How To Use Picoprobes And Flexible Pitch Probes Advanced TLP/HMM Solutions This application note describes the four point Kelvin standard TLP method with discrete current sensor, fixed pitch and variable pitch wafer level probe tips. L is the length of the 50 Ω interconnection cable between the transformer-based current sensor and the DUT. The propagation velocity of the pulse signal in the 50 Ω cable is approximately c v = √ ≈ 0.2 m/ns r 1 Four Point Kelvin TLP Method Fig. 1 shows the four point Kelvin TLP method including dc leakage measurement. This setup is widely used for standard TLP measurements with high accuracy because of separated pulse force and pulse sense channel. A transformer-based pulse current sensor (e.g. Tektronix CT-1) is used in the pulse force channel. A tiny 4950 Ω resistor which is integrated in front of the pulse sense probe tip acts as voltage divider 100 : 1 = (4950 Ω + 50 Ω)/50 Ω together with the 50 Ω transmission line and 50 Ω input impedance of the digital oscilloscope. where speed of light c ≈ 2.998 · 108 m/s and r ≈ 2.1 is relative dielectric constant of PTFE (Teflon) based insulator of the 50 Ω coaxial cable. The readout of the current sensor is wrong in the first time up to: t = 2· GPIB SMU DUTILeakageITest t L ... ... v ... Leakage DUT GPIB Transformer4Based PulseICurrentISensor SwitchIControl DUTIPulseIForce Pulse 5- Ω PulseIOutput 5- Ω DUT SWITCH PAD PAD DUTIPulseISense VDUTI(t) GPIB IDUTI(t) time in [ns] distance between current sensor and DUT in [m] 0.2 m/ns pulse signal propagation velocity 1. Propagation velocity in the cable is v ≈ 0.2 m/ns, according Eqn. 1 4995IkΩ 5- Ω PulseISense (2) Let’s investigate the following example test conditions: 5- Ω DIGITAL OSCILLOSCOPE L v 5- Ω Picoprobe ModelIy- LI<I-9yIm 5- Ω 5- Ω PC + System Control Software LEAKAGE TEST TLP GENERATOR (1) 5IkΩ Picoprobe ModelIy- 2. Distance ”L” from DUT to current sensor (Fig. 2) TinyIresistorIbuiltIinIfrontIof theIpulseIsenseIprobeItip9 Current 5- Ω Ay 3. Rise time of the TLP is set to 100 ps and pulse width is set to 100 ns Figure 1: Four point Kelvin TLP method including dc leakage measurement. 4. TLP voltage is set to VPulse = 1000 V 5. Device-under-test (DUT) is a 5 Ω resistor 1.1 Why the current sensor should be connected to the DUT as close as possible? For this load condition we expect Fig. 2 is a simplified view of the block diagram shown in Fig. 1. The probe tips with equivalent circuit are shown in Fig. 9. TLPtGenerator PulsetSignal Propagation Transformer-Based PulsetCurrenttSensor L 50 Ω 50 Ω DUT PAD PAD IDUT VDUT DUTtPulsetSense 50 Ω 5tkΩ Picoprobe Modelt10 4.95tkΩ Tinytresistortbuilttintfronttof thetpulsetsensetprobettip. Figure 2: TLP generator, pulse force channel, pulse sense channel and current sensor output. Application Note AN-010 Rev. 1.4 September 4, 2014 IDUT = VPulse · 5 Ω = 90.9 V 50 Ω + 5 Ω VPulse = 18.2 A 50 Ω + 5 Ω (3) (4) 1.1.1 Test Case 1: L = 0 m 50 Ω DUTtPulsetForce CurrenttSensortOutput = 50 Ω Picoprobe Modelt10 50tΩ VPulse VDUT If L = 0 m, which means the current sensor is mounted directly at the DUT (which is not possible in reality), we would measure the following DUT voltage and current waveforms, as expected by Eqn. 3 and 4, confirmed by TLP simulation results1 shown in Fig. 3. Fig. 4 shows the same plot but with higher time resolution up to t = 20 ns. As a summary, with L = 0 m the voltage and current waveforms occur immediately at same time t = 0 ns with correct value. 1 TLP Page 1 of 4 wave reflection simulation including cable losses. High Power Pulse Instruments GmbH Internet: www.hppi.de E-mail: [email protected] How To Use Picoprobes And Flexible Pitch Probes 100 100 90 90 80 80 VDUT [V], IDUT [A] VDUT [V], IDUT [A] Advanced TLP/HMM Solutions 70 60 VDUT IDUT 50 40 30 70 60 40 30 20 20 10 10 0 -10 0 10 20 30 40 50 60 70 80 90 100 110 120 Time [ns] VDUT IDUT 50 0 -2 Figure 3: Simulated TLP DUT waveforms with L = 0 m condition. 100 0 2 4 6 8 10 12 Time [ns] 14 16 18 20 Figure 5: Simulated TLP DUT waveforms with L = 0.2 m condition. readout of the current waveform at t > 2 ns onwards. 90 VDUT [V], IDUT [A] 80 70 1.1.3 Test Case 3: L = 1 m 60 VDUT IDUT 50 Now we assume L = 1 m to review the expected waveforms in Fig. 6. 40 30 20 100 10 90 80 -2 0 2 4 6 8 10 12 Time [ns] 14 16 18 20 Figure 4: Simulated TLP DUT waveforms with L = 0 m condition. Detail view up to t = 20 ns. VDUT [V], IDUT [A] 0 70 60 VDUT IDUT 50 40 30 20 1.1.2 Test Case 2: L = 0.2 m 10 Now we assume L = 0.2 m, which means the current sensor is mounted 0.2 m far away from the DUT in Fig. 2. In this case we would measure the waveforms shown in Fig. 5. At t = 0 the propagating pulse signal (v ≈ 0.2 m/ns) in the DUT pulse force line (Fig. 2) hits the current sensor. The readout of the current sensor at 0 < t < 2 ns will result into VPulse I(t) = = 10 A (5) 2 · 50 Ω 0<t<2 ns After 1 ns, at t = 1 ns, the ropagating pulse signal hits the DUT. Therefore immediately the expected correct voltage reading of VDUT = 90.9 V occurs. However, the propagating pulse signal (the reflected wave) needs to go back to the current sensor for another 1 ns. Only if incident and reflected signals are available for superposition at the current sensor, we can expect the correct Application Note AN-010 Rev. 1.4 September 4, 2014 0 -2 0 2 4 6 8 10 12 Time [ns] 14 16 18 20 Figure 6: Simulated TLP DUT waveforms with L = 1 m condition. The correct current readout occurs at t > 10 ns, as predicted by Eqn. 2. 1.1.4 Test Case 4: L = 1 m and Rise Time tr = 10 ns Finally, we assume L = 1 m and a TLP rise time of tr = 10 ns instead of tr = 100 ps of the previous test cases. In general the same effects occur as shown in Sect. 1.1.3 but the waveforms are filtered and delayed by the 10 ns rise time filter, which give a result as shown in Fig. 7. Page 2 of 4 High Power Pulse Instruments GmbH Internet: www.hppi.de E-mail: [email protected] How To Use Picoprobes And Flexible Pitch Probes Advanced TLP/HMM Solutions tips (see also Fig. 2). On the left sided probe tip clearly the molded resistor can be identified. The mold compound is a little bit larger in comparison to the right sided 50 Ω probe tip without built-in resistor. 100 90 VDUT [V], IDUT [A] 80 70 60 VDUT (L=1m) VDUT (L=0m) IDUT (L=1m) IDUT (L=0m) 50 40 30 Pulse Sense 20 10 0 -10 Tiny resistor 4950 50 0 10 20 30 40 50 60 Time [ns] 70 80 (molded) Pulse Force 50 Signal 90 100 GND Figure 7: Simulated TLP DUT waveforms with L = 0 m, 1 m and TLP rise time of tr = 10 ns. 1.1.5 Summary Test Cases The previous investigations show that it is recommended to mount the current sensor as close as possible nearby the DUT. L = 0.1 . . . 0.15 m is a good choice and compromise. If this is not possible a wrong readout of the current sensor is expected in the first time defined by Eqn. 2 and the delay of voltage and current waveform readout needs to be considered. Figure 9: Fixed pitch picoprobes. Probing a reference device on the TLP calibration substrate. Detail view of Fig. 8 including equivalent circuit of the probe tips. 3 Flexible Pitch Probe Tips 2 Fixed Pitch Probe Tips Fig. 8 shows a typical fixed pitch (200 µm) probetips from GGB. Please refer to http://ggb.com/10.html for the model 10 probe tip holder and description of replacement probe tips. http://ggb.com/PdfIndex_files/mod10.pdf Picoprobe Model 10 (Probetip Holder) Sometimes the fixed pitch is a limitation in case of devices to be measured changes often and have different pad pitch and dimensions. For these cases the flexible pitch setup has been developed. 10-5k(0502)-125-W-1 GF-A Picoprobe Model 10 (Probetip Holder) 10- 50/30-125-W-1 Pulse Pulse GF-A Sense Force GND Tiny resistor 4950 (molded) Pulse Sense Probe Tip 10-5K(0502)-125-2-L-200 Figure 10: Flexible pitch setup using GGB 1050/30-125-W-1 (pulse force) and GGB 10-5k(0502)-125-W-1 (pulse sense) replacement probe tips and GF-A flexible pitch ground fixture. Pulse Force Probe Tip 10-50/30-125-W-2-R-200 Figure 8: Fixed pitch picoprobes. Probing a reference device on the TLP calibration substrate. Fig. 9 shows the detail view of Fig. 8 including the equivalent circuit of the GS-type (ground-signal) probe Application Note AN-010 Rev. 1.4 September 4, 2014 GND The 10-50/30-125-W-1 (pulse force) and 105k(0502)-125-W-1 (pulse sense) probe tips have a center conductor/needle but no ground needle. The ground contact is made using a clamp, short flexible Page 3 of 4 High Power Pulse Instruments GmbH Internet: www.hppi.de E-mail: [email protected] How To Use Picoprobes And Flexible Pitch Probes Advanced TLP/HMM Solutions wire, nozzle for a 0.5 mm needle and a separate probe arm for the ground needle. Despite the ground connection using a clamp with wire the pulse signal quality remains very good for rise times down to 200 ps and below. Figure 11: Ground fixture GF-A assembly: clamp mounted on picoprobes, flexible wire and nozzle including 0.5 mm tungsten needle. Figure 13: Please make sure that the probe is clamped in the groove. Fig. 11 shows the setup with removed micropositioners for the ground needles. Figure 12: Ground fixture assembly, two times, for pulse sense and pulse force: clamp, flexible wire and nozzle including 0.5 mm tungsten needle. Figure 14: Detail view: ground clamp. Fig. 12 shows the ground fixture comprising the clamp, flexible wire and nozzle including 0.5 mm tungsten needle. The clamp needs to be fixed on the picoprobe tip. The tungsten needle shall be mounted on a separate DC probe arm including micropositioner (not supplied by HPPI). Fig. 13 shows the clamp which has a groove inside the jaws. Please make sure that the probe tip is clamped in the groove in order to have stable position and good electrical contact. Fig. 15 shows the clamp fixed on the probe tip. The final result of the assembly is shown in Fig. 10 Figure 15: Two ground fixtures GF-A clamped on the probe tips. Application Note AN-010 Rev. 1.4 September 4, 2014 Page 4 of 4 High Power Pulse Instruments GmbH Internet: www.hppi.de E-mail: [email protected]
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