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Chip Scale Review January • February • 2018

[ChipScaleReview.com]

Reducing wafer test time

By Klemens Reitinger

[ERS electronic GmbH]

uring a typical thermal wafer

test set up, the temperature will

influence every other boundary

condition, like the mechanical accuracy or

electrical performance. There are different

ways to deal with these influences, but they

all require long “adaption times” in which

the test cell is just waiting for the system to

reach the right temperature. This article will

explain why it is so important to generate new

solutions that reduce the expensive waiting

time in the overall test process, and how

ERS’ new thermal wafer test chuck system is

designed to improve this longstanding issue.

Data and discussion about a new generation

of thermal chucks will be presented that

will confirm how the technology can help

shorten the time needed for a test set up to be

“ready for test.” The article concludes with

a discussion of the outlook for the future of

semiconductor testing in general, and also

as it relates to emerging products such as

microelectromechanical systems (MEMS)

and sensors.

Background discussion

Wafer test is an integral part of every

wafer fabrication line today (

Figure 1

).

There are numerous steps performed on

a silicon wafer – 300mm diameter being

the industry standard today – until the

devices are fully functional. A common

characteristic of all these process steps

is that they are performed at the wafer

level, so ever y process step creates

hundreds or even thousands of chips,

which is cost-effective. However, as soon

as the wafer is singulated into individual

devices, the subsequent development

becomes substantially more expensive.

The next steps are usually performed on

a tiny piece of silicon, which is a time-

consuming process operated by intricate

and costly machinery.

To know the status and performance

of every die on the wafer previous to

singulation is an important element to

improve yield. Therefore, the wafer has

to go through a process called wafer

test, that ensures that only the good

dies are further processed. The wafer

test is performed in a test cell, which

consists of an elect r ical tester (the

most expensive piece of equipment), an

electromechanical interface to the wafer

(a probe card), a wafer chuck (in this

case, the source of temperature for the

wafer test) and a wafer manipulator, also

called a wafer prober. As we are talking

about semiconductors, it is obvious, that

temperature is a key parameter for this

test. Today, there are many tendencies

suggesting that temperature is getting

more and more impor tant, and that,

at some poi nt it may even become

the most important parameter of all.

Additionally, the temperature range is

getting wider and wider, as many of

the microelectronic devices are now

being used in rougher environments,

for example, for autonomous driving,

I nter net of Th i ngs ( IoT), or power

electronics for regenerative

energy harvesting.

Howeve r, d e a l i ng w i t h

temperature is one of the less

explored areas in electrical

testing of a device, one of the

reasons being that a testing

process usually means a lot

of nonproduct ive t ime. I n

the past, the test time was

very simply calculated by the

time the chuck needs for the

temperature transition, e.g.,

from 25°C to 200°C. Now,

as the structures are getting

smaller and smaller, the simple transition

time of the chuck is subject to temperature

influences that significantly slows down

the process. With an increasing demand

for smaller measurements, the industry is

now facing complex challenges to a much

greater extent than before.

Mechanical accuracy

If you change the temperat ure on

the chuck top from 25°C to 200°C, the

environment, especially the bottom side

of the chuck, will also get very hot. This

leads to mechanical thermal drift in the

area of the wafer prober and probe card.

One might think this temperature range

is too “small” to have any major effect

on the wafer test, however, this is not

the case.

The linear coeff icient of thermal

expansion (CTE) for different materials

is very low, especially for ceramics or

metals. The value for stainless steel

is 14x10

-6

K

-1

, wh ich means t hat by

changing the temperature of a 1mm

piece of steel by 1°C, the length will

change by 0.000014mm. This might

not sound like a lot, but if you calculate

a temperature change of 100°C on a

100mm long device, it will amount to

a 0.14mm change, which is 140µm. If

you assume that a pad size of 50µm has

to be contacted by a needle, it becomes

obvious that this is far too much for a

stable operation.

Today, there are multiple ways to deal

with the problem noted above, and the

D

Figure 1:

A typical semiconductor manufacturing

process.

Figure 2:

Soak time comparison of a regular chuck system and the

new solution.