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SEMI MF1529-1104 © SEMI 2004 7 Table 2 Nominal Values of the Standard Resisto r and of the Center-leg Resis tor, r , for the Analog Circuit Appropriate to Various Sheet Resi stance Range #1 The resistance shall be wit hi…

SEMI MF1529-1104 © SEMI 2004 6

Figure 1

Schematic of Measurement Circuit Showing Use of a Standard Resistor

and a Probe Wired for Configuration A

NOTE 7: to semiconductor effects occurring at higher

measurement currents are avoided. However, for certain

applications of interest, such as metal films and very heavily

doped silicon substrates, it will generally be necessary to

accept measurements with fewer significant figures or else to

use a current supply with an output above the common

maximum of 100 mA, or else to use a voltmeter with sub-

microvolt resolution.

7.5.1.2 Standard Resistor — Used to monitor the value

of the measurement current if the current supply does

not meet the accuracy of settability given in Section

7.5.1.1. The standard resistor shall be selected to give a

potential difference of 0.5 to 5 times that measured

across the specimen. This requires the standard resistor

to have a value from 2.5 to 25 times the sheet resistance

of the layer. The value of the standard resistor must be

known at least to four significant digits.

7.5.1.3 Switches — Double pole, double throw switch

for reversing the direction of the current, and four pole,

double throw switch for changing the probe

configuration. The switching functions may be

accomplished by wafer switches or relays. Isolation

between all switch poles or relays must be 10

times the

sheet resistance of the layer being measured; isolation

of 10

, or greater is recommended.

7.5.1.4 Electronic Voltmeter — To read the potential

difference across the specimen and standard resistor, or

if calibrated in conjunction with the current source, to

read the voltage-current ratio directly. The voltmeter

shall be capable of measuring dc voltages between 1

and 100 mV full scale, and be able to resolve the

measured voltages to 0.01%, or better. The meter must

have an input impedance of at least 10

.

7.5.2 Analog Test Circuit — Five resistors connected as

shown in Figure 2 shall be used according to the

measurement electronics qualification procedure of

Section 11.3 for evaluating the accuracy and precision

of the electronics in the presence of large series

resistors simulating the probe contact resistances.

Several circuits of this type may be needed with the

resistance of the central resistor, r, of each being,

selected according to the expected sheet resistance of

the layer to be measured, as listed in Table 2.

Figure 2

Analog Test Circuit, Simulating the Contact

Resistances in a Four-Probe Measurement

7.5.3 Conductivity-Type Instrument — Apparatus in

accordance with Method A of SEMI MF42.

7.6 Computer Control — It is intended that this test

method will be under control of a computer for

positioning the sample at each of the intended

measurement sites, lowering the probes, and performing

all necessary control of circuit switching, setting of

current values, and measuring and logging voltages. It

is beyond the scope of this test method to specify

details of the computer-based automation.

SEMI MF1529-1104 © SEMI 2004 7

Table 2 Nominal Values of the Standard Resistor

and of the Center-leg Resistor, r, for the Analog

Circuit Appropriate to Various Sheet Resistance

Range

The resistance shall be within a range from one-half to twice the value listed

and its value shall be known to 0.05%.

8 Reagents and Materials

8.1 Purity of Reagents — All chemicals for which such

specifications exist shall conform to the assay and

impurity levels of Grade 1 SEMI specifications for

these specific chemicals. Other grades may be used,

provided it is first determined that the chemical is of

sufficiently high purity to permit its use without

lessening the accuracy of the test.

8.2 Purity of Water — Reference to water shall be

understood to mean Type E-3 or better deionized (DI)

water as described in ASTM Guide D 5127.

8.3 Qualification Wafers

8.3.1 Polished Silicon Wafers — Of any convenient

diameter for making probe impressions to be inspected

for general probe related damage and contact size and

shape. It is useful to divide the surface into rectangular

regions by use of scribe lines or similar technique to aid

in locating a particular series of impressions under the

inspection microscope.

8.3.2 One, or more, wafers from each of the processes

to be evaluated, for testing the electrical suitability of a

given probe for the intended evaluation.

8.4 Reagents for Surface Treatment — If surface

treatment is required, the following chemicals may be

needed:

8.4.1 Buffered HF — 10:1 or more dilute, in

accordance with SEMI C23.

8.4.2 2-Propanol — In accordance with SEMI C41.

8.4.3 Acetone — In accordance with SEMI C19.

8.4.4 Filtered dry nitrogen.

9 Sampling

9.1 In the case of referee measurements, it is left to the

parties to the test to agree upon the number of wafers

from a batch, and their selection procedure, as well as

the number and location of test positions on each wafer.

9.2 In the case of non-referee measurements, for

example, process control or research applications, it is

left to the user of this test method to determine the

number and location of test positions on each wafer.

10 Suitability of Test Specimen

10.1 The front and back surfaces of the wafer to be

measured should be tested for conductivity type using

Method A of SEMI MF42. If they are of the same

conductivity type, a thin sheet of insulating material,

such as mica, should be placed between the wafer and

the stage. This test is not necessary if the front and

back surfaces are known to be of opposite conductivity

type or if the layer is fully isolated from the substrate

by a dielectric layer.

10.2 If the wafer to be measured was fabricated by a

process that uses “finger” clamps or other types of

clamping that intrude into the top surface area of the

water, the wafer is unsuitable for use with sampling

plans that require measurements within several probe

spacings from the wafer perimeter.

11 Preparation of Apparatus

11.1 Visual Inspection of Probe Impressions — This

inspection should be performed when a new, rebuilt or

reconditioned probe is first installed to get an initial

indication of the mechanical performance of the probes.

Once a probe is installed, meets the visual inspection

criteria, and is left mounted, visual inspection of probe

impressions is generally not needed; functional probe

performance tests detailed in Section 11.2 generally

suffice to qualify the probe for continued use. Further

visual inspection of probe impressions is advised,

however, when a probe has trouble meeting the

requirements of Section 11.2.1. More routine

inspection of probe impressions is also advised if

probes are interchanged routinely for special

applications, thus increasing the risk of changes in the

alignment or rigidity of the probe mounting.

11.1.1 After selecting a probe for the intended

application, make a series of at least 10 probe

impressions on a polished silicon surface in steps of 50

to 125 m (0.002 to 0.005 in.). Examine the

impressions from each of the pins to determine that

there is no probe skidding, no cracks or fracture lines

surrounding any of the impressions, and that the

impressions are generally compact in nature (see Figure

3).

11.1.2 If fracture lines are seen, the probe must be

replaced, or conditioned on surfaces of ceramic,

sapphire, lapped silicon or other suitable, non-

Sheet Resistance,



Analog and Standard

Resistor,



<2.5 1

2–25 10

20–250 100

200–2,500 1,000

2,000–25,000 10,000

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)