Technique for measurements of semiconductor devices between 2 and 300 K

1596 lEEE TRANSACTIONS ON ELECTRON DEVICES. VOL. 36, NO. 8. AUGUST 1989

Technique for Measurements of Semiconductor Devices Between 2 and 300 K OTTMAR W. KINDL AND WERNER A. LANGHEINRICH

Abstract-During investigations of a CRYO-CMOS technology suitable for very low temperatures, we acquired substantial experience concerning cryogenic measurement techniques. This paper describes our test assembly, which enables computer-aided acquisition of measurement data in the temperature range between 2 and 300 K.

I. INTRODUCTION HE subject of our investigations is the development T of cold readout electronics for focal plane IR-detector

arrays aboard the European Infrared Space Observatory I S 0 [ l ] , [2]. The readout electronics of the telescope camera will be operated inside a dewar at temperatures between 2 and 14 K. Since the behavior of standard MOS devices can show severe degradation at temperatures below about 30 K, we made principal investigations on a suitable CRYO-CMOS technology [3].

Our research was based on a large number of measurements between room temperature and 2 K. In order to provide measurement conditions that are similar to our ap- plication, we constructed the special test assembly that is to be described in this paper.

TI. TEST EQUIPMENT A schematic block diagram of our test assembly is given

in Fig. 1. The most important component is a HD-3 dewar from Infrared Laboratories Incorporated, in which the devices to be measured are mounted.

The dewar is cooled with liquid helium (LHe), which is supplied out of a storage can using a special helium transfer tube. A rotary vane vacuum pump (DUO 030A, Pfeiffer, West Germany) enables cooling below the liquidation point of helium at atmospheric pressure by means of vaporization heat.

As a heat sink and for thermal shielding against the warm environment, the dewar also has to be supplied with liquid nitrogen (LN,).

Thermal isolation is done by evacuating the dewar case with a second pumping unit, which consists of another rotary vane vacuum pump and a turbo molecular pump (TSH 172, Pfeiffer). Control of the pressure is possible with a suitable manometer gauge (PKG 020, Balzers).

Manuscript received August 27, 1988; revised March 3, 1989. This work was supported in part by the German Ministry of Science and Technology.

The authors are with the Institute of Semiconductor Electronics, Tech- nical University of Darmstadt, Schlossgartenstr. 8, D-61, Darmstadt, Ger- many.

IEEE Log Number 8928092.

- 4 Pump U n i t

!I rn !I !I DUO 0 3 0 A P f e i f f e r

Manometer Gauge

He 1 i u m 1 Dewar I PKG 020 aalzers

vacuum Pump U n i t

E l e c t r i c a l

E q u i p m e n t H D - 3 T S H 172 I n f r a r e a P f e l f f e r

L a b o r a t o r i e s I n c

Fig. 1. Block diagram of the test assembly

The cold electronics inside of the dewar and the measurement instrumentation outside of the dewar are linked via a connector plug.

111. HELIUM DEWAR A . Internal Construction

As indicated in Fig. 2, our all-metal cryostat consists of a helium can, which is thermally isolated by evacuation of the dewar case and by a liquid-nitrogen-cooled radiation shield.

The bottom of the helium can is a solid copper plate with a thickness of about 0.35 in ( = 9 mm). The copper plate forms the “cold work surface” of the dewar. Access to this area is obtained by removing the bottom plates of the dewar case and the radiation shield.

We installed a test adapter for 40 pin ceramic dual-in- line packages on the cold work surface. The dual-in-line packages are bolted directly to a chip-mounting block, which is in intimate contact with the helium container (see Fig. 3).

B. Electrical Interconnections

The heat input through the electrical interconnections between the cold work area and the outer electrical plug of the dewar has to be reduced as far as possible. Other- wise the samples would never quite attain the bath temperature of the helium can.

Therefore, all wiring should be of small diameter ( < 100 pm) and of a material with a low thermal con- ductivity (constantan or manganin). This leads to some disadvantages concerning the electrical resistance of the

0018-9383/89/0800-1536$01 .OO O 1989 IEEE

KINDL AND LANGHEINRICH: TECHNIQUE FOR MEASUREMENTS OF SEMICONDUCTOR DEVICES

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Fig. 2. Cross section of the helium dewar.

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wiring, but for most of the measurements on MOS devices the influence of the parasitic resistance is negligible.

In our test assembly we blocked the heat flow with two so-called ‘‘temperature fixpoints. ” From the outer electrical plug, which is almost at room temperature, manganin wires are led to the temperature fixpoint 1 . Fixpoint 1 is simply an aluminum cylinder where the wires have been wrapped around several times. It is bolted to the radiation shield and “shortcuts” the heat flow on to the liquid-nitrogen can.

From fixpoint 1 the wires are led to an epoxy board that supports the wiring and is part of fixpoint 2. Since the

Helium Can

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Fig. 4. Schematic of temperature fixpoint 2. Superconducting NbTi wires block the heat flow into the cold work area.

epoxy board is practically a thermal isolator, direct mounting on the cold work surface does not introduce any heat leak. The second part of fixpoint 2 is an A1 block, where varnish-isolated Cu wires have been embedded (see Fig. 4). It is bolted to the cold work surface and is therefore at helium temperature. The interconnections between the A1 block and the epoxy board are made out of NbTi wires. Due to the fact that NbTi becomes superconducting at temperatures below about 9.5 K, their thermal conduc-

1538 IEEE TRANSACTIONS ON ELECTRON DEVICES. VOL. 36. NO. 8. AUGUST 1989

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, Fig. 5 . Characteristics of the temperature sensors.

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tivity is reduced and they inhibit the heat flow from the liquid-nitrogen-cooled environment into the

Fig. 6. Circuitry for the temperature measurements. The parasitic resistance of the internal dewar wiring has no influence on the evaluation of

cold work area. the temperature.

From fixpoint 2 the wires are connected to a test socket with gold-covered contacts. The test socket connects the wiring to the dual-in-line packages of the samples.

C. Temperature Sensors

IV. ELECTRICAL MEASUREMENT EQUIPMENT From the outer electrical plug of the dewar, we used a

short shielded cable to connect a so-called “BNC distribution board” (see Fig. 7). The board provides BNC con-

Since it is necessary to know the actual temperatures of the samples, two temperature sensors have been built into the chip-mounting block of the cold work surface.

For the temperature range between room-temperature and 20 K, we use a Pt-100 platinum resistor with a guar- anteed accuracy of 1 percent.

Temperatures between 2 and 20 K are measured with an Allan-Bradley carbon resistor. The carbon resistor was calibrated with a so-called “fixpoint device” using the T, temperatures of superconducting materials as a reference. Carbon resistors are cheap and enable accurate temperature measurements below 20 K because of a very strong temperature dependence (see Fig. 5). The disadvantages of carbon resistors are that they are sensitive to temperature shocks and show long time drifts so that the calibra- tion has to be repeated from time to time.

As indicated in Fig. 6, the temperature sensors are operated in a Kelvin arrangement. Therefore, the parasitic resistance of the internal wiring has no influence on the measurement. Two external sources provide highly accurate constant currents. The voltage drop over the resistors enables the evaluation of the corresponding temperature.

To minimize the heat input due to the temperature measurements, current is switched on to only one of the sensors at any one time and only for the short time necessary to take a sample of the voltage drop. Since data rates in the range of about 6 sampledmin are sufficient for temperature control, the duty cycle is so small that the heat input due to the power dissipation in the temperature sensors is almost negligible. Furthermore, the setting of the currents limits the peak power dissipation during the sample period to a maximum of about 20 pW at 2 K.

nectors for the electrical links to the cold work area and forms a standard interface for all the electrical measurement equipment and the current sources for the temperature measurements.

Control of the current sources is assigned to two channels of a “relay switching unit.” The other channels of this component are available for multiplexing purposes between several measurement setups.

The relay unit receives its instructions over an IEC-bus interface from the controller of the system. Furthermore, the IEC-bus provides access to the “semiconductor tester” and the other electrical components of the system.

The semiconductor tester consists of three program- mable voltage sources, one current source, an ammeter for currents from 1 pA to 20 mA, an electrometer, a highly accurate digital voltmeter, and a full Kelvin relay matrix for set up of internal connections and multiplexing purposes. The tester provides all necessary facilities for the measurement of static characteristics of MOS devices.

In addition, the voltmeter of the tester is used for the temperature measurements. The control computer calcu- lates the temperature from the measured voltage drops and the characteristics of the sensors.

For our purposes, we often used an HP 4145A Semi- conductor Parameter Analyzer as a very flexible instru- ment for measurement and immediate graphic display of static device characteristics.

Dynamic measurements were made using an HP 3300B/ 3570A Network Analyzer and a Tektronics 2221 Digital Storage Oscilloscope. An HP 7550A Plotter was available for documentation of measured data.

Depending on the specific measurements, a lot of other electrical measurement equipment can be integrated into

KINDL A N D LANGHEINRICH. TECHNIQUE FOR MEASUREMENTS OF SEMICONDUCTOR DEVICES

the IEC-bus. IKl

1539

Measurements at 300 K

Fig. 7 . Block diagram of the electrical measurement equipment.

done to check the correct operation of the devices under 1 - test and obtain reference values at 300 K.

tie Pumping - Measurements below 4 2 K

10 10

with liquid nitrogen. During precooling the pressure should decrease by about two decades. Due to the large heat capacity of the dewar, proper precooling takes about 1 h. Measurements at 77 K are possible as soon as the dewar has reached the temperature of liquid nitrogen.

Before cooling continues with the transfer of liquid helium, all precoolant must be removed out of the helium can, either by pouring out or by transferring it. A temperature rise to about 80 K indicates that the dewar is ready for helium transfer. The helium transfer tube should be flushed with helium gas and slowly inserted into the supply dewar. Sealing the top of the supply dewar traps the evolving helium gas and starts to build up pressure, which vents through the transfer tube. As the transfer tube cools, it should be inserted into the dewar until it almost touches bottom. When the temperature comes down to 4.2 K, liquid helium is collecting in the dewar. The transfer should be continued for several minutes until a sudden jet of very cold gas signals that the helium can is filled. Then

the pressure applied to the supply dewar is released and the tube withdrawn.

After the measurements at 4.2 K have been made, temperatures below the liquidation point can be obtained by pumping the helium. A slow pump-down over a period of about 1 h is recommended, since going too fast may simply pump all the helium out of the dewar. If ice forms on the outside of the pump fitting, it indicates that the pumping rate is too large. With our equipment we reached a minimum of about 2.3 K. Measurements in this temperature range should be done quickly because pumping con- sumes a lot of helium.

Return to equilibrium at 4.2 K has to be performed using He gas for pressure balancing. Depending on the quantity of liquid helium that remained in the dewar, it may take up to several hours before all of the liquid helium is con- sumed and warmup starts.

If the thermal isolation is held at a high level by refill- ing the nitrogen can and maintaining a good vacuum, up

1540 IEEE TRANSACTIONS ON ELECTRON DEVICES. VOL. 36. NO. 8. AUGUST 1989

I 0 5 10 15

Drain Source Voltage V,, [VI

Fig. 9 . Output characteristic of an NMOS device at different temperatures. The drain current ID shows a kink below temperatures of about 30 K.

to 12 h may elapse before the temperature of LN2 is reached. Warmup to room temperature takes typically two to three days.

Experiments showed that measurements are possible during warmup, too. We found that the thermal capacity of our dewar is quite large, so that a small amount of energy introduced during the measurements does not dis- turb the system too much. To minimize the energy input we use computer support to do the measurements as fast as possible. If the power dissipation is limited to a maximum of about 5 mW and the measurements are done in a few minutes, the relative variation of the temperature is, even in the worst case, less than 5 percent. For most of the measurements on MOS devices, it is no problem to keep within these limits.

Since the computer has full access to the temperature sensors and the IEC-bus instrumentation, it was possible to develop a software package that repeats a user-defined measurement sequence during warmup at selectable temperature values. Due to the full automatic acquisition of measurement data, we are able to study the variation of device characteristics versus temperature over the full range from 4.2 to 300 K.

VI. EXPERIMENTAL A detailed presentation of the results of our measure-

ments is not the subject of this paper, but will be pub- lished elsewhere. Just to give some examples that illus- trate the possibilities of our test assembly, we refer to the current kink effect [3], [13], [14].

As illustrated in Fig. 9 this anomalous rise of the drain current occurs at temperatures below about 30 K. Accord- ing to Fig. 10 this corresponds to the on-set of substrate freeze-out.

We found that the current kink effect is related to other low-temperature anomalies, too. For example, low-frequency noise shows a maximum if the drain source voltage is set to the specific value that marks the current kink. In a small interval around a “critical” value, the increase of noise is so significant that it is possible to detect the maximum directly on the screen of an oscilloscope. In

103) I O 100

Temperature T [K]

Fig. 10. Substrate freeze-out observed at an n-well resistor with a surface and a junction depth of about 6 concentration of about 1 x 10l6

Lcm .

(Kink)

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--Jm I

1 0 - 2 0 0 100 1000 10000 10

Frequency f [Hz]

Fig. 11. Noise equivalent spectral power density versus frequency of an NMOS device at 300 K and LHe temperature for different drain-source voltages.

small-signal analysis the noise maximum is observed as a floating of the drain current dc level, since the spectral noise distribution is dominated by very low frequencies (see Fig. 11).

VII. SUMMARY AND CONCLUSIONS A computer-controlled test measurement setup was pre-

sented that enables full automatic acquisition of device characteristics in the temperature range from 2 to 300 K. The required equipment, the special construction of a suitable cold work area inside of a helium dewar, and a typical cooling procedure were discussed in detail. Some typical measurement data give an impression of the studies of device behavior versus temperature that are possible with our test assembly.

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131 0. Kindl, W. Langheinrich, and G. Fischer, “Cry0 CMOS technology,” in Proc. Symp. Low Temperature Electronics and High Tem-

KINDL AND LANGHEINRICH: TECHNIQUE FOR MEASUREMENTS O F SEMICONDUCTOR DEVICES 1541

perature Superconductors, vol. 88-9. Electrochem. Soc., 1987, p. 518.

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*

Ottmar W. Kindl was born in Darmstadt, West Germany, on April 9 , 1958. He received the M Sc. degree In electncal communication engi- neering from the Technical University of Darm- stadt, West Germany, in 1984 He is currently working toward the Ph D. degree at the Solid State Electronics Laboratories of that university. His interests are in the field of CRYO-CMOS technology.

*

Werner A. Langheinrich was born on January 12, 1934 in Ulm, West Germany. He received the Diplom degree in 1959 and the Dr rer.nat degree in 1962 in the field of physical chemistry from the University of Stuttgart

He then joined the Research and Development Laboratories of AEG-Telefunken Company and was head of silicon device technology Since 1977, he has been a Professor at the Technical University of Darmstadt, West Germany. He is a cofounder of the Solid State Electronics Labora-

tories of that university

Documents

Technique for measurements of semiconductor devices between 2 and 300 K