NISTIR 7345 

Screening Candidates 

for 30 nm Spheres

Michelle K. Donnelly 

Jiann C. Yang 
NISTIR 7345 

Screening Candidates 

 for 30 nm Spheres 

Michelle K. Donnelly 

Jiann C. Yang 

Building and Fire Research Laboratory 

 August 2006 

U.S. Department of Commerce

Carlos M. Gutierrez, Secretary

Technology Administration

William Jeffrey, Acting Under Secretary for Technology

National Institute of Standards and Technology

 William Jeffrey, Director
Abstract 

This paper describes the preliminary screening process that was conducted to identify 

potential candidate particles available from commercial sources to be used as National 

Institute of Standards and Technology (NIST) Standard Reference Material (SRM) for 

30 nm spheres. Five different samples were obtained and measured using the NIST 

particle calibration facility. The sample with the narrowest size distribution and a 

preliminary peak particle diameter of 30.1 nm was identified as a potential candidate to 

become the NIST 30 nm SRM. A secondary candidate, with a fairly narrow distribution 

and a peak particle diameter of 24.2 nm was also identified. Further studies to address 

accuracy issues and ensure repeatability are recommended.

Introduction 

An initial screening process to identify candidate particles that could be used as the NIST 

Standard Reference Material (SRM) 30 nm spheres was conducted. NIST currently has 

polystyrene latex (PSL) spheres available as SRMs as small as 60 nm. The SRM 

particles are used primarily by the semiconductor industry to calibrate and test metrology 

instruments. The Semiconductor Industry Association has developed an International 

Technology Roadmap for Semiconductors (ITRS) specifying the need for smaller 

reference materials as semiconductor components and devices become smaller [1]. The 

ITRS discusses critical semiconductor components scaling to 32 nm or smaller by as 

early as 2008. 

In response to the need for accurately sized particles, NIST has developed a calibration 

facility to accurately measure particles in the submicrometer range. The calibration 

facility uses electrostatic differential mobility analysis to measure the effective diameter 

of aerosolized particles. Compared to other methods, such as light scattering or electron 

microscopy, differential mobility analysis has lower uncertainty for measuring small 

particles and offers good statistics, because each test measures thousands of particles. 

Details regarding the calibration facility, including theory of operation and uncertainty 

assessments are included in the document by Donnelly and Mulholland [1]. Updates to 

the facility including the use of an electrospray aerosol generator and added uncertainty 

issues are discussed in Mulholland, et al. [2]. 

Process 

NIST obtains commercially available submicron particles and accurately measures their 

size. To obtain the best possible sample to serve as the SRM for 30 nm particles 

numerous companies were contacted. Samples of monosize particles suspended in liquid, 

with a nominal diameter of 30 nm ± 5 nm were acquired. One of the difficulties in 

obtaining the samples was that despite industry needs, many of the companies did not 

manufacture particles this small in diameter. In the end, five candidate samples were 

obtained. 
The samples were measured using the NIST particle calibration facility. A dilute liquid 

suspension containing the particles is placed into an electrospray aerosol generator where 

the particles are aerosolized. The particles then travel through an electrostatic classifier 

where they are separated by size using a controlled electric field. A monodisperse 

aerosol leaves the classifier and the particles are counted by a condensation particle 

counter. A computerized data acquisition system records the particle count and electrical 

field strength to determine the size and distribution of each sample. 

Prior to screening the 30 nm samples, the system was calibrated using the SRM 1963, 

with a diameter of 100.7 nm, whose measurement is traceable to the length standard. The 

system was checked to verify that the equipment was working properly and 

measurements were within the uncertainty range. During sample testing, calibration runs 

of the SRM 1963 were performed both before and after the sample measurements to 

correct for atmospheric conditions and any possible drift in the flows. Each candidate 

sample was tested twice and, the peak diameter was measured relative to the SRM 1963. 

Results 

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

15 20 25 30 35 40 45 50

Sample A

Sample B

Sample C

Sample D

Sample E

Normalize

d Count

Diameter (nm)

Figure 1. Particle size distribution as a function of the normalized count. 
The five candidate samples for the 30 nm SRM were measured using the NIST 

calibration facility, and the peak particle diameter was computed for each sample. The 

peak particle diameter represents the modal diameter, which is the peak in the number 

size distribution. The particle count for diameters between 15 nm and 50 nm was 

measured. Figure 1 shows the size distributions for the five candidate samples. 

Representative data points are included to distinguish the samples. Samples A, B, C, and 

D all have distinct peaks for the diameter range measured. The plot of Sample E shows 

that there was no discernable peak in the particle diameter over the size range of 15 nm to 

50 nm, as the number concentration continually increased over the size range examined. 

The preliminary, uncorrected mean peak diameters and the estimated uncertainty in the 

diameter measurements are listed in Table 1. For Sample E, a peak was not identified in 

this range. 

Table 1. Preliminary mean peak diameter and estimated uncertainty. 

Particle Sample Preliminary Peak Diameter (nm) Estimated Uncertainty (nm) 

A 32.1 0.9 

B 30.1 0.8 

C 24.2 1.0 

D 29.2 1.4 

E undetermined N/A 

When selecting a sample to be used as an SRM, it is desirable for the peak of the 

distribution to be close to the target diameter, in this case 30 nm. Peak size, however, is 

not the only criteria for a good sample. The size distribution should be fairly symmetric, 

with no shoulders or ledges on the sides of the peak. A narrow distribution culminating 

with a steep peak is most desirable. A quantitative way to define the narrowness of the 

sample peaks is to compare the widths of the distributions as a function of the normalized 

particle count. The widths of the distribution were found using the plots of the particle 

size distributions. The width of the distribution was measured for each sample at the 

normalized particle count of 0.8 and 0.6. Figure 2 shows an example of the distribution 

width measurements conducted for Sample B. The measured widths at 0.8 and 0.6 for all 

of the samples, excluding sample E, are listed in Table 2. Of the nominally 30 nm 

samples tested, sample B had the narrowest distribution, measuring 4.7 nm at 0.8 and 

7.2 nm at 0.6. For reference, the width of the distribution for the 100 nm SRM 

(SRM 1963) is 3.0 nm at 0.8 and 4.5 nm at 0.6. 

Table 2. Width of the distribution at normalized particle count. 

Sample Width (nm) at 0.8 Width (nm) at 0.6 

A 9.0 13.9 

B 4.7 7.2 

C 6.5 9.4 

D 11.5 16.7 

E N/A N/A 
0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

20 25 30 35 40

Norm

aliz

e

d Cou

nt

Diameter (nm)

Sample B

width = 4.7 nm

width = 7.2 nm

Figure 2. Graph of the width of the distribution for Sample B. 

Based on the criteria for selecting a sample to become the nominal 30 nm SRM, 

Sample B is the best candidate, because it has the narrowest and most symmetrical size 

distribution. The preliminary peak diameter of 30.1 nm is also very close to the target 

diameter of 30 nm. The second choice would be Sample C. Its size distribution is 

slightly wider than Sample B, but is fairly symmetrical. Also, the peak diameter of 

Sample C is small, measuring 24.2 nm, so it would be a nominally 25 nm sample. 

For screening purposes, two measurement runs were performed for each sample in order 

to obtain preliminary measurements of the peak particle size and the distribution. When 

determining the certified measurements for the SRM certificate, a series of at least 10 

measurements are performed to obtain accurate statistics and uncertainty. Additionally, 

the results given here for the peak diameters are the preliminary, uncorrected 

measurements used to screen the candidates. It is possible to have a small drift of the 

electrospray generator flow during the sample run which can slightly skew the 

measurement. Error due to this drift can be minimized and corrected by taking drift 

measurements during the sample test, but it was not done here because the error is small 

(approximately 0.07 nm) and this was preliminary screening. Another source of error 
that can effect the measurements is the transfer function of the electrostatic classifier. 

The transfer function is the probability that a particle entering the classifier will exit 

through the passage at the end of the differential mobility analyzer [3]. Due to the 

operating principle of the system as the particle size increases, slightly more of the larger 

particles are counted, shifting the peak count to a larger number. Because the particles 

are nominally the same size, the transfer function correction would be nominally the 

same for all of the samples and would not be necessary during the preliminary screening. 

Conclusions 

Five candidate samples were obtained and tested, and four of the five had peak diameters 

within 6 nm of the target diameter of 30 nm. The size distributions of these four samples 

were fairly symmetrical, though not as narrow as the 100 nm SRM. Of the samples 

tested, sample B is the best choice to become the 30 nm particle standard. It has the 

narrowest size distribution with a steep symmetrical peak. The preliminary, uncorrected 

peak particle diameter for sample B is 30.1 nm. The estimated uncertainty is ± 0.8 nm. 

Recommendations 

The interest for smaller SRM particles is present in the semiconductor industry. In order 

to have a well defined SRM in the 30 nm range, continuing work is needed. Multiple 

samples of the prime candidate should be obtained and measured for repeatability. 

Numerous measurements should be performed on varying days, and accounting for drift 

within the measurement should be taken. The issue of the transfer function would need 

to be investigated and the uncertainty computed for the 30 nm size. Also, the possible 

deposition of residue on the particles would need to be considered. Additional 

measurements using other methods, such as light scattering would be helpful in 

supplementing the work. 

References 

[1] Semiconductor Industry Association. International Technology Roadmap for 

Semiconductors (ITRS): 2005 Edition, available at http://public.itrs.net. 

[2] Donnelly, M. K., and Mulholland, G. W. “Particle Size Measurements for 

Spheres With Diameters of 50 nm to 400 nm,” NISTIR 6935; August 2003. 

[3] Mulholland, G. W., Donnelly, M. K., Hagwood, C., Kukuck, S. R., and Hackley, 

V. A., “Measurement of 100 nm and 60 nm Particle Standards by Differential 

Mobility Analysis,” Journal of Research of the National Institute of Standards 

and Technology, Vol. 111, No. 4, 257-312, July-August 2006. 

[4] Donnelly, M. K., Mulholland, G. W. and Winchester, M.R. “NIST Calibration 

Facility for Sizing Spheres Sustpended in Liquids,” AIP Conference Proceedings 

683, Characterization and Metrology for ULSI Technology: 2003 International 

Conference, Austin, Texas, 313-317, March 2003.