Agilent 4142B Specifications Sheet

HP 4142B Application

aa

HEWLETT
PACKARD

HIGH SPEED MEASUREMENT OF FET

Reprinted from the IEEE proceedings of

Microelectronic Test Structures Conference

March 1989

ANALOG FEEDBACKUNIT

;-- ---- ------ ------------

Vth

AT LOW Id

Proc. IEEE 1989 Int. Conference on Microelectronic Test Structures, Vol 2,

NO.

1, March 1989

31

HIGH SPEED MEASUREMENT OF FET Kh AT LOW

HIDEYUKI NORIMATSU

Yokogawa Hewlett Packard

9-l,Takakura-cho, Hachioji, Tokyo, Japan

Phone: [phone]

Abstract,: High speed measurement of FET threshold volt­age at low drain current has been achieved by using an analog
feedback method with proper guarding techniques. The method
can be oplied to process control test structure FETs and/or to
characterize parasitic FETs which may be formed in high den­sity hlOSLS1 circuits.

Introduction

It is popular to monitor LSI manufacturing process by mea-

suring characteristics of test devices. There are many measure-

ment items such as leakage current, breakdown voltage, transis­tor characteristics etc. Measurement of FET threshold voltage
(C;,) is very useful, but it is time consuming. Fast and accurate

I/t,, measurement at lower drain current is required especially

for recent high density device/process evaluation.

For spot

Vt,,

measurements, the gate voltage has to be con­trolled to set the drain current to a defined value. To control
the gate kroltage, two different methods are available, digital

feedback and analog feedback. Digital feedback is widely used
because of its easy and stable operation. But measurement re­quires a great deal of time. Analog feedback allows high speed
and accurate measurement, but it needs some technical skill to

Id

get optimum circuit parameters without oscillation problems.
We have developed a system to provide fast, accurate a.nd sta­ble

Vth

measurements using analog feedback method.

Analog Feedback unit (AFU)

Fig.1 shows a block diagram of the system (analog block),
which consists of two Source/Monitor Units (SMUs) and an
Analog Feedback Unit (AFU). AFU controls the Search chan­nel SMU so that the monitor value of the Sense channel SMU
sets to preset value. SMU has two modes of operations, voltage
source and current source. In the Vth measurement, voltage

source mode is used. The output current and voltage of SMUs
can be measured by the A/D converter, which is not shown in

Fig.1, in the instrument.

Measurement steps are as follows:

(1) Set gate voltage to the search start value, and apply

drain voltage.

(2) A voltage ramp, of a selected slew rate, is applied to the
gate searching for a target value. In this mode the analog
feedback loop is open. (Search mode)

IMcasurement system

I

,_______--___-___-_-____________________----------------

1
1 I

1 I

I

Search SMU

feedback loop

I

Semr SMU

Pig.1 ScIlcmat,ic drawing of ;malog block

CH26Y3-0/X9/0000-031$01 .00~:198’) IEEE

’ I

I

I