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SEMI MF1529-1104 © SEMI 2004 9 where: r fi and r ri = the calculated values for the analog box resistor, with current in th e forward an d reverse directions, res pectively, for the i th measurement, R s = the value of t…

SEMI MF1529-1104 © SEMI 2004 8

contaminating material until sets of impressions can be

made that do not exhibit fracture. After a conditioning

process, it is useful to clean the probe tips with

methanol on a cotton swab to loosen debris that may

have collected in the tip.

11.1.3 If probe skidding is seen, tighten or otherwise

adjust the probe clamp and probe lowering mechanism

to eliminate skidding.

11.1.4 If non-compact probe impressions are seen,

acceptable data may result, but the probe tip(s) are

generally in an advanced state of wear and may not be

stable with use.

11.2 Probe Performance Verification — This test must

be performed before any series of referee

measurements, or if a type of layer is being measured

for which there is not past experience regarding proper

selection of probe radius, load and conditioning. It

qualifies a probe for taking highly repeatable

measurements, as are needed for mapping spatial

variations of sheet resistance, but does not ensure

accuracy of measured value. It should be performed

separately for each type of layer to be measured.

11.2.1 Select a wafer of the type to be checked for

uniformity. At five different locations that are

reasonably well separated on the wafer, make a series

of 10 measurements following the procedure of

Sections 12.4 to 12.6, 13.1, and 13.2, using steps no

larger than 100 m between each of the 10

measurement positions. Calculate the average and

standard deviation for each of the five sets of

measurements. For a probe to be satisfactory for use in

measuring layers of this type, the standard deviation

must be no more than 0.1% of the average for at least

four of these sets.

11.3 Qualification of Measurement Electronics — The

suitability and accuracy of the measurement electronics

shall be verified immediately prior to a referee

measurement unless the equipment is separately

demonstrated to be under statistical control for

measurements of that process. For non-referee

applications, use of the analog circuits is helpful for

troubleshooting or performance monitoring of the

electronics.

11.3.1 Analog Test Circuit

11.3.1.1 With the current supply short-circuited or

turned off, attach the current leads from the analog test

circuit of the appropriate resistance value to the current

supply and connect the potential leads designated V (see

Figure 2) from the analog circuit to the input of the

voltage measuring instrumentation. Allow sufficient

time for the electronics to warm up in accordance with

the manufacturers instructions.

a b c d

All probe impressions were made with steps of about 50

m between impressions, and using probes loaded more

heavily than would normally be done for measuring thin

films; this was done to provide better photographic detail.

Figure 3

Photographs of Three Indentations Each from (a) a

Satisfactory Probe Tip, (b) a Badly Worn Probe Tip,

Probe Tip Showing Skidding

NOTE 8: Caution: Constant-current power supplies often

operate at output voltages of several hundred volts when not

connected to a load. Any changes of connection to a

constant-current supply should be made either with the

current supply turned off or with its output short-circuited.

11.3.2 With the current initially in either direction (to

be called “forward”) adjust its value to give a measured

voltage of between 7 and 12 mV (see Table 1 for

nominal values). Record the current, I

, through the

analog box, or measure the potential, V

, across the

appropriate standard resistor connected in series with

the analog box. Measure the potential, V

, across the

analog box. Reverse the direction of the current and

record the current, I

, or measure the potential V

across the standard resistor; measure the potential, V

across the analog box. Repeat this procedure until ten

sets of data have been taken. All values measured and

recorded must be known to at least four significant

figures.

11.3.3 Calculations for the Analog Test Circuit

11.3.3.1 Calculate the resistance of the analog box

resistor for both forward and reverse directions of

current using the appropriate form of the equations

according to whether, or not, a standard resistor was

used as follows:

sar

saf

r  and (1)

SEMI MF1529-1104 © SEMI 2004 9

where:

and r

= the calculated values for the analog box

resistor, with current in the forward an

reverse directions, respectively, for the

measurement,

the value of the standard resistor, ,

and V

= the potentials measured across the

analog box in the forward and reverse

directions, respectively,

and I

= the values of the current in the forward

and reverse directions, and

and V

= the potentials measured across the

standard resistor in the forward an

reverse directions.

11.3.3.2 For each of the ten pairs, r

and r

, calculate

the average, r

)(

rifimi

rrr  (2)

11.3.3.3 Calculate the overall average,

of these ten

values of r





mim

(3)

11.3.3.4 Calculate the sample standard deviation, s

, of

the ten values as follows:









mmia

rrs

(4)

11.3.4 Requirements for the Analog Test Circuit — For

the electrical equipment to be suitable for referee

measurements at the sheet resistance value just

simulated, it must meet the following requirements:

11.3.4.1 The value of

must be within 0.25% of the

known value of the resistor, r, and

11.3.4.2 The value of the standard deviation, s

must be

no greater than 0.1% of

NOTE 9: If not previously known, the value of the analog

circuit resistor may be determined by using the procedure of

Sections 11.3.1 to 11.3.3.2 but measuring the potential across

the analog box using the connections V in Figure 2.

12 Procedure

12.1 Specimen Preparation

12.1.1 If the specimens have been kept in a clean, non-

contaminating atmosphere, or are to be measured within

3 h after fabrication, proceed to Section 12.2.

12.1.2 Remove possible organic contaminants that may

arise from the storage container as follows: Rinse the

specimen in acetone for 1 min. Remove. Immediately

immerse in 2-propanol for 1 min. Remove. Blow dry

with filtered dry nitrogen. Repeat if necessary until

specimen is free from visible stains, streaking or other

visual evidence of residue.

12.2 Mounting and Checking the Wafer

12.2.1 Using vacuum paddle, tweezers, or robotic

arrangement, mount the wafer on the stage, so that the

wafer center is within 1 mm of the stage center. Clamp

the wafer to the stage with vacuum or other means.

Lower the probes onto the wafer. If not known

measure the electrical resistance between any of the

probe pins and the stage to verify that the electrical

isolation is at least 10

.

12.2.2 Allow sufficient time for the wafer's temperature

to equilibrate with that of the stage. Thirty seconds is

sufficient if the wafer had been held at room

temperature prior to mounting. Longer times are

necessary if the wafer was recently removed from a

reactor or other elevated temperature process.

NOTE 10: The absolute value of the wafer temperature

during measurement affects the absolute values of sheet

resistance, particularly for lightly doped layers, but it should

not affect the measurement of uniformity of sheet resistance

values, as long as the temperature is constant within 0.2°C

during measurement of any wafer. However, for referee

measurements or process control applications, for which the

absolute sheet resistance values are likely to be an important

part of the measurement results, it is important to maintain the

stage temperature within a narrow range (e.g., 23 ± 1°C) or to

develop an empirical relation between sheet resistance and

measurement temperature for each layer fabrication process

of interest.

12.3 Measurement Site Selection — Using the

measurement site selection agreed to for a referee

measurement or that decided upon for process control

or other application, follow Sections 12.4 through 12.6

at each measurement site before raising the probe and

moving to the next site.

NOTE 11: Various site selection plans may be chosen

according to the application needs. These include, but are not

limited to: diameter scans with step sizes appropriate to the

spatial resolution needed, sparse sampling extensions of the 5-

point and 9-point plans described in SEMI MF81, and area

sampling plans with the size of the area and number of points

chosen by measurement time constraints and process

information requirements, such as the circular or rectangular

patterns of SEMI MF1618. This test method makes no

recommendation about choice of sampling plans, since user

needs and interests are widely varied.

NOTE 12: If wafer diameter scans are chosen, the probe

should not be oriented so that the pins lie exactly along the

diameter being scanned. Doing so would risk noisy

measurements if probes were placed in previous measurement

locations. This may be avoided by slightly misaligning the

probe pins with respect to the diameter.

12.4 Measuring in Configuration A to Obtain R

—

Connect the probe to the electronics so that the current

will flow through the outer pins (designated Pins 1 and

4), and that the specimen voltage will be measured by

the digital voltmeter (DVM) using the inner pins (Pins

2 and 3), Figure 4. Lower the probe into the wafer.

Start with the current in either direction (called

“forward”), and adjust the current to obtain a specimen

voltage between 7 and 12 mV, inclusive. It is

recommended that the current value be increased from

low levels until the required voltage is obtained, rather

than lowering the current from large values. Measure

the current (using the proper standard resistor, or by

monitoring the set point of a calibrated current supply),

and the specimen voltage, each to a resolution of at

least 0.01% of the value. Record the current reading as

f (1-4)

, and the voltage reading as V

f (2-3)

. Reverse the

direction of the current and again measure the specimen

voltage and current to the same resolution. Record the

current readings as I

r (1-4)

and the voltage reading as

r (2-3)

. The subscripts used for current and voltage are

those of the pins that perform the current-carrying and

voltage-measuring functions (see Caution, Note 8).

12.5 Measuring in Configuration B to Obtain R

—

Without raising the probe from the measurement site,

connect the probe to the current supply so the current

flows through one of the outer probes, and the

nonadjacent inner probe, Figure 5. (There are two ways

of doing this that should provide equivalent results.)

Connect the remaining two probes to the voltmeter,

making sure that the wiring polarity corresponds to that

used for the connections to the current supply. With the

current in the forward direction, measure and record the

current as I

f (1-3)

and the voltage as V

f (2-4)

), with the same

resolution as in Section 12.4. Reverse the direction of

the current; measure and record the current I

r (1-3)

and

the voltage as V

r (2-4)

NOTE 13: The notation in Section 12.5 assumes Pins 1 and 3

were used for current, and Pins 2 and 4 were used for

specimen voltage measurement. If the other choice of wiring

the probe for Configuration B were used, the subscript

notation in Section 12.5 (and Section 13.1.2) would be

changed accordingly.

12.6 Moving to the Next Measurement Site

12.6.1 Turn off, or short circuit, the output of the

current supply; raise the probe and move the stage to

the next measurement site. Follow the procedure of

Sections 12.4 and 12.5 at this and all other sites.

Figure 4

Schematic Wiring of Four-point Probe for

Measurement in Traditional Configuration A

Figure 5

Schematic Wiring of Four-point Probe in One of the

Two Choices for Configuration B