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SEMI G78-0699 © SE MI 1999 4 is to be entered into the d ata collection tab le as pa rt of the application of this met hod. 6.2.4 The alg orithm collects data fro m 9 wafers out of a lot of 10. Three set ups are perform …

SEMI G78-0699 © SEMI 19993

5.17 scrub mark — mark left by the probe in the

bonding pad or bump after the probe card has touched

down on the wafer.

5.18 temperature testing — testing of devices at a

controlled temperature level other than ambient.

5.19 test — one complete run-throu gh of the data

collection portion of this document on one automated

wafer prober.

5.20 tester — specialized computer controlled system

designed to test integrated circuits.

5.21 prober communications protocol (see Section

9.6) — means of transmitting data between the tester

and the prober. Common methods are:

• TTL

• RS-232c

• GPIB

5.22 test head — package of electronics (part of the

Tester) which interacts both electrically and

mechanically with the probe card, typically through the

Prober Tester Interface (PTI).

5.23 throughput — rate at which die can be cycled, by

the automatic wafer prober, for the purpose of being

tested. It is inherent in the functionality of an automatic

wafer prober (or any other motorized positioning

device) that accuracy, repeatability and throughput are

intimately and inseparably related.

5.24 touch down — contact between the probe card

and the wafer. This user may choose to define this as

either first electrical or first mechanical contact.

5.25 wafer — semiconductor substrate upon which

multiple die are fabricated.

5.26 wafer boat or wafer cassette — carrier for

multiple wafers.

5.27 wafer chuck — platform withi n an automatic

wafer prober that supports and transports the wafer.

The chuck may contain the means for controlled

temperature testing.

5.28 x, y, z and

— motions relativ e to the center of

the Probe Card when standing in front of the Prober:

• Motion to the right is motion in the positive X-

direction.

• Motion towards the back of the prober is motion in

the positive Y-direction.

• Motion away from the floor is the positive Z-

direction.

• Motion revolving around a Z-axis passing through

the center of the probe card is θ-motion. Motion in

the counter-clockwise direction when facing down

from above the prober is motion in the positive θ-

direction.

5.29 z-clearance — distance between the user defined

initial contact point and the top surface of the wafer

during that portion of the wafer prober’s cycle when the

wafer chuck is moving the wafer between DUTs.

6 Summary of Method

6.1 Objective – A SEMI Probe Sta ndards Task Force

has defined this method. That task force consisted of

members from semiconductor wafer probe system users

and wafer probe system suppliers. This method, in part

a guide to collection of data, is aimed at a specific set of

wafer probe system parameters: accuracy, repeatability

and throughput. This method is a tool that creates

comparative data. That data will act as a criterion by

which multiple wafer probe systems can be judged

competitively.

6.2 Probe System Accuracy and Throughput – A wafer

probe system will have needle placement error due to

the probe system electromechanical systems, and

additional needle placement error due to the probe card

physical alignment of the needles. The degree of

accuracy to which the probe system can place the

chuck, the effectiveness of the probe system's bond pad

to needle alignment, the physical alignment of the

probe card needles in their X and Y plane, and the

probe system's vision resolution and accuracy (x

microns of distance per pixel) will be the factors that

influence a probe system's overall placement

performance.

6.2.1 Probe system comparative acc uracy and

repeatability are established in this method using probe

mark data acquired manually with a vision system or, if

available, through use of an automated probe mark data

analysis system. Acquired data is analyzed with the

Probe Mark Data Analysis algorithm contained in this

method.

6.2.1.1 The intention of this data colle ction exercise is

not to make a deterministic conclusion establishing a

probe system’s accuracy. Data analysis results are only

meaningful in the context of a comparative analysis of

multiple probe systems.

6.2.2 Probe system throughput is be st determined

using time measurements made using a stopwatch.

Other approaches can be applied, such as:

• the time stamp and log file (if available) on the

probe system under evaluation.

• time tracking within the device test program

employed for testing the prober.

6.2.3 Any of these approaches is via ble for measuring

time intervals during probe system operation. That data

SEMI G78-0699 © SEMI 1999 4

is to be entered into the data collection table as part of

the application of this method.

6.2.4 The algorithm collects data from 9 wafers out of

a lot of 10. Three setups are performed using 3 wafers

per setup. All dice will be probed, but data will be taken

only on twelve of the die on each wafer. Twenty-four

pads on each die are evaluated (see Figure 4). Overall,

2592 die pads are analyzed in a lot.

6.2.5 When all the data is collected and analyzed, each

probe system will have an average die offset value.

This will be a comparative representation of accuracy

consisting of average offsets for all evaluated die. This

comparative representation of accuracy is a measure of

how accurate the probe systems place the chuck, and

thus the probe needles, to the center point on the die

pads, consistent with normal operation of the probe

systems.

6.2.6 In general, when the probe marks are viewed

across all dice, there will be a data spread, or

distribution of error points for each wafer.

Repeatability is the ± 3σ variation of all die offsets,

identified in this method as the 3σ calculation of

Normalized Die Offset.

6.3 Probe Card Issues – Probe card construction

variability and probe card usage at temperatures other

than ambient are important considerations. The

following sources of error should be kept in mind:

• In a hot chuck environment the probe card and

needles will experience a high percentage of the

elevated chuck temperature. The material selection

for the probe card will determine how it expands

and contracts due to the temperatures applied.

Probe systems can be equipped with programmable

preheat (soak) times. Longer preheat times will

reduce probe card variability while decreasing

throughput.

• Needle construction can result in excessive error

due to bending of the needles when excessive

probe system z-stage overdrive is applied. The

number of needles and the selection of needle

technology, i.e., cantilever versus vertical, is

another variable, having a noticeable influence on

measurement results.

• Since each user of this method is not confined to a

standard for probe card construction, the user of

this method is advised to choose a probe card and

vendor with good integrity, and to use that same

probe card when evaluating multiple probe

systems. The assertion here is that the same probe

card used to evaluate multiple systems will react in

a repeatable manner under varying environmental

conditions. This assumes there is no excessive

probe card needle wear during the multiple

evaluations, and that needle alignment is verified

or achieved before each execution of the

procedure.

6.3.1 Probe mark scrub length can vary due to several

factors including:

 variations in the flatness of the chuck and

stage travel that are not compensated by the z-

sensor mapping algorithms

 by hard spots in the aluminum pads, or

 by variations in probe tip geometry, etc.

6.3.2 To minimize the impact of this variation in the

probe mark analysis algorithm, use only scrub mark

location data that is taken normal to the direction of

scrub. Distances measured normal to the orientation of

the scrub mark are generally accepted to be

significantly more stable than that which is taken

parallel to the scrub mark (see Figure 2).

6.3.3 In summary, the probe card itself can be a

limiting factor when making needle placement accuracy

measurements, especially at varying environmental

conditions. Material selection, vendor to vendor

variation, construction of a probe card (blade, epoxy

ring, vertical, etc.), especially with varying

environmental conditions, will create inconsistent

analysis results, unless care is taken with the

application of this method. The precautions discussed

here are meant to promote consistent and accurate

evaluation results for this method.

6.3.4 Nevertheless, the precautions mentioned here

could ALSO be an important basis for using this

method. As an example, a probe card is typically

designed for XY positional placement and planarity,

and is expected to meet customer specification

requirements in normal operation at room temperatures.

This method could serve as a means of establishing

numerical results that represents the effects that

temperature or probe card construction variability have

on probe card specification requirements.

6.4 The PMA Algorithm – A best-f it rectangle can be

drawn around the scrub mark and the passivation

opening for each pad, reference Figure 3.

NOTE: Applying a best-fit rectangle around the passivation

opening may prove difficult for certain vision systems.

Application of the best-fit rectangle around the die pad metal

is an acceptable alternative. The center position of the best-fit

rectangle around the scrub mark will represent the center of

the scrub mark. The center of the die pad is the center of the

best—fit rectangle around the passivation opening. The

distance between the two centers is the pad offset. Pad offset

is computed from the four values left, right, top, and bottom,

reference Figure 2.

X offset =

(right – left)/2 + left

Die pad

Cente

Lef

Y Pads- Scrub mark &

scrub mark cente

Die pad

Cente

Top

Bottom

X Pads- Scrub mark &

scrub mark cente

Y offset =

(top – bottom)/2 + bottom

Right

Offset

Figure 2

X and Y Offset

Scrub mark

and scrub mark

center

Die pad

Passivation opening and best fit

rectangle

Best fit

rectangle

Die pad center

Figure 3

Scrub Mark Analysis

6.4.1 Vision system measurements will be made

establishing offset distances from the die pad center to

the scrub mark. The stability of the measurement is

greater when made perpendicular to the direction of the

scrub mark. This will establish X and Y offsets via

measurement of offsets for pads in the X and Y plane of

the die.

6.4.2 Once the offsets are established, a two-step

procedure will manipulate 1296 X and 1296 Y offset

values. The end result of the algorithm will be a

relative measure of the total probe system accuracy and

repeatability for the pads analyzed. Figure 5 represents

a visual summary of the method.

6.5 Considerations of Scale — When evaluating the

suitability of a particular probe system to probe a

particular size of bond pad, or to probe accurately at a

particular pad-to-pad pitch, a good rule of thumb is that

the prober’s positional accuracy, as stated in its

specifications, should be 1/10 that of the scale of the

features to be probed. For example, if the probe system

being evaluated has an overall placement accuracy of

5µm, it would be inappropriate to analyze wafers using

this method and this probe system for dice that have

pads less than 50 µm square (this would be less than

10x).

6.5.1 Regardless, when this method is used for

evaluation of multiple systems, it is essential that die

and bond pad size/pitch consistency be maintained from

evaluation to evaluation if the results are to be

meaningful.

6.5.2 The same 10× rule applies to the vision system

or automated Probe Mark Analysis system employed to

make the pad offset measurements. These systems

should have a pixel resolution that is at least 10× finer

than the die pad dimensions associated with the scrub

marks being measured.

6.6 Conclusion — It is assumed th at the user of this

method has a wafer probe system or systems, or is

planning to make a selection from the various systems

available in the market place, and requires objective

comparative analysis for accuracy, repeatability and

throughput.

6.6.1 It should be clear that this method employs a

three-step process of probing, probe mark data

collection, and data analysis.

6.6.2 When probing, every die on ea ch of the ten

wafers is to be “tested” and probed. Of those ten

wafers, the last nine with 12 die per wafer will be used

for scrub mark data collection. At least 24 pads per die

will be used for the analysis. The first wafer is meant

only to allow stabilization for the probe system and its

probe card.