defined to have zero electrical length even though the Line must have some practical length for probe contact. The 1 picosecond coplanar line on Cascade’s ISS is the recommended choice for the LRM tine standard. Defining the LRM tine standard to have zero offset length sets the reference plane at the center of the tine after calibration. However, the reference plane is usually desired at the probe tips. The reference plane can be set at the probe tips by specifying, in the HP 8510B Port Extensions feature, a negative test port delay equal to half the Line standard’s delay. Using Port Extensions in this manner subtracts only the Line standard’s phase and not its loss; however, the loss of coplanar lines as short as those recommended here 1 ps.) is typically negligible. LRM REFLECT The LRM Reflect standard has exactly the same require ments as the TRL Reflect. Like TRL, LRM only requires that the reflection at each probe tip be the same. No modification of the TRL Reflect standard definition needs to be performed before it can be used as an LRM Reflect. On-wafer opens and shorts serve as good Reflect standards since they are highly reflective. The simplest Reflect is an open created by lifting each probe tip at least 10 mils above the wafer. This reflect eliminates probe placement errors since the probes do not contact the substrate. However, measuring the same on-wafer short at each probe tip offers the most identical reflection provided each probe tip’s overtravel is identical. LRM MATCH The LRM Match standard is a pair of on-wafer loads de fined to have a fixed impedance. imperfect reflection is measured as part of the calibration’s raw directivity. Pair of Loads vs Single Load Either a pair of loads or a single load may be used as the LRM Match. LRM assumed the reflection of the Match standard at each probe tip is identical. Since most on-wafer loads are nearly identical, LRM calibrations using a pair of loads are highly accurate. When using a pair of loads, simultaneous probe contact to each load must be made before the HP 8510 measures each load. Probes fixed in position by probe cards must use a pair of loads. Probes mounted on independent arms can use a single, high-quality load to attain an even more accurate calibration. If the probe overlap is the same for both connections, a single load will provide a more identical match at each probe tip. However, using a single load in this manner is less convenient since a single load requires manual probe switching during its measurement. To measure a single load at both probe tips, (1) contact port 1 probe tip to the load, and make Averaging the active function by pressing its softkey, (2) press the softkey labeled “Line: LRM Match” at the TRL 2-port Cal Menu, (3) The HP 8510 will begin measuring the load contacted to the Port 1 probe as a normal TRL Line standard. Averaging Factor to a high number (eg. 4000) to slow down the sweep. (5) lift port 1 probe up and contact the port 2 probe on the (6) Reset the Averaging Factor to its original value, and let SUMMARY The tables and comments above provide enough information to implement an LRM calibration using the HP 8510B and on-wafer calibration standards such as those on Cascade Microtech’s latest ISS.