IN order to characterize the

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1 D E S G N F E A T U R E VERFY WAFER-PROBE REFERENCE PLANES FOR MMC TESTNG Three techniques for verification of planar calibration standards agree within ko.1 ps of electrical length. N order to characterize the accuracy of on-wafer MMC measurements, it is important to understand microwave wafer-probe calibrations over a matrix of planartransmission-line types, sizes, and substrate materials. One of the calibration issues that arises is the electrical lengths of the planar calibration standards relative to each other and relative to the device under test. Here, three techniques for verification of planar calibration standards are presented and are shown to agree within ko.1 ps of electrical length. This. study is limited to the coplanar waveguide (CPW) geometry of alumina and sapphire impedance standard substrates (SSs) currently available for wafer-probe calibration. The effects of one-port impedance standards on one-port vector network analyzer (VNA) calibrations have been widely reported, along KETH JONES, Senior Microwave Engineer, and ERC STRD, President, Cascade Microtech, nc.. P.O. Box 1589, Beaverton, OR ; (503) k l.ooj Deviation from linear phase - 3* J $ f 0.20V g a f h Deviation from linear magnitude , 4.60 k hlaanitudel-., -8*D lhedltterenoeinektdcallengthbetweena wallshoftandacoptanarshottcanbe cak&tedwiththoakjofaloox4cakm&i. _-Conductor wall -i-w mg Coplanar short has extra length AT - with examples of typical errors in short-open-load calibrations. Placement of probe tips on a short calibration standard and, to a lesser extent, placement of the tips on a load standard will affect the resulting calibration. For a ground-signal-ground probe-tip configuration, the effect of variations in shorts manifests itself as a variation in electrical length. Varying the placement of the load results in a slight shift in series load inductance. To verify the reactances of the short, open, and load, this study utilized an open or shorted stub as a fourth standard. Typical deviation from linear phase and magnitude of

2 D E S G N F E A T U R E Wall short Offset length ()-pm 3. These offset short standards were used to relate the coplanar short delay to the actual shorting bar. The delays of five different offset short standards are plotted against their mechanical lengths. Step 1: Perform two.porf calibration in 35mm coax w> S, m Step 2: Measure adapter lengths -> >> 3 with shorts and opens 2 G-7 ED> Step 3: Measure length of both adapters plus through A D Z0Z through Step 4: Through length = l- S, -S, with respect to one-port standards 4. Three steps are necessary for achieving the unterminatlng technique, which measures transmission and reflection reference planes from a coaxial reference. Ps a 50-a, open-stub measurement and the reactances of the standards are shown in Fig. 1. Once the reactances of the standards are understood for a given probe placement, the probe s electrical length can be determined and the reference planes can be defined by relating electrical length to mechanical length. This was accomplished through utilization of three calibration techniques: scale modeling, short-open-load-through (SOLT), and through-reflect-line (TRL). A ZERO=LENGTH SHORT n order to determine the electrical length of a planar short standard, some assumptions must be made about what constitutes a zero- Knowing reactances of standards for probe placement, electrical length can be determined and reference planes defined. length short. TEM media with an infinite shorting wall provide a good model. For CPW, a conduction wall was used to approximate a zerolength short. The difference in electrical length between a wall short and a coplanar short can then be calculated. A loo-x-scale model was used to make the comparison (Fig. 2). A 0.4- ps delay was found for the coplanar short. This delay is equivalent to a 52-pm offset of the wall short, assuming a propagation velocity of 3. Reprnted wth permlsslon from Microwaves 8 RF - April 1988 Copyrght 1988 VNU Business Publications. nc.

3 D E S G N F E A T U R E 130 pm/ps. The next step is to relate this coplanar short delay to the actual shorting bar used on the SS (Fig. 3). These structures were realized in thin-film gold on 25-mil alumina. Probe overlap in each case is 1 mil. Linearity of the data through zero shows that the 0.4-ps delay holds for the shorting bar-probe combination. This exercise also confirms the propagation velocity at 130 pm/ps. Dispersion of the coplanar short was not observed and can be attributed to the small dimensions of the structure with respect to a wavelength. Coaxial standards, for example, may operate to within 90 percent of the frequency of the first waveguide mode. This permits significant evanescent moding to occur near the upper band edge. n contrast, the 0.4-ps delay of the coplanar short amounts to only l/50 of a wavelength at 50 GHz. TWO APPROACHES For two-port calibrations, a minimum-length coplanar through connection is added to the one-port standards. t is important that the reference planes for the reflection and transmission calibrations coincide. To verify the reference plane locations, two approaches were used: approximate unterminating and TRL calibration. Cascade Microtech has developed the approximate unterminating approach from the already established unterminating approach. n the unterminating approach (Fig. 4), a two-port coaxial calibration is used to characterize the electrical length of the probe on each port while it is terminated with a reflect standard. This length is compared with the length of both probes contacting a through standard. The two measurements then differ by the length of the through standard with respect to the reflect standard. However, imperfections in the probe generate first-order reflections that tend to obscure the delay information from the one-port measurennent. The approximate unterminating echnique was employed to improve he probe electrical length data. f.n assumption is made that the robe s output reflection coeffiient, S,,, is much less than 1, the 6. TRL calibration provides verification of the re!ference plane locations via use of these St andards. i-eflection coefficient equation may be simplified with a binomial expan- S ion (Fig. 5). The first-order reflection term is removed vectorially by S ubtracting the short from the open i-1eflection coefficients (hence the name approximate unterminatir1g"). The resulting signature corre- to the dispersion of the CPW Slponds ir1 the probe. This signature agrees very well with the transmission ps Reflect > Probe in air ( Reference plane is placed in midpoint of shorter line, then rotated out 0.5 ps each. * Know reference planes from propagation velocity (e.g., 0.5 ps X 130 pm/ps - 65 km from midpoint). tep 3: Measure and store r,,, with r = -1 (short r Ls r Adapter s 12 Determining S,, of adapter itep 1: Calibrate in media A itep 2: Measure and store r,,, with r = 1 (open; -1 = s,, t s,: s,, t s;, Gsume S,, is much less than 1) ll * =s,, ts,: s,,-s;, tep 4: Subtract short from open A-r: = 2S2: n the approximate untermmanng- tecnmque, the firstorder reflection term is removed vectortally by subtracting the short from the open reflection coefficients. surement signature, allowing accurate subtraction of electrical lengths. The measured electrical length of the through standard is within ko.1 ps of the distance between the probe tips divided by the CPW phase velocity, when a 1-mil probe overlap is used. NDEPENDENT VERFCATON TRL calibrations were performed with an automatic vector network analyzer as an independent verification of the reference plane locations. The same 1-ps through standard, a 25-ps transmission line, and an open circuit were used as standards (Fig. 6). The reflect standards are computed as part of the TRL calibration when the through standard is used to set the reference plane. The only requirement is that the reflect offsets be the same for both ports. This permits the short or open standards to be measured after calibration. The phase of the short standard measured after a TRL calibration showed error of 0.03 ps. Although the TRL technique offers the most accuracy for delay measurements, it is directly linked

4 D E S G N F E A T U R E to the characteristic impedance, Z,, of the transmission line as an impedance standard. This is an advantage for TEM media such as coaxial line since accurate air-line standards can be realized. However, coplanar transmission lines are dispersive and will degrade impedance data. The dispersive Z, makes the lumpedresistive standards on the SS appear dispersive. Coupled lines 7. n the model of the probe overlap, the section of bond pad under the probe tip can be modeled as either a section of coupled line, or as mutual inductance and mutual capacitance in a lumped model. Coupling effects of probe tip and bond pad modifies electrical length of the probe and the reference plane. w For measurements on the wafer, the TRL technique can remove the offset uncertainty of SOLT standards used in later tests. The three methods produce reference plane measurements that agree to within +O.l ps, locating the reference plane from 0 to 1 mil beyond the contact tips (see table). n comparing these methods, a &5-pm probe placement error should be added to the measured numbers. n the model of the probe overlap on a DUT s bond pads (Fig. 7), the probe tip couples to the section of the bond pad directly underneath it. This can be modeled as a section of coupled line, or mutual inductance and mutual capacitance in a lumped model. This coupling effect modifies the electrical length of the probe and the reference plane. t is important to remove the repeatability variable by precise probe placement. As probe overlap is increased on a shorting plane, the effective electrical length decreases. This effect u25.urn overlap ElOO.um overlap Model from measured data 25 ph 25 ph Delay =k = 1 ps Delay = l/2( Zo C] = ps Reference plane determination 8. The effects of overlap on twoqx~ri measurements are apparent when the degree of overlap is changed from 25 pm to 100 pm. Reference plane measured location Method Reference (in front of probe tip) Offset shorts Wall shcxt = zero length +27 ccrn Coax calibration with TEM standards set +4 pm unterminating approach reference plane TRL (LRL) rneflect 1 = rreflect2 +0 firm two-port measurements. A 1-ps through standard was measured with 1-mil overlaps, and an equiva- lent circuit was extracted. The through standard was then completely overlapped, effectively removing the series inductance and leaving only the shunt capacitance. The extracted equivalent circuit continues until an asymptote is reached at approximately -2-ps shortening, with 1-mil overlap defined as zero. Since the probe slopes away from the shorting plane, the coupling decreases to zero with further overlap, and therefore the effeet vanishes. Fie. 8 shows overlar, effects on

5 r D E S G N F E A T U R E shows no series inductance and a slight decrease in shunt capacitance caused by coupling with the probe. n FET measurement, this effect will manifest itself as variations in the parasitic gate and drain inductances, which can be extracted. The slight capacitance change is swamped by the much larger FET capacitance. Evidence of this effect can be illustrated when a FET is measured at the innermost edge of its 2-mil bond pads and corresponding equivalent circuit extraction, and then with the probes at the outermost edge of the bond pads (Fig. 9). The additional rotation in the input reflection coefficient, S,,, is apparent. Also, the equivalent circuit shows a 30-pH increase in the gate inductance, Lo, and drain inductance, LD, while the capacitances remain essentially constant. * (4 For Further Readinig ~ Eric Strid, Extract more accurate FE e uwalent cucults. MSN, Monohthic Technology Supp ement. Oct pp Eric Strid, Planar impedance standards and accuracy considerations in vector network analysis. Proceedings ofthr Automatic RF Techniques Group, 1986 A CASCADE MCROTECH 9. n comparing FET pertormclnce measured at the innermost edge of the 2.mil pads (a), and then at the outermost edge (b), the increased is apparent, and the equivalent circutt shws a 3&pH increase in gate inductance and dmin mductance, while capacitances remain constant. (b)