SOLAR CELL SURFACE INSPECTION USING 3D PROFILOMETRY

Transcription

1 SOLAR CELL SURFACE INSPECTION USING 3D PROFILOMETRY Prepared by Benjamin Mell 6 Morgan, Ste16, Irvine CA P: F: nanovea.com Today's standard for tomorrow's materials. 21 NANOVEA

2 INTRO: With the economy in its current tailspin and gas prices at record highs, the solar energy industry has been called upon to provide solutions for our energy crisis. Global concerns about the atmosphere and the effort to reduce greenhouse gas emissions are other reasons there has been a large acceleration of growth in the solar energy industry. Solar cell production is increasing at just under % yearly. The overall price tag for the buyer will be the most important factor for the solar energy industry to be successful. This task of decreasing the overall cost to the consumer can be achieved, thus producing higher profits for the solar cell manufacturer, only if the efficiency and quality of solar cell materials continues to improve. IMPORTANCE OF SURFACE METROLOGY INSPECTION FOR QUALITY CONTROL The white light axial chromatism technique utilized by Nanovea s 3D Non Contact Profilometers has become widely known as a vital bench top research and verification tool. It is now the Automated Optical Inspection (AOI) environments that have begun to acknowledge Nanovea s superior capability. Unlike the traditional vision and laser systems that typically sacrifice one feature for another, speed or resolution, Nanovea provides both. Vision and laser systems also have very limited automated surface measurement options, whereas Nanovea s system provides a wide range including: Profile, Dimension, Roughness, Shape & Form, Flatness & Planarity, Volume Area, Step-Height Depth and Thickness. And keep in mind, the technique utilized by Nanovea s inspection system has the superior ability to measure nearly any material surface and zero influence from sample reflectivity or absorption. Nanovea s standard inspection system stage speed can reach 1m/s, up to times faster than most optical systems in its class. It is equipped with a 31KHz white light axial chromatism sensor and XY measurement area of 4mm x 6mm, which at maximum stage speed can measure 1 point every 32μm and traverse the full 4mm in less than 1 sec. (higher resolution can be obtained with proportionally slower stage speeds). In addition, the system allows Nanovea PRVision (auto-recognition) on precisely chosen surface features with little to no user interaction through user-friendly software. To reach speeds suitable for more time constraint production and quality control environments a line sensor can be used providing 1 x 18 array of measurement points and can scan up to 18 lines per second to create and overall scan rate of up to 324, points per second. The combination of superior overall features makes Nanovea s HS1 system unquestionably the instrument of choice for inspection demands found in Semiconductor, Microelectronics, Solar and Medical industries among many others. Nanovea s inspection capability can be provided for stand alone or inline integration and is the most competitively priced inspection option in the industry. MEASUREMENT OBJECTIVE In this application, the Nanovea HS1 is used to measure two different types of solar material (photovoltaic material and glass) and calculate their respective surface roughness, surface features and wafer bow. The shape of trace lines on the photovoltaic material will also be measured. With a maximum stage speed of 1 meter per second, the HS1, unlike stylus profilometers, can easily handle high production throughput applications, where many measurements are needed in quick fashion. 2

3 TEST RESULTS: SOLAR MATERIAL ROUGHNESS VALUES Ssk Sku Sq Sp Sv Sz Sa GLASS nm 74 nm nm 14.7 nm nm PHOTOVOLTAIC μm 4.12 μm μm 7.89 μm μm GLASS FALSE COLOR HEIGHT REPRESENTATION mm mm nm GLASS EXTRACTED PROFILE nm Length = 2 mm Pt = 14.1 nm Scale = 1 nm mm 3

4 GLASS 3D SURFACE nm PHOTOVOLTAIC FALSE COLOR HEIGHT REPRESENTATION mm mm

5 PHOTOVOLTAIC EXTRACTED PROFILE Length = 2 mm Pt = 13.1 Scale = mm PHOTOVOLTAIC 3D SURFACE

6 PRINTED TRACE EXTRACTED PROFILE Length = 1 mm Pt = 4.9 Scale = mm PRINTED TRACE PITCH MEASUREMENT mm Horizontal Distance: -1 = 2.14mm 2-3 = 2.4mm PRINTED TRACE HEIGHT MEASUREMENT mm Height Difference: -1 = 21.3μm 2-3 = 26.12μm 4- = 27.7μm 6-7 = 26.73μm 6

7 PRINTED TRACE WIDTH MEASUREMENT (ZOOMED ON TWO TRACE LINES) mm Horizontal Distance: -1 = 168.9μm 2-3 = 21.4μm PRINTED TRACE FALSE COLOR HEIGHT REPRESENTATION PRINTED TRACE 3D SURFACE 7

8 PYRAMID STRUCTURES COMMONLY FOUND ON PHOTOVOLTAIC MATERIALS D REPRESENTATION OF WAFER BOWING mm

9 TEST DISCUSSION: From the surface roughness parameters, it can be observed that, as expected, the glass surface was much smoother than the photovoltaic material surface. As stated previously, glass solar material is as smooth as possible to avoid scattering and absorption of light. The printed trace lines can quickly and easily be measured for height, width, and pitch. All of these measurements are very significant quality control inspections needed to evolve and improve solar cell efficiency and performance. CONCLUSION: The HS1 can be used to provide precise measurements at multiple stages within solar manufacturing. Due to the superior measurement technique of Nanovea s Profilometers, light absorbing materials can easily be measured along with the flexibility to measure glass roughness and wafer bowing; all with a single non-contact instrument. These measurements can be automated by creating and running a Macro measurement in the Nanovea 3D Software for a quick and very efficient method in which to measure a large quantity of materials (samples). Monitoring surface roughness, feature dimensions and flatness of solar cell material are extremely useful for quality control inspection and regulating the various production processes. The Nanovea HS1 is a perfect choice for quality control inspections of the surface of solar materials, which can lead to higher efficiencies and increased profits for solar cell manufacturers. For more information visit Nanovea Automated Optical Inspection 9

10 MEASUREMENT PRINCIPLE: The Chromatic Confocal technique uses a white light source, where light passes through an objective lens with a high degree of chromatic aberration. The refractive index of the objective lens will vary in relation to the wavelength of the light. In effect, each separate wavelength of the incident white light will re-focus at a different distance from the lens (different height). When the measured sample is within the range of possible heights, a single monochromatic point will be focalized to form the image. Due to the confocal configuration of the system, only the focused wavelength will pass through the spatial filter with high efficiency, thus causing all other wavelengths to be out of focus. The spectral analysis is done using a diffraction grating. This technique deviates each wavelength at a different position, intercepting a line of CCD, which in turn indicates the position of the maximum intensity and allows direct correspondence to the Z height position. Unlike the errors caused by probe contact or the manipulative Interferometry technique, Chromatic Confocal technology measures height directly from the detection of the wavelength that hits the surface of the sample in focus. It is a direct measurement with no mathematical software manipulation. This provides unmatched accuracy on the surface measured because a data point is either measured accurately without software interpretation or not at all. The software completes the unmeasured point but the user is fully aware of it and can have confidence that there are no hidden artifacts created by software guessing. Nanovea optical pens have zero influence from sample reflectivity or absorption. Variations require no sample preparation and have advanced ability to measure high surface angles. Capable of large Z measurement ranges. Measure any material: transparent or opaque, specular or diffusive, polished or rough. Measurement includes: Profile Dimension, Roughness Finish Texture, Shape Form Topography, Flatness Warpage Planarity, Volume Area, Step-Height Depth Thickness and many others. A.1

11 1 z(x, y) dxdy A A 1 A A z2 (x, y)dxdy 1 Sq 3 [1 A A z3 (x, y)dxdy] 1 Sq 4 [1 A A z4 (x, y)dxdy] A.2