On-Wafer Vector Network Analyzer Calibration and Measurements The Vector Network Analyzer or VNA has become the workhorse of most network measurements above 1 GHz. Getting the best on-wafer mea- surement results requires a solid understanding of measurement system components and their interaction. This application note is intended to introduce on-wafer vector measure- Port-l b_ ) Port-2 ments and provide references for fur- +Cable y3 DUT _ ther study. a2 VNA Calibration Fundamentals The representative VNA block diagram (see Figure 1) shows the key elements of the analyzer. The Figure 1. Main hardware blocks in a Vector Network Analyzer (VNA). RF source at the top provides the system ends and the DUT begins. erence plane. If we ignore any non- device-under-test (DUT) stimulus. In on-wafer VNA measurements idealities of the cables, couplers, A forward/reverse switch directs the this boundary is known as the “ref- mixers, and so forth, then the ratios RF energy to either DUT port. The erence plane” of the measurement of wave amplitudes (a’s and b’s) test set uses directional couplers or and will often be located at the inside of the machine correspond bridges to pick off the forward and probe tips. A typical wafer probing directly to the DUT's S-parameters. reverse waves traveling to and from VNA setup is shown in Figures 2 For example: each port, and down-converts these and 3. signals to four IF sections. Each IF b,/a, = S, 1 of the DUT section filters, amplifies, and digi- Ideally the system will measure tizes the signals for further digital the characteristics of whatever is VNA calibration is the process of processing and display. connected to the measurement ref- measuring devices with known or A VNA measures vector ratios of reflected or transmitted energy to energy incident upon the DUT. AS a stimulus-response measurement, a VNA measurement determines the properties of devices rather than the properties of signals. Signals would be measured by instruments such as oscilloscopes or spectrum analyzers. A significant challenge in stimu- lus-response measurements is defin- ing exactly where the measurement Figure 2. A typical RF device characterization setup is shown above. On the left is a vector net- work analyzer, in this case an Agilent 8510 system. Cables from the test set ports route over the ® probe station to a pair of microwave probes which are aligned through the microscope or camera CASCADE system onto calibration substrates and test devices. The right side of this picture shows the PC that controls the prober and runs the calibration and measurement software. 1 Figure 3. Close-up of microwave probes and cables on positioners providing probe planarity adjust- ment and cable Figure 5. The Smith Chart is a polar graph of reflection coefficient with grid lines showing normalized impedance values. partly known characteristics and tors or at the ends of cables. 1 +l- = r+ix using these measurements to estab- Calibration functions allow the zL =zo - 1 -I- lish the measurement reference user to store measurements of stan- [ 1 planes. Calibration also corrects for dards, compute the error models, the imperfections of the measure- and automatically apply corrections V reflected zL - zll ment system. These imperfections to DUT measurements. = r= Z not only include the non-ideal V incident L + z. nature of cables and probes, but Where the ohmmeter’s calibration also the internal characteristics of simply used a short circuit (con- where Z,= the reference imped- the VNA itself necting the test leads together) to ance of the measurement system determine the extra resistance term The VNA is calibrated in much in the error model, a VNA uses The Smith chart (see Figure 5) is the same manner that the “zero” multiple calibration standards - a polar graph of the reflection coef- function on your ohmmeter sub- typically open circuits, short cir- ficient with grid lines showing the tracts out the resistance of the test cuits, loads, and through (thru) normalized impedance values. All leads. On an ohmmeter, when you connections. passive impedances are represented activate the “zero” function, it in a compact unit circle reflection stores a resistance measurement Measuring Impedance format. Typically the vna auto- which is then subtracted from all mates the display of a wide variety future measurements. The obvious The measured ratios of the ai and of formats: Smith Chart, polar, and error model is simply a resistance in bi wave amplitudes are called linear and log magnitude formats. series with the test port. Corrected S-parameters. S-parameters are just VNA measurements are referred to one type of network representation Calibrated VNA Accuracy as “deembedded.” used for linear, small-signal, a.c. analysis. As with any corrected measure- A VNA does the same thing as ment, the absolute accuracy of a the ohmmeter but uses a more A reflection ratio at a device port, calibrated VNA measurement is complex error model with several with all other ports terminated, is determined by the techniques and terms for each frequency point. The also known as a reflection coeffi- completeness of the error model measurement system is described as cient. The load impedance, Z,, is used, the accuracy of the descrip- an ideal VNA with an error adapter related to the reflection coefficient, tion of the reference devices (cali- network that models all of the sys- I?, by the bilinear transform pair as bration standards), and the repeata- tem’s non-idealities: directivity of follows: bility of the measurement system. the couplers, imperfect match look- ing back into the reflectometer (test set ports), imperfect frequency P 1 response of the reflectometers and Forward the transmission between ports, and Error DUT the crosstalk between ports. This Adapter Pl error model is shown in Figure 4. VNA's rely on calibration for Port 2 accuracy even in measurements 3 with the reference plane defined at the instrument front panel connec- Figure 4. Error model for a typical network analyzer. All errors are modeled by an error adapter in front of an otherwise perfect system. 2 If performed improperly, calibra- VNA error models are based VNA Calibration Options and tion can introduce errors. The upon the use of S-parameter repre- Standards impedances used in the calibration sentations of network properties. S- The VNA will measure the must be accurately known and parameters are signal flow and uncorrected S-parameters if not cal- entered. Assumption of ideal transmission line based. Only a sin- ibrated, though not very accurately. behavior of standards is a recipe for gle mode of propagation at device Uncorrected measurements are errors, but worse things can hap- terminals is assumed. Situations rarely used (see Figure 7). pen. For example, improperly that violate this assumption such as entering a description of a short using waveguides that can propa- circuit or open circuit standard will gate multiple modes, radiation, or lead to useless results that may not parasitic coupling between net- always be obviously wrong. The works other than at the device ter- VNA will believe everything that minals are not properly handled. you tell it. Accurate descriptions of l Convenient the electrical behavior of the vari- If a second mode, radiation, or l Generally not accurate ous calibration standards must be extra parasitic doesn’t change for l No errors removed supplied to the VNA. any and all devices being measured then it will drop out with the cali- Figure 7. Uncorrected VNA. Corrected measurements rely on bration. Usually, however, these A “response” calibration is simply the repeatability of the measure- extra modes have behavior that is a vector magnitude and phase nor- ment system. Just as an ohmmeter DUT dependent and VNA calibra- malization of a transmission or cannot correct for a changing resis- tion will not account for the reflection measurement, used only tance of its test leads or drift in the effects. A clean, well-designed ohmmeter circuitry, a VNA cannot probing system (including die pad at low frequencies (See Figure 8). correct for random errors such as pattern) with good quality trans- 0 0 noise or dynamic range, cable mission-like interconnections will thru repeatability, or instrument drift. minimize these errors as much as Any change in the measurement possible (see Figure 6). due to, for instance, drift of the VNA test set, thermally induced An extensive discussion of VNA cable length changes, or even the calibration and accuracy is included l Use when highest effects of noise due to VNA in Calibration and Accuracy accuracy is not require’ dynamic range can invalidate the Factors Summit High-Frequency l Removes frequency response error correction. Sensitivity of cable elec- Probe Station Reference Manual. l trical performance (such as phase Figure 8. Response calibration. delay) to environmental changes is a significant element of quality and Typically a full calibration of all the suitability for VNA use. error parameters is used as a refer- ence or to assure the highest accuracy (see Figure 9). 2 ground contact pitch 100 - 150 pm signal DUT I-0 4I I Highest accuracy for P-port 100 - 150 pm devices Removes these errors - directivity - source match - load match ground - reflection tracking - transmission tracking - crosstalk Figure 6. Recommended microwave pad pattern for each measurement port. Figure 9. Full 2-port calibration. On-wafer standards, fabricated on the same wafer as the DUT, are sometimes desirable since the probe-to-standard transition can be designed to be very similar to the transition to the DUT. This is helpful at frequencies above 20 GHz since a primitive transi- tion may introduce extra parasitics or modes which, since they are occurring at the probe-tip refer- ence plane, may not be corrected by calibration.