Recent progress on the measurement methodology of surface roughness inside the PFA tubes
Hitoshi Imamura
Product R&D Department, Chemicals Division
Daikin Industries, Ltd.
Preface
PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer) is a perfluoro elastomer-based fluoropolymer that can be melt-processed (1). Its molecular weight is in the hundreds of thousands, and its melt viscosity is 0.01 - 0.1MPa・s (380°C). Various PFA products optimized for compression molding, extrusion molding, injection molding, etc. are commercially available. Due to its excellent purity and chemical resistance, PFA is increasingly in demand as a material for components such as chemical liquid tubes, joints, valves, pumps, filter housings and wafer carriers in semiconductor plants (2) Recently, due to the miniaturization of semiconductor chips, contaminating particles that affect the yield are also nano-sized.
Surface roughness of PFA tubes
Fluoropolymers used in this study
1. Material properties
The surface roughness of tubes was evaluated using commercially available PFA listed in Fluoropolymer Handbook compiled by the Japan Fluoropolymer Industry Association (3). The tube was processed by an extruder with a cylinder diameter of 30mmφ using the external sizing method with water-cooling bath. The molding temperature conditions are described in Table 1.
Table.1 Raw material properties evaluated for PFA tubes
2. SEM observation of inner surface of PFA tube
SEM observations were performed as in Fig.1 on PFA1 (NEOFLON PFA SH, manufactured by Daikin Industries, Ltd.) with a spherulite size of ~15µm (15,000 nm) and commercially available PFA2 with spherulite size ~5 µm. Apparently from visual observation, unevenness of both two tubes made from different material grades are identical in spite of the difference of the size of the spherulites Also, the surface roughness of “on the boundary” and “off the boundary” didn’t show the significant difference. This indicates it is necessary to quantify the unevenness of the “intra-spherulite” rather than that of the “inter-spherulite”. SEM observations of the boundaries between spherulites showed that there were no valleys at the boundary and the boundary area has a surface structure similar to that of the overall spherulite surface, which is also shown in Fig.1.Fig.1 SEM observation of PFA tube inner surface and spherulite boundary
Comment: There is no groove at the boundary. Surface condition equivalent to spherulite surface roughness.
Methods for measuring surface roughness
1. Current method
Conventionally, surface roughness of the inner surface of PFA tubes has been measured by the carapace-like pattern formed by spherulites in the range of evaluation length of 400-4000µm. This method of measuring surface roughness is described in the SEMI standard (F57-0301), JIS standard (B0601) and ISO standard (4287) for semiconductor components. For most cases, the values in the commercial tube for semiconductor application in some catalogs are measured in the same way.
Other general example of the measurement method of the surface roughness on the processed products are a stylus type surface roughness meter and the non-contact laser method. SEMI standard (F57-0301), a representative standard for PFA components for semiconductors, states that the surface roughness of the inner surface of a PFA tube should be Ra ≤ 0.25µm (250 nm). For measurement of the inner surface roughness of a PFA tube, the tube is cut, and its inner surface roughness is measured in the longitudinal direction with a stylus type surface roughness meter. According to the JIS standard (B0601), the ISO standard (4287) and other standards which specify the measurement method for surface roughness, the evaluation length of the roughness curve is determined by the size of surface roughness Ra. A detailed explanation is given in literature (4) in the journal of the Japan Society for Precision Engineering. In the standards, the evaluation length is specified depending on the surface roughness Ra (µm).
If 0.1<Ra≦2, the evaluation length is 4mm. If 0.02<Ra≦0.1, the evaluation length is 1.25mm. Furthermore, if Ra≦0.02, the evaluation length is 0.4mm.
If the evaluation length is 0.4 to 4mm, i.e., 400 to 4000µm, and the spherulite size is 4 to 50µm, the surface roughness Ra can be determined as an unevenness dependent on the spherulite size.
2. TEM observation of PFA tube cross section
Across-section of a 1-inch tube was observed by high acceleration voltage transmission electron microscopy (TEM) using frozen ultrathin sliced piece samples. An example of observation of PFA3 is shown in Fig.3. This cross-sectional TEM image shows that the inner surface of the tube has fine unevenness ranging from a few nanometers to tens of nanometers.Fig.2 Calculation method of arithmetic average roughness Ra
[ Arithmetic mean roughness Ra ]
Evaluation length ℓ is extracted from the roughness curve in the direction of the average line, and the absolute values of the deviations from the average line of this extracted portion to the measurement curve are summed and averaged.
Fig.3 Freezing ultrathin section TEM of cross section of PFA tube
3. Measurement by atomic force microscopy (AFM)
For nano-sized contaminating particles, nano-sized roughness of the spherulite surface must be studied. The author measured the surface roughness of a tube in a 3μm square measurement area, with an evaluation length smaller than the spherulite size, by an AFM under the following conditions. Surface roughness Ra was measured using an AFM5200S (HITACHI High-Tech) large high-precision probe microscope unit, in the dynamic force mode. A measurement area of 3μm of the sample surface was measured with a scanning speed of 1Hz, x-y direction 256×256 division, and cantilever SI-DF-20 (Si, f=134kHz, C=16N/m). The AFM topographic image was calculated by performing an automatic inclination correction process.
Meanwhile, as stated in the literature (2), the quality of injection-molded products depends greatly on the surface roughness of the mold metal. There is no carapace-like pattern of spherulites on the surface that has contacted the mold. There is also no spherulite surface on a surface cut by machining. These surfaces are rougher than the inner surface of an extruded tube. The SEMI standard (F57-0301) for the surface roughness of the inner surface of a PFA tube is Ra ≤ 0.25µm, while that for injection molded products is Ra ≤ 0.38μm and that for machined products is Ra ≤ 0.62µm. In these cases, surface roughness may be increased by scratches, and conventional method is suitable as the evaluation method.
4. Comparison of PFA materials and various surface roughness measurement methods
The surface roughness Ra of spherulite surfaces of tubes made with PFA1, PFA2, and PFA3 was compared by AFM. The measurement results are shown in Fig.4, Fig.5, and >Fig.6. Surface roughness measurements of various measurement areas, i.e. different evaluation lengths, are summarized in Table 2. With an evaluation length of 3μm, the results for PFA1 and PFA2 tubes were Ra = 2.8(nm). For PFA3, the result was Ra = 5.9(nm). Conventionally, discussion on surface roughness assumes that foreign contaminating objects sized 20 to 60nm locates and sticks in the valleys at the boundaries between spherulites. But this is unrealistic as illustrated in Fig.7 if an actual scale were taken into account. The measurement of roughness of the spherulite surface proposed in this paper is suitable for imagining foreign objects sized 2 to 6nm adhering to uneven surfaces. This is assessed to be a realistic measuring method.Fig.4 AFM observation of the inner surface of the PFA tube
Fig.5 AFM observation of the inner surface of the PFA tube
Fig.6 AFM observation of the inner surface of the PFA tube ( 3μm square area )
Table.2 Summary of PFA tube surface roughness measurement results>
Note*
1) In the table, Ra represents surface roughness, which is calculated by extracting the evaluation length from the roughness curve and summing and averaging the absolute values of the deviations from the average line of this portion to the measurement curve.
2) Rp-v (or sometimes represented as Ry) is calculated by extracting the evaluation length from the roughness curve and summing Yp from the average line of this portion to the highest peak and Yv to the lowest bottom, i.e. Rp-v = Yp + Yv
Fig.7 Spherulite size and surface roughness image of spherulite surface
5. Analysis of nano-size surface roughness
The unevenness on spherulite surfaces are formed by lamellar bundles, or bundles of lamellar crystals consisting of folded molecular chains. The size of the lamellar bundle is thought to determine the size of the nm-size unevenness (smoothness). PFA is a copolymer of TFE and PAVE and the size of the lamellar bundle may vary depending on the type of PAVE and the amount of nucleating agent, and it has been realized that this leads to variation in nano-level smoothness. Figure 8 shows the result of SEM observation of spherulite surfaces by Elionix ERA-9000.
Fig.8 SEM observation of spherulite surface of PFA tube
Fig.9 Electron beam three-dimensional roughness analysis
( Releationship between valley depth Rv and valley number )
Conclusion
References
- (1) Takaomi Satokawa, Ed., Fluoropolymer Handbook, Nikkan Kogyo Shimbun (1990), pp. 3-341
- (2) Hitoshi Imamura: Clean Technology, November issue, 5 (2002)
- (3) Japan Fluoropolymer Industry Association, Fluoropolymer Handbook, (revised 13th edition, 2014
- (4) Journal of the Japan Society for Precision Engineering, Vol. 78, No. 4, (2012), pp.301-304
Note** This technical report was originally published in the December 2019 issue of Clean Technology, to which some additions have been made.
Note***All the data shown in this report are not guaranteed.