H. Kristiansen, K. Redford, Z. L. Zhang, J. Y. He, M.

Fleissner, P: I. Dahl, Development and characterisation of

micrometer sized polymer particles with extremely narrow

size distribution. 12th IEEE International Symposium on

Advanced Packaging Materials: Processes, Properties and

Interfaces (ISAPM 2007), San Jose/Ailicon Valley, USA,

Oct. 3-5, 2007.

Development and Characterisation of Micrometer Sized Polymer Particles with Extremely Narrow Size Distribution

H. Kristiansen1, K. Redford1, Z. Zhang2, J. Hee2, M. Fleissner3 and P. I. Dahl3

1: Conpart AS 2: NTNU

3: SINTEF Material and Chemistry helge@conpart.no

Abstract This paper describes size characterisation and

mechanical characterisation of micrometer sized polymer particles with an extremely narrow size distribution. Typical applications for this type of particles are as conductive particles (plated with metal) in Anisotropic Conductive Films (ACF) or as spacers for LCD or chip stacking.

A number of different instruments and techniques have been investigated to be able to measure size, size distribution and frequency of “off-sized” particles. Due to very tight specifications, none of the instruments were able to provide the full set of information needed, and a combination of different techniques is needed.

Also greatly improved technique for mechanical characterisation of such polymer particles are reported in this paper. Adapting a commercial Nano-Indenter and optimising sample preparation and testing procedure have obtained very reproducible and consistent results.

Introduction Conpart AS is a Norwegian company specialising in the

manufacturing, characterization and application of polymer particles with a size range from a micrometer and upwards. The unique manufacturing process gives very interesting properties for use in electronics and micro-system applications such as conductive particles in anisotropic conductive adhesives (ACA/ACF) and as spacers in LCD or chip stacking. Some of the unique properties of these particles includes an extremely narrow size distribution, a vide variety of possible chemical compositions and hence tailor-making mechanical and thermal properties. Coating of the particles with different surface group as well as metals or oxides is possible. It is therefore possible to manufacture particles with a large range of different mechanical, electrical, optical and other properties to tailor-make particles for different applications and needs.

The manufacturing process is based on the patented Ugelstad process[1]. The process is based on seed polymerisation and yields particles with a very narrow size distribution, with only a small fraction of “off-sized” particles. Off-size particle is here defined as a particle with diameter more than 10 % larger than the nominal diameter. An advanced post processing technique has been developed to reduce the amount of “off-size” particles down to ppm levels. A SEM photograph of such particles is shown in Figure 1

Two of the most demanding applications for such particles today are as spacers for flat panel displays and as

conductive particles for anisotropic conductive film (ACF), where the polymer core is plated with metal. For both these applications the most critical properties are the particle size and size distribution together with mechanical properties [2, 3]. A detailed description of the ACF bonding process is outside the scope of this paper and can be found elsewhere [2, 4]

Typically such particles have been obtained by classifying from a particle lot with a wide size distribution. Such a process becomes very much more difficult as the particle size is reduced and the required width of the size distribution is reduced.

Characterisation of particle size A typical batch of particles after polymerisation will

have a size distribution looking somewhat like Figure 2. The main peak corresponds to the main population of particles, which will typically be within ±0.2 micron from the targeted particles size (for particles smaller than 30 micron). The next two peaks represent particles with significantly larger diameters. These are typically particles with double or triple volume compared to the main peak. Also small fractions of much larger particles as well as small clusters and bulk polymerisation can be present but these can easily be removed by traditional screening. In the characterisation of the particle size we are looking for three parameters. One is the mean size of the particles in the main peak, which we call nominal particle size. The next parameter is the amount of “off-size” particles in the finished lot. For a number of applications an important part of the post processing of the particles is to remove “off-

Figure 1: SEM picture of 3.25 micron acrylic particles

1-4244-1338-9/07/$25.00 ©2007 IEEE 135 IEEE Advanced Packaging Materials Symposium

size” particles (particles with a diameter > 1.1 times the nominal diameter). The last parameter is to measure the width of the size distribution. This is typically measured as the coefficient of variation (CV), which expresses the standard deviation as a percentage of the sample mean. The fact that the particles are spherical simplifies the characterisation as the size of a particle is fully described by one scalar (the diameter of the particle). This also means that the orientation of the particle with respect to the “observer” is not an issue. We have tested a number of instruments and techniques to be able to determine the three parameters.

Laser diffraction Laser diffraction is the most widely used technique for

particle size analysis. Instruments employing this technique are considered easy to use and particularly attractive for their capability to analyse over a broad size range in a variety of dispersion media. In laser diffraction particle size analysis, a representative cloud or ‘ensemble’ of particles passes through a broadened beam of laser light which scatters the incident light onto a Fourier lens. This lens focuses the scattered light onto a detector array. A particle size distribution is inferred from the collected diffracted light data, using a rather complex inversion algorithm [5] including parameters as complex refractive index.

A sample of particles was tested using a Mastersizer 2000 from Malvern. Particles were dispersed in water and put into the test chamber. Different parameters were used for the inversion algorithm by varying the real and imaginary part of the refractive index and also using “multiple narrow modes”, and “spherical particles” as constraints in the mathematical inversion. Some of the results are shown in Figure 3.

To obtain reasonable results it was necessary to “force” the instrument to operate in “multiple narrow modes”, or other vice it was looking for a wide size distribution. It was also clear that the results were very dependent on the

refractive index, mostly on the imaginary part (absorption). A good estimate for this value can not easily be obtain from literature, and for practical purposes we were forced to turn the problem around and look for refractive values that gave one single peak. For most of the values tested, we obtained a size distribution with two or more peaks. Once the settings were optimized we obtained a single peak distribution with a mean size of approximately 3.8 micron which is similar to what we obtain with optical microscopy and SEM. However, the measured size distribution is significantly wider than the real distribution, and the technique is not able to determine the presence of “off-sized” particles.

Coulter counter The Coulter counter (named after Wallace Coulter) is probably the most accurate instrument for particle sizing available today. Particles suspended in a weak electrolyte solution are drawn through a small aperture, separating two electrodes between which an electric current flows. The voltage applied across the aperture creates a "sensing zone". As particles pass through the aperture (or "sensing zone"), they displace their own volume of electrolyte, momentarily increasing the impedance of the aperture. This change in impedance is used to calculate the volume of the particle through the sensing zone. However, particle size analysers using the electrical-zone method assume that particles pass through an orifice tube one at a time. In fact, two or more particles will sometimes pass through the orifice tube roughly simultaneously. This is called coincidence, and is a source of errors in the measurements from these devices [6]. Coincidence reduces the observed number of particles and

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Figure 3: Results from laser diffraction. Top and middle: Particle size distribution for nR: 1.5+i0.01 and nR: 1.5+i0.001 respectively. Bottom comparing measurement (green) and mathematical model (red) for nR: 1.5+i0.001.

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Figure 2: Typical particle size distribution before Confine™ process (note the logarithmic scale)

increases the observed average particle size. The first effect can be corrected simply and is not critical for our application. Correction of the second effect is much more difficult and is also much more critical for determining the fraction of off-sized particles. Also surface conductivity of the particles can influence the measurement results. However, the highly insulating properties of the polymer particles are assumed to have negligible influence on the result.

Figure 4: Results from Coulter counter

The same batch of particles was tested in a Multisizer 3

(Beckman Coulter), and the results are shown in Figure 4. Approximately 10,000 particles are counted and binned into 300 different size bins between 1.5 and 7.5 micron. This process was repeated 3 times. The mean particle size was found to be 3.801, 3.803 and 3.806 micron respectively with a standard deviation of between 0.070 and 0.072 micron. This gives a CV of 1.86%. The absolute size is obtained by calibrating with a “reference” particle that which size has been measured in optical microscope.

A very careful analysis reveals that there are a few counts (less than 10) with a diameter above 4.5 micron. It is, however, impossible to know whether these are due to actual off-size particles or due to coincidence. There is also a number of counts at smaller diameter particles. Such particles are very rarely observed in microscopy, and we believe the present counts are due to “contamination” from the sample fluid. Such contamination can be caused by tiny air-bubbles or dust from the atmosphere.

Optical microscopy The fact that the particles tend to form “crystal like”

structures when dried out from a suspension can be used to measure the absolute size of the particles. A row of particles, typically 10 is measured. In this way, the uncertainty related to the wavelength of light is reduced by an order of magnitude. The particles measured in Figure 5 are from the same batch as those shown in Figure 1.

Optical microscopy can also be used to look for “off-size” particles. However, it is not easy to use this technique to obtain statistical data. This is partly because the eye is drawn towards such particles because they break up the

symmetry, but also because these particles tend to occupy certain positions during the sample preparation.

Flow Particle Image Analyser A flow particle image analyser combines a microfluidic

device with advanced image processing. The purpose of the microfluidic device is to confine the flow of the particle dispersion into a very thin and defined layer between two liquid sheet flows. An attached optical microscope can therefore accurately image the particles. This system is able to image and analyse of the order of 10,000 particles per minute. Each particle image is stored together with information regarding “effective” diameter and eccentricity (deviation from circular image). Due to limitations in resolution caused by optical effects, this instrument can not be used to measure accurate size or size distribution. However, due to the attached image, it is easy to discriminate between a single particle with double volume and two particles in close proximity.

A number of tests have been made using a Sysmex FPIA-3000 instrument from Malvern instruments. Figure 6 shows a typical plot of particle diameters found in a sample

Figure 5: Optical micrograph used to measure particle size

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Figure 6: Plot of particle diameters observed by Flow particle Image Analyser

of 450,000 particles. There are a number of interesting observations in this plot. First of all, the width of the main peak is very much wider than the real size distribution of the particle sample (as observed with Coulter counter and microscopy. This is due to optical effects. Also the nominal particle size has been shifted from 4.83 (measured in microscopy) to more than 5.0 micron. Calibration is performed with standard divinylbenzene particles which have different composition and optical properties to our particles. In the results we also observe some particles at smaller size (2, 3 and 4 micron). These particles are contaminations from previous tests of 3.0 and 3.8 micron products and a 2 micron calibration sample. This illustrates one of the problems with this instrument. All the test samples (water dispersions) are put into the same sample holder and passed through the same plastic tube and valve before entering into the test zone. A tine fraction of the particles adhere to the tube wall during this process. Even though we do a lot of cleaning between each sample we can not avoid that there still are contamination left in the tubing when testing a new particle batch.

On the other side of the curve we can see a small peak (around 30 particles) with size around 6.3 micron and 3 more particles at around 7 micron. These are real “off-size” particles, corresponding to double and tipple volume.

Scanning Electron Microscopy SEM can certainly also be used to obtain size

information about the particles. Given a well calibrated instrument this is a very accurate technique. Statistical analysis will be very time consuming and are also subjected to similar problems regarding sample preparation as optical microscopy. In addition there is a need to coat the particles with a conductive layer (typically gold). The electrical contact resistance to the substrate also limits the accelerating voltage and probe current that can be used before Joule heating will destroy the particle.

“Environmental” effects Comparing different techniques for size determination

clearly showed that the particle size is dependent on the procedure used for the measurement, even when the instruments that are used are well calibrated. The variations are quite significant, giving variations from 4.68 to 5.0 micron for a nominally 4.8 micron particles. This corresponds to approximately ±3 %. These results are partly due to influence from the “environment” and partly due to the nature of the instrument.

When polymer particles are suspended in a dispersing agent (water in our case), there will develop equilibrium between the water outside the particle and inside the particle. This results in a slight swelling of the particle. This swelling will be dependent on the “osmolality” of the water (that is the amount of dissolved molecules etc..) as well as the chemical composition and cross linking of the particle. Pure water will give a higher swelling ratio than water with dissolved molecules, that is, the particles will be larger. On the other hand, when water is removed (dried out), the particle will slightly shrink. Also water with a high “osmolality” (for instance the electrolyte used in the

Coulter) will tend to extract water from the particle. This effect will of cause depend on the concentration of the electrolyte and the nature of the particle.

Table 1: Comparison of measured particle size using different characterisation techniques

Microscopic techniques are number based techniques.

That is, each particle will have the same contribution to the statistics, irrespective of it´s size. However, Coulter Counter is a volume-based technique, meaning that the larger particles will have a stronger impact on the statistics. The influence on the mean size will therefore also scale with the CV of the size distribution. Table 1 shows the results regarding particle size obtained from different techniques.

Mechanical properties To measure the mechanical properties of the particles we

are using a modified nanoindenter from Hysitron (Triboindenter). This instrument is equipped with a high resolution load – displacement cell and a diamond indenter tip which has a polished flat with a diameter of approximately 100 micron. This equipment makes it possible to indent single particle with very high accuracy and reproducibility [7].

The particles are distributed onto a polished silicon wafer, and single particles with sufficient distance form nearest neighbours are selected for indentation. A lot of work has been done to optimise the experimental procedure. As a consequence, the measurements give highly reproducible and consistent results.

In Figure 7 we compare force-deformation curve for three particles from the same manufacturing batch. The results are very reproducible which is typically the case for all the tests from a single batch. At a load close to 9.0 mN, which correspond to approximately 1 GPa, the particles

Technique Measured size

Comments

Coulter 4.93 (±0.02) Partly dried out due to electrolyte, however, this is counteracted by the “volume based” technique that tends to increase the size

“Dry” microscopy

4.84 (±0.05) Dry particles

“Wet” microscopy

5.00 (±0.05) Measured in pure water (max swelling)

SEM (array) 4.89 (±0.02) Dry particles (due to high vacuum) Sticking to substrate, causing some particles to loose contact with each other (creating small gaps)

SEM (single) High magnification

4.68 (±0.1) Highly dried out particles Possibly also “distorted” by high temperature due to Joule heating by the E-beam

crack. Due to this cracking, the “recovery” of the particle is negligible during the de-loading.

In Figure 8, we have compared the stress versus strain relationship of three types of particles with different size (3.27, 3.81 and 4.85 micron). The stress is calculated as applied force divided by cross sectional area of the particle (at the centre) and the strain is the ratio between the deformation and the diameter of the particle. The graph shows that particles with different sizes (and from different batches) have a very similar stress-strain relationship. For the larger particles, the maximum load of 10 mN is not sufficient to destroy the particle. For these we observe a high recovery.

Conclusions We have developed and characterized polymer particles

in the 3 to 5 micron range that are well suited for different applications within microelectronics and microsystem technology. Due to the extreme narrow size distribution we have not been able to find a single instrument capable of characterizing the particle size, size distribution and frequency of “off-sized” particles. A Coulter counter is the best choice with respect to measurement of size distribution, whereas this instrument has problems to find off-sized

particles due to the problem with coincidence. FPIA is suitable to look for “off-sized” particles but as we approach ppm levels, problems with cross-contamination from previous batches becomes a big problem.

Acknowledgments The authors acknowledge the financial contribution to

this project from the Norwegian Research Counsel under the NANOMAT program.

References 1. PCT-Application No. PCT/GB00/01334 filed 10. April

2000 2. H. Kristiansen, Z. L. Zhang and J. Liu,

“Characterization of Mechanical Properties of Metal-coated Polymer Spheres for Anisotropic Conductive Adhesive”, IEEE Advanced Packaging Materials 2005, 0-7803-9085-7/05, Sec 8-2

3. S. Ichikawa, K. Suekuni, M. Ishimaru, H. Nakatani, T. Unate, A.nakasuga “Investigation of Cell-Gap Defects Using Gap Simulation”, IEICE Transactions on Electronics 2006 E89-C(10):1390-1394

4. J.H. Constable, "Analysis of the Constriction Resistance in an ACF Bond," IEEE Transactions on Components, Packaging and Manufacturing Technology, Sept. 2006, Volume: 29, Issue: 3, page(s): 494- 501

5. Laser diffraction paper 6. E. J. W. Ynn, M. J. Hounslow, “Coincidence correction

for electrical-zone (Coulter-counter) particle size analysers”, Powder technology, 1997, vol. 93, no2, pp. 163-175

7. J. Y. He, Z. L. Zhang, H. Kristiansen, “Mechanical properties of nanostructured polymer particles for anisotropic conductive adhesives”, Int. J. Mat. Res. (formerly Z. Metallkd.) 98 (2007) 5

Figure 8: Stress versus strain for three different particle sizes

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Figure 7: Measured force versus deformation curve. Particle size is 3.25 micron