Fig. 1 Illustration of the non-contact probe setup for on-chip

device and integrated circuit characterization using conventional VNA frequency extenders [2].

S11 S21

Non-Contact Probe Calibration for THz-frequency Device

Characterization

Cosan Caglayan, Georgios C. Trichopoulos, and Kubilay Sertel

The Ohio State University, ElectroScience Laboratory, Department of Electrical and Computer Eng.

1330 Kinnear Rd., Columbus, OH 43212

E-mail:{caglayan.1,trichopoulos.1,sertel.1}@osu.edu

Abstract — We present a non-contact, on-wafer, broadband

device and component testing methodology scalable to the

THz band. The “contactless” probe setup is based on

radiative coupling of vector network analyzer test ports onto

the coplanar waveguide environment of monolithic devices

and integrated circuits. Efficient power coupling is achieved

via planar, broadband, antennas that act as the “virtual”

probe-tips on the chip under test. For accurate S-parameter

measurements, repeatable errors in the setup are calibrated.

In this paper, we demonstrate for the first time experimental

validation of the calibration of the new non-contact probes

for the 325-500 GHz band (using WR 2.2 frequency

extenders and a standard vector network analyzer as the

backend). This non-contact probe setup is accurate, low-cost

and is readily scalable down to the mmW band and up to the

THz band (60GHz-3THz).

I. INTRODUCTION

Recent advances in high-speed material systems and

accurate nano-fabrication technologies are enabling

ultrafast electronic devices that can provide electronic gain

at frequencies approaching 1THz [1]. With these new

ultrafast transistors, THz monolithic integrated circuits

(TMICs) and integrated systems are becoming a reality.

Such systems are badly needed for cost-effective

implementation of many potential THz applications, such

as biomedical imaging, security screening, and ultrahigh-

data-rate communications.

Conventionally, contact probes are used to characterize

on-chip performance of devices and circuits; however,

these are extremely limited in performance (e.g.

bandwidth) and suffer from high-cost and fragility issues

when scaled to THz frequencies. As such, device testing in

the 0.3-3 THz remains a specialty of a few research groups

that can shoulder the high cost of operating and

maintaining such setups.

To truly enable an accurate, low-cost and easy-to-use

testing setup for the THz band device characterization, we

recently proposed [2] a novel technique that enables non-

contact characterization of on-chip components. As

described in [2] (and illustrated in Fig. 1), efficient

coupling between the test ports of a standard vector

network analyzer (VNA) and the on-chip environment of

the device under test is achieved using broadband,

butterfly-shaped slot antennas [3] that are integrated with

the co-planar waveguide (CPW) environment on the chip.

The antennas are fabricated monolithically and are

optimized for i) wide bandwidth, ii) optimal impedance

match to the on-chip environment and iii) optimal

polarization and pattern match for robust quasi-optical

coupling to the VNA ports. A conceptual illustration of

the quasi-optical coupling sub-system is shown in Fig.1.

As seen, the incident beam launched from a horn antenna

attached to THz VNA module is focused on the THz

antennas via an extended hemispherical lens. The antennas

are fabricated on the device plane and are connected to

device under test through a coplanar waveguide. The

signal transmitted through the device is coupled to the

receiving VNA port via a second antenna connected to the

output port of the device. We note here that the actual

incident and transmitted beam angles are very small (<10

degrees), allowing for effective coupling to planar

antennas at broadside. As demonstrated below, this

simple, yet versatile configuration is shown to operate

effectively in the 325-500GHz band and can be scaled up

Fig.2 Simplified, 5-error-term model for the calibration of non-contact probes using a 1.5-port VNA extender setup [4]

b1

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00 e

11 DUT

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978-1-4799-2935-1/13/$31.00 ©2013 IEEE

to 3THz and down to 60GHz to cover the entire mmW-

THz regime.

II. CALIBRATION OF THE NON-CONTACT THZ PROBES

Conventional 2-port calibration algorithms (such as

through-reflect-line or short, open, line, through) are not

applicable for THz-frequency on-chip calibration due to

the accuracy and reliability of the standard loads.

Moreover, due to high costs associated with frequency

extension modules, often on a 1.5-port system must be

used. In another words, only S11 and S21 can be measured

using a Tx-Rx module for port 1 and an Rx-only module

for port 2. As such, conventional 2-port calibration

algorithms are not suitable. In this case, the standard 12-

term error model can be simplified to a 5-error term model

(see Fig. 2) and the unknown error terms can be extracted

by means of 1-port calibration techniques and an

additional “through” measurement.

The first step in calibrating the 1.5-port system is to

determine 1-port error parameters e00, e11, and e01, e10 (see

Fig.2) using at least three different terminations.

Subsequently, e22 and e10, e20 can be found using a simple

through measurement, as described in detail in [4].

Non-repeatable errors (such as quasi-optical alignment

of the on-chip antennas with the VNA extender ports) are

expected to have significant effect on calibration accuracy,

given the quasi-optical nature of the overall setup. Since

the beam spot on the planar antennas is sensitive to

alignment errors (this is akin to the contact-probe landing

accuracy), illumination of each non-contact probe slightly

differs from one another. Thus, in order to improve

calibration accuracy, an over-determined calibration with

more than three standards is performed. This can be

readily achieved using multiple, shorted CPW lines as

calibration standards as described previously in [5].

However, different from [5], the contact-probe landing

pads are replaced by planar butterfly antennas in our

setup.

We recently implemented the proposed non-contact

probe setup and demonstrated its performance for the 325-

500GHz band. In order to verify the self-consistency of

the calibration method, we re-measured each of the 5

standards, while using the remaining four standards as the

calibration set. The comparisons of “re-measured”

standards with full-wave simulated (HFSS) standards are

shown on the Smith Chart in Fig. 3. As seen, excellent

accuracy can be achieved in reproducing the expected

response of the shorted CPW lines of different lengths.

III. CONCLUSION

We demonstrated -for the first time- a non-contact

approach for device and circuit characterization in the

THz band. A simple calibration procedure was adopted for

use with a through-reflect configured 1.5-port THz VNA

system. Calibration and measurement accuracy can be

further improved by minimizing the losses within the

system, thus improving its dynamic range. Owing to the

non-contact nature of the new setup, our probe is free from

fragility and wear/tear issues of traditional contact-based

probes. More importantly, they can be easily scaled

beyond 900GHz where there is no existing solution for on-

chip device and IC testing

ACKNOWLEDGEMENT

This work is supported by ONR MURI Program:

DATE (Devices & Architecture for THz Electronics),

N00014 11-1-0077.)

REFERENCES

[1] W. R. Deal , K. Leong , V. Radisic , S. Sarkozy , B. Gorospe , J. Lee , P. H. Liu , W. Yoshida , J. Zhou , M. Lange , R. Lai and X. B. Mei "Low noise amplification at 0.67 THz using 30 nm InP HEMTs", IEEE Microw.Wireless Compon. Lett., vol. 21, no. 7, pp.368 -370 2011

[2] C. Caglayan, G. C. Trichopoulos, K. Sertel, “On-Wafer Device Characterization with Non-Contact Probes in the THz Band”, IEEE International Symposium on Antennas and Propagation (APSURSI), 2013, July 2013

[3] G. C. Trichopoulos, H. L. Mosbacker, D. Burdette, K. Sertel, "A Broadband Focal Plane Array Camera for Real-time THz Imaging Applications," IEEE Transactions on Antennas and Propagation, , vol. 61, no. 4, pp. 1733-1740, April 2013

[4] D.Rytting, “Network Analyzer error models and calibration methods” September 1998, Hewlett-Packard Company

[5] L. Chen, C. Zhang. T. J. Reck, A.Arsenovic, M. Bauwens, C. Groppi, A. W. Lichtenberger, R. M. Weikle, N.S. Barker "Terahertz Micromachined On-Wafer Probes: Repeatability and Reliability" IEEE Transactions on Microwave Theory and Techniques, vol. 60, no. 9, pp. 2894-2902, Sept. 2011

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Short+ 27μm

Short+ 54μm

Short+ 82 μm

Short+ 108 μm

Fig.3 Smith Chart Representation of Re-measured

Standards: Black dashed lines are HFSS simulations, thin

solid lines are measurements from 325-500 GHz

Short (Reference)