SURFACE WAVE PHENOMENON IN WAFER PROBING ENVIRONMENTS EDWARD M. GODSHALK Cascade Microtech, Inc. Beaverton, OR ABSTRACT Investigation of microwave and millimeter wave propagation in dielectric slabs and along copla- nar transmission lines on dielectric slabs, reveal effects that may be explained by surface wave phenomenon. These surface waves can be transmitted and received with wafer probes and influ- ence the transmission characteristics of coplanar transmission lines. This paper presents measured data showing the presence of surface waves and how they interact with wafer probes and coplanar waveguide transmission lines. Methods for minimizing these in- teractions are explored and quantified. A discussion of surface wave effects on wafer calibrations is included I INTRODUCTION By their nature practically all wafer probing measurements involve some sort of dielectric slab (i.e. Alumina or GaAs), both in the calibration and actual measurement itself. These dielectric slabs may be viewed as open-boundary waveguides which support modes of propagation called surface waves (1). The two lowest order Transverse Electric (TE) and Transverse Magnetic (TM) modes are shown in Figure 1 for a dielectric sheet of thickness 2T. They are the TE0, TM0, TE1, and TM1 modes. The first two modes are classified as even modes due to their symmetry about the y axis, and similarly the latter two are odd modes. In all cases the field strength of the mode falls off exponentially outside the dielectric (i.e. |x|>|T|) hence the term surface waves. Note that if a metal layer is placed in the z-y plane where. x=0 the TE0 and TM1 modes will not be supported with these boundary conditions. This condition models the case of a dielectric slab of height T on a ground plane, as is common in many wafer probing situations. Mode charts for a .010" (0.254 mm) thick alumina (A12O3) slab are shown in Figure 2. Notice that the TE0 and TM0 modes have no cutoff frequency, where as the TE1 and TM0 modes do. Surface waves explain much of the phenomena discussed in this paper. In Section II two types of wafer probes are used to launch surface wave modes. Results are shown that indicate the pres- ence of these modes in a alumina slab. In Section III the interaction of Coplanar Waveguide 1 (CPW) with surface waves is explored, and methods for minimizing these interactions are pre- sented. Conclusions are presented in Section IV. 2 II WAFER PROBE INTERACTION WITH SURFACE WAVES The two most common styles of wafer probe used today in microwave and millimeter wave wafer probing are the Ground-Signal (GS) and Ground-Signal-Ground (GSG) probe. Signals typ- ically originate in a network analyzer and reach the probe via either a coax or waveguide (2). The GS or GSG probe converts this signal into either a slot line or a coplanar field pattern, re- spectively. The probe tips of the GS and GSG probes are shown in Figure 3. The GS probe slot line field pat- tern effectively creates an electric dipole. The dipole will generate a transverse electric field pat- tern in a dielectric slab when the probe tip is brought into contact with the slab surface. This will launch TE modes that are allowed in the dielectric. TM modes are also possible if the electric field can penetrate deep enough into the slab to couple into these modes. The GSG probe has two opposed dipoles that tend to cancel the transverse electric field of each other, but can gener- ate a net transverse magnetic field if the electric fields are bent into the dielectric slab as shown. This will couple to the TM modes. Experimental verification of the above hypothesis was achieved by first measuring the crosstalk (S21) between a GS and SG probe and then between a pair of GSG probes. An ideal pair of wafer probes would have no crosstalk, since this eliminates the possibility of one probe perturb ing the other. The GS/SG probes were 100 um pitch (separation between the two nickel fingers) and the GSG probes were 150 um pitch. Typical pitches range from 100 to 250 um for probes used above 26.5 GHz. The probes were calibrated; the GS/SG probes with SOLT (Short-Open- Load-Thru) and the GSG probes with LRM (Line-Reflect-Match). In each case three tests were performed: (i) the two probe tips in air 0.15” (0.38 cm) above a piece of RF absorber, (ii) both tips on a .010” (.254mm) alumina dielectric slab (dielectric con- stant of 9.9) suspended in air 0.15” (0.38 cm) above RF absorber, and (iii) both tips on the alu- mina slab with the bottom surface of the slab placed on a aluminum plate. In all tests the probe tips were separated by .004” (0.10 mm). These three tests are designed to generate (i) no surface waves, (ii) the TE0 and TM0 surface wave modes, and (iii) only the TM0 mode. The results are shown in Figures 4 and 5. The data for the GS/SG pair is only valid to 40 GHz, since the coax connector on this probe overmodes above this frequency. The increased noise above 45 GHz in the GSG data is from the system noise floor of the network analyzer and should not be interpre- ted at surface wave phenomenon. 3 In test (i) the GS/SG probe. pair has a crosstalk of -42 to -39 dB from 10 to 40 GHz compared to - 53 to -49 dB for the GSG pair. Presumably the dipole cancelation in the GSG probe results in this additional 10 dB of isolation compared to the GS/SG probes. In test (ii) the GS/SG probes show an increase of about 4 dB in crosstalk due to coupling to the surface wave modes. When the ground plane is added in test (iii) the crosstalk increased only 0 to 3 dB supporting the hy- pothesis that the dipole couples strongest to the TE0 mode, which is now suppressed. In the GSG case crosstalk increased 7 to 8 dB for tests (ii) and (iii) relative to test (i) versus only 4 dB for the GS/SG probes, although it should be noted that the absolute crosstalk level is still lower for the GSG case. The increase is nearly identical for tests (ii) and (iii) which suggests the TM0 mode is dominant for a GSG probe since only it propagates in both tests. This conclusion is supported by field overlap integral calculations which predict greater coupling to the TM0 mode relative to the TE0 mode for coplanar waveguide on a dielectric substrate (3). The GSG probes seem to couple tighter to the TM0 mode than the GS/SG probes do to the TE0 mode, based on the relative increases in crosstalk noted For completeness, the suspended dielectric slab measurements should also be done on .020” thick alumina slabs, since a .020” suspended dielectric slab has the same vs. frequency re- sponse for the TM0 and TE1 modes as a .010" grounded slab. These measurements were per- formed and resulted in similar data to the .010" suspended slab experiment with the exception of 38-46 GHz for the GS/SG case. In this range, there was a pronounced null in S21 reaching -50 dB at approximately 42 GHz. A plausible explanation is that the .020” alumina slab is thick at 45 GHz, where is the wavelength in alumina. Around this frequency, the open boundary condition at the slab bottom may be transformed into a virtual short circuit at the slab top sur- face. This would effectively place a short circuit at the probe tips, explaining why resulting cross- talk is even lower than when no surface waves are present in test (i). 4 This data has application to the question of whether it is better to have the probe tips in air or on a substrate when it is desired to terminate the probe in a zero length open reference termination, such as during calibration. It has been found that the reflection coefficient is high enough to use as an open in either case. An ideal open would have no perturbation from external objects, in- cluding the other probe. Thus, performing the open calibration with the probe tips in air will be more ideal, since this will minimize crosstalk and result in better isolation. III COPLANAR WAVEGUIDE INTERACTION WITH SURFACE WAVES A cross section of a Coplanar Waveguide (CPW) on a dielectric slab is shown in Figure 6. It has a ground plane of width A, gaps of width G, and a signal line of width W. Dispersion and radia- tion loss are minimized for the CPW line when the ground-to-ground spacing (W+2G) is small compared to the dielectric wavelength and substrate thickness (3). Dispersion occurs when the CPW mode couples to the surface waves via radiated energy (4,5,6). The (W+2G) separation be- tween the ground planes may be thought of as an antenna aperture, hence reducing this quantity reduces the radiated energy available for coupling to surface waves. Reducing the substrate height, T, increases the cutoff frequency of higher order surface wave modes. By picking T small enough these modes are pushed above the frequency range of inter- est. The TE0 and TM0 have no cutoff frequency, hence they must be dealt with in an other man- ner. The TE0 mode can be eliminated by adding a ground plane to the substrate bottom, but this may result in microstrip modes (4). Some insight as to how the CPW interacts with surface waves may be gained by studying Figure 2.