Microelectronics Reliability 48 (2008) 1440–1443

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Microelectronics Reliability

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Testing semiconductor devices at extremely high operating temperatures

Peter Borthen *, Gerhard WachutkaInstitute for Physics of Electrotechnology, Technische Universität München, Arcisstrasse 21, 80290 München, Germany

a r t i c l e i n f o

Article history:Received 30 June 2008Available online 10 August 2008

0026-2714/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.microrel.2008.07.037

* Corresponding author. Tel.: +49 89 289 23107; faE-mail address: borthen@tep.ei.tum.de (P. Borthen

a b s t r a c t

We developed a dedicated measurement set-up for the electrical and electrothermal characterization ofsemiconductor devices and microsystems under very high-temperature conditions. The set-up consists ofseveral modules comprising a vacuum system as basic unit and a number of alternative sample stages.Currently it enables measurements in the temperature range between room temperature and about700 �C. We give a detailed description of the measurement system, sample mounting techniques, andexemplary measurements on SiC devices.

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1. Introduction

Operation at high temperature constitutes, among other stressconditions such as high pressure, reactive chemical environment,radiation, vibration or mechanical stress [1–3], one of the mostimportant factors which limit the lifetime and the reliability ofelectronic or mechatronic components. This becomes the more rel-evant, the more we face an ever-growing field of applications ofmicrosystems and electronic systems in very hot environments,the realization of which requires devices that are able to operateunder such harsh conditions. Well-known examples of high-tem-perature applications are encountered in combustion engines,brake systems for cars and aircrafts, turbines, and oil or geothermalwell drilling [4–7]. Another challenging field, where high-temper-ature ruggedness is indispensable, is space and planetary explora-tion [8,9]. This development is accompanied by an intensiveresearch on semiconductor materials such as silicon carbide [10],GaAs or diamond [11,12], which are used as basis for high-temper-ature devices. The market for high-temperature electronics, sen-sors and actuators is expected to reach $15 billion in the nearfuture [13].

As a consequence, the characterization and test of electronicand mechatronic microsystems in a temperature range up to600 �C and higher has increasingly gained practical relevance.However, commercial probe stations usually do not allow for mea-surements at such high temperatures. So far, only rarely experi-mental equipment suited for the electrical characterization atvery high temperatures has been reported in more detail. A smallchamber for Hall-measurements up to 400 �C was described in[14]. In [15,16] a modified commercial high-temperature ovensfor measurements on samples at temperatures as high as 500 �C

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x: +49 89 289 23134.).

were described. RF measurements on wafers at temperatures upto 500 �C are described in [17]. There are also several recent reportson measurements at temperatures up to 600 �C [18–20] or even1050 �C [12]; however, only few experimental details are men-tioned there.

In the following we describe a versatile, modular experimentalset-up for the electrical and thermal characterization of microsys-tems and semiconductor devices under extremely high tempera-tures. Notable features of this measurement system are the easyadaptation to specific measurement requirements, the simple ex-change of the samples, and the fast heating-up of the sample stage.To this end, we also developed a novel sample mounting and con-tacting method, which enables a stable and reliable electrical con-nection of the sample to the external circuitry even at extremelyhigh temperatures. Both, the measurement system and the samplemounting method is expected to be applicable for the electricalcharacterization of semiconductor devices up to at least 800 �C.

2. Experimental set-up

The experimental set-up consists of two main building blocks: avacuum system and a set of measuring instruments controlled by acomputer. The measurements are performed under high vacuumconditions which provides an excellent thermal isolation of the de-vice under test and, at the same time, prevents the degradation ofthe sample and the internal system units due to the exposure tothe oxygen in the surrounding air. The vacuum system consistsof Trinos modular vacuum chambers [21] with an inner diameterof 40 cm and a Leybold PT 151 pumping unit [22] consisting of arotary pump and a turbomolecular pump (Fig. 1). The minimumachievable pressure in the vacuum chamber is about 1 mPa, whichis fully sufficient for the purpose of this system. Inside the vacuumchamber, different exchangeable measuring platforms can beplaced depending on the kind of the measurement task. Fig. 2shows a general-purpose prober unit and a platform with a circuit

Fig. 1. Experimental set-up: vacuum system (left) and measuring instruments(right).

Fig. 2. High-temperature sample prober with the heating unit in

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for high-current pulsed measurements. Both systems are in-housedevelopments.

The sample stage of the general-purpose prober unit is made ofa circular plate (molybdenum, copper or Robax�-glass ceramic)with a resistive heating coil underneath. The heating unit is sur-rounded by several concentric heat shields made of polishedmolybdenum foils. Currently, with this unit a maximum tempera-ture of at least 700 �C can be attained.

Stainless steel manipulators with coax-cable (RG402) cantile-vers fixed on top of the manipulators are used for contacting thesamples. The inner conductors of the semirigid coax-cables aremade of steel wires plated with copper and silver. Their ends werebent and sharpened to form tips with radii of about 50 lm. Fortemperature sensing a PT100 platinum resistor gauge is used,which is placed in the vicinity of the sample. A temperature controland power supply circuit reads and converts the PT100 resistancevalues. For temperatures above 700 �C thermoelectric temperaturesensors are employed instead. All measured data is collected by aPC using a PCI-6014 A/D-converter (National Instruments). Thewhole heating system is controlled by a LabVIEW� program devel-oped for the specific measurement requirements. The electricalcharacterization of the samples is supported by a Hewlett–PackardHP4142B modular parameter analyzer, which is also connected tothe PC and controlled by LabVIEW� software [23].

3. Sample mounting

A reliable die attachment and stable electrical contacts areindispensable prerequisites for measurement at elevated tempera-tures. In order to prevent the lift-off of the chip and to maintain agood thermal and, if needed, electrical contact to the substrate thedifferences in the coefficients of thermal expansion between thesubstrate and the chip should be kept as small as possible. For ex-tremely high temperatures the only reliable solution consists in theuse of pure metallic substrates and chip bond materials. Goingabove 800 �C, the use of a refractory metal substrate becomes man-datory, because other metals exhibit an intolerably high vapor

the center surrounded by several molybdenum heat shields.

Fig. 4. Electrical resistance as a function of temperature for four SiC-TLM-teststructures with varying distance d between the contacts. Inset: The resistancevalues for 20 �C, 200 �C and 500 �C as a function of the contact distance.

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pressure or even start melting. Up to 800 �C other metals may beused as well, but it is advisable to use pure metals such as copperor silver.

Measurements under vacuum conditions require a very goodthermal contact between the die and the substrate, even if no elec-trical contact is necessary. In the case that there is a proper metal-lization layer deposited at the backside of the die, the so-calledNTV-technique can be applied [24]. With this method, a thin por-ous silver foil between the die and the metallic substrate is sin-tered under elevated pressure and temperature. As a result, ametallic bond is formed that shows very good electrical and ther-mal properties. We have used this bonding technique for themounting of different exemplary samples measured repeatedlyup to 500 �C with no signs of failure. We guess that this die bond-ing method will work properly up to 800 �C (tests are currentlybeing made).

Another method to achieve good thermal and electrical diebonds is the use of indium or an indium–silver alloy as connectinglayer. With a melting temperature of only about 160 �C indiumforms a liquid at higher temperatures. But, due to the high boilingtemperature of more than 2000 �C, the vapor pressure of indiumremains relatively low up to about 800 �C. This makes it possibleto use a thin liquid indium layer for die bonding under vacuumconditions, yielding a very good thermal and electrical connection.We used indium-bonded SiC samples for measurements performedup to 700 �C as shown in Figs. 5 and 6.

The use of standard bond pads for electrical contacts becomesvery problematic at higher temperatures, since the thermal degra-dation of the tip of the manipulators will inevitably cause an inter-ruption of the contact, which may even lead to a damage of the teststructure. Whenever possible, wire bonds are the better option:standard aluminum bond wires are usable up to 500 �C, and goldwires up to 800 �C.

Fig. 5. Forward characteristics of a SiC-pin diode (linear scale). RT (A): firstmeasurement at room temperature; RT (B): second measurement at roomtemperature after cooling down from 700 �C.

Fig. 6. Forward characteristics of the SiC-pin diode from Fig. 5 (logarithmic scale).Fig. 3. Sample mounting and contacting method for extremely high-temperaturemeasurements on test samples: (a) top view; (b) sectional view.

P. Borthen, G. Wachutka / Microelectronics Reliability 48 (2008) 1440–1443 1443

Fig. 3 shows a sample mounting stage developed recently in ourlaboratory for measurement up to 800 �C. It consists of a molybde-num disc (diameter 20 mm) and a glass-ceramic ring with a hole inits center accepting the central part of the molybdenum substratewith the test chips and the temperature sensors. The glass-ceramicring has several small, concentrically arranged holes (diameter1.2 mm) for silver contact wires. These wires are used as externalbond pads for the bond wires and can be easily contacted by meansof the manipulator tips.

4. Exemplary high-temperature measurements

Results for two SiC samples (both from SiCED GmbH) are shownin Figs. 4–6. The first example (Fig. 4) shows results obtained froma TLM (transmission line method)-test structure [25]. The mea-sured electrical resistance as a function of the contact distance d(inset, Fig. 4) allows to extract the contact and the sheet resistanceof the material as a function of temperature.

Figs. 5 and 6 display the forward characteristics of a SiC pindiode for temperatures between 18 �C and 700 �C. This diode canbe operated up to about 15 A [26], but the maximum current mea-sured here was restricted to 0.1 A in order to avoid self-heating ofthe samples. The diode was die-bonded using a thin indium filmbetween the chip and the molybdenum substrate. At 700 �C thestandard metallization at the anode side (Al) was most probablymolten. Nevertheless, the diode still performed quite well aftercooling it down again to room temperature with, however, signsof degradation which most probably are due to the partially meltedupper metallization. More detailed results and a comparison withdevice simulations illustrating the high-temperature behavior ofSiC-pin diodes may be found in [27].

Acknowledgements

This work was funded by the Dr. Johannes Heidenhain-StiftungGmbH and the BMBF under the contract 01 M 3079E. We wouldlike to thank the SiCED GmbH, Erlangen, Germany, for providingthe SiC samples used in our tests.

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