|Micro Laser Assisted Machining and Thermal Imaging:
Preferential Heating of the High Pressure Metallic Phase of Silicon
Lei Dong, University of North Carolina at Charlotte, Charlotte, NC 28223, U.S.A.
John A. Patten, Western Michigan University, Kalamazoo, MI 49008, U.S.A.
Experiments to demonstrate micro-laser-assisted machining (µ-LAM) of silicon were carried out to study the thermal softening effect of this heating technique. Preferential heating and resultant thermal softening, of the high pressure metallic phase of silicon, assists with plastic deformation and ductile material removal, such as occurs in a machining process.
The principle of operation, apparatus and experimental results of laser heating of the high pressure metallic of silicon are first introduced. Real time thermal images of a small volume (µm3) of the heated high-pressure metallic phase of silicon (-tin Si or Si-II) were taken using a mid-wavelength (2-6 µm), high spatial resolution (<5 µm) IR thermal camera. The thermal images indicate temperature increases restricted to the laser heated high-pressure phase material and not in the surrounding material. The results provide additional supporting evidence of the localized thermal softening that promotes a ductile response of hard- brittle materials.
Thermal-related issues in precision machining processes have always been a challenging topic. This paper presents experimental results for laser heating and thermal softening within a small tool-work piece contact zone (µm3) during a micro scale scratching/machining process. Applications for ductile regime machining, at the nano to micro scale, of hard-brittle materials are provided, reviewed and evaluated, and future work is discussed in this paper.
Keywords: Advanced/Hybrid Processes, Ductile Cutting/Machining, Micro/Nano machining/fabrication, Heat Treatment and Surface Engineering, Semiconductors and Ceramics.
Semiconductor materials and ceramics are important engineering materials. Semiconductor materials are vital to the microelectronics industry. Ceramics have many desirable properties, such as excellent wear resistance, chemical stability, and the superb ability to retain strength at elevated temperatures. These outstanding properties make them good candidates for long lasting components in harsh or extreme working environments, such as seals and bearings in engines . However, the undesirable machinability, i.e. difficult to machine nature of these materials, remains a major obstacle that limits the wider application of these hard, but brittle, materials. The plastic deformation in these brittle materials at room temperature is much less, or at a smaller size scale, than in metals, which means they are prone to fracture during material deformation and removal processes. Surface cracks generated during machining are routinely and subsequently removed via lapping and polishing, which significantly increases the machining cost .
Laser assisted machining (LAM) methods use lasers as the high energy-heating source to thermally soften or vaporize (ablate) the material to achieve material removal. Industrial applications for these laser processes usually operate at very high power level (kW or higher). The LAM technique used for semiconductor and ceramic material removal is considered a rough material removal process, as the surface roughness is on the order of micrometers . The heat affected zone for LAM processes can be controlled within a few millimeters in diameter and several micrometers deep.
The nano-micro level machining regime allows the use of low power lasers (mW), as contrasted to other LAM systems that operate on the macro scale using kW power lasers. The micro level system described in this paper is unique in that the combination of laser radiation (wavelength) and material properties (semiconductors and ceramics [4, 5]), coupled with the optically transparent diamond tooling, allows for the preferential heating of the zone or volume of material directly involved in the chip formation process during machining.
This paper reports on the demonstrated feasibility of using an IR laser to preferentially heat and soften the high pressure metallic phase of silicon during scratching, as a prelude to a µ-LAM system . This method can be adapted to machining other semiconductor and ceramic materials by choosing a laser with a suitable wavelength and a sufficient power level for each material and processing condition.
Laser-Diamond Hybrid System Design
A 5m radius single crystal diamond tip is attached to the facet of an IR laser diode (1480 nm, Furukawa) ferrule using UV-cured epoxy (Figure 1 a and b). Total laser power is measured using a high resolution (1 mW resolution) laser power meter. The maximum power decreases from its original 350mW to approximately 150 mW (at maximum laser diving current 1400mA). Approximately 60% of the laser power is lost after attaching the diamond tip to the ferrule. A detailed investigation into this power loss and laser beam profile measurements is provided elsewhere .
Preferential Heating of High Pressure Metallic Phase of Silicon: Experiments and Results
By properly choosing a laser source, at a wavelength that is transparent to covalent silicon, the transformed metallic silicon (having optical properties similar to a metal) is expected to block and partially absorb the laser energy. Thus the potential exists for detecting and possibly heating the high pressure metallic phase of silicon, without affecting the bulk covalent silicon.
In-situ IR detection of the high pressure metallic silicon has been demonstrated in precious work . Using the same apparatus as the in-situ detection experiments, two sets of IR preferential heating experiments are carried out. Figure 2 shows the results of the ‘scratch and stay’ test, for the scratch groove depth measurements under various laser power levels and heating times. Figure 3 shows the results of the ‘scratching speed’ test, with results for the scratch groove depth under various laser powers and scratching speeds. The depth of the scratch groove is a measure of the material’s “resistance to penetration” or (scratch) hardness, and is used to determine the relative amount or degree of thermal softening due to the laser heating.
ig 1 Diamond tip attachment for IR heating. (a) Diamond tip attached to the ferrule with UV-epoxy (b) close up on the diamond tip
There is an obvious transition point, observed in Figures 2 and 3, where a significant or measurable heating effect takes place. In the ‘scratch and stay’ test (Figure 2), at 120 seconds the transition occurs when going from 500 to 1000 mA of laser current. In the ‘scratching speed’ test, a transition occurs at the intermediate speed (0.02 mm/s) as the laser current (power) is increased from 250 mA to 1000 mA. These transition points or regions appear to result from a combination of resident time or scratching speed in conjunction with laser power. For example, in the ‘scratch and stay’ test, as shown in Figure 2, for conditions at or above (to the right of) 120 seconds and 1000 mA, result in considerable enhanced ductility, compared to conditions of 120 seconds and 500 mA and below (to the left). In Figure 3, there is an apparent transition ‘range’ which occurs at the intermediate speed (0.02 mm/s) and over the current range of 250 mA to 1000mA. At the fastest speed, (.305 mm/s) there is very little measured thermal softening effect. At the slowest speed (0.002 mm/s), the power level has little effect on the softening effect, suggesting that a saturation point has been reached.
Of particular significance is that the combination of parameters, i.e. laser wavelength selected for specific materials (such as silicon and silicon carbide), permits heating and thermal softening of the chip formation zone, while leaving the balance of the work piece unaffected (thermally). This is accomplished by selectively heating the high pressure phase transformed (HPPT) work piece material that forms as a result of the intimate contact at the interface with the cutting tool . Extremely high pressures (HP), > 10 GPa, are developed at the tool-work piece interface, due to the high hardness of these materials (semiconductors and ceramics), combined with the high hardness of the diamond tool and the extremely sharp cutting edge (radius measured in nanometers). Of particular importance is that these high pressure phases are generally ductile and metallic in nature, in contrast the nominally brittle nature of semiconductors and ceramics.
This preferential heating and resultant thermal softening will assist with plastic deformation and material removal in a ductile manner, such as in a scratching or machining process. The resultant softened material is expected to contribute to reduced tool wear and enhance the material removal rate, as the material is more ductile and thus easier to plastically deform.
Fig 2. Scratch and Stay Test: AFM groove depth measurements. The error bars are the standard deviation of the AFM measurements for each scratching condition. Load used in all tests is 25mN
Fig 3. Scratching Speed Test: AFM groove depth measurements. The error bars are the standard deviation of the AFM measurements for each scratching condition. Load used in all tests is 25 mN
IR thermal imaging of heating the high pressure metallic phase of silicon
A PhoenixTM camera system with Real-Time Imaging Electronics (RTIE) is used to take thermal images of the interface area between the diamond tip and the deformed silicon under the diamond tip. The camera head is configured with a large format (640x512 pixels) sensor in the MWIR (middle wavelength infrared, 2.0-6.0 µm) wavelengths.
The experimental setup for thermal imaging of the laser heated high pressure metallic phase of silicon is shown in Figure 4. The diamond tip, with a preload of 40mN, is set on the silicon wafer and remains stationary. The silicon wafer, which is fixed on a load cell, is moved at a constant speed by a linear motor. After the diamond tip is set on the wafer with the applied load, the movement of the wafer generates the scratch. The scratching stops when the diamond tip reaches the edge of the wafer, where the focal point of the IR camera is preset, as shown in Figure 4. The diamond tip stays at the end of the scratch groove for a period of 20 seconds with the laser running at its maximum output power (400mW, which reduced to 130mW after going through the diamond tip).
Fig 4. Schematic of the experimental arrangement for in-situ thermal imaging
The IR camera records the stationary diamond tip for the entire 20 second time period. The IR thermal images that represent the changing heat distribution of the diamond tip and the adjacent silicon are shown in Figures 5-10. The x and y axes in these Figures are pixel numbers. The color scale in these images is the relative amount of photon energy in each pixel. The camera, for each image frame, automatically adjusts the scale. Therefore, the photon energy distribution is not directly comparable from one image to the next, as the scale changes with each image.
A total of 3500 frames are taken by the IR camera during the 20 seconds of IR heating. In the fist 15 seconds, only the profile of the diamond tip and some radiation from under the tip are seen and the thermal images show little change from one frame to the next over this time interval. In the last 5 seconds, the silicon under the diamond tip experiences dramatic change, as shown in Figures 5-10. These Figures represent sequential images or stages that occur during the final 5 seconds of heating. The thermal images of the diamond tip in the first 15 seconds show that the highest photon intensity occurs within the diamond tip (probably from the laser itself) and there is little radiation emitted from the silicon under the diamond tip. After 15 seconds, a zone in the silicon (estimated depth of 200nm below the silicon surface and about 100 nm thick) beneath the diamond tip appears to radiate and the photon intensity in this area progressively increases (from Figure 6 through 10). The heated area gradually increases in extent and relative intensity. By the end of the measuring period (20 seconds), the highest photon intensity zone has shifted from within the diamond tip (as shown in Figure 5, which is presumably the laser radiation) to the heated area under the diamond tip (refer to Figures 9 and 10).
Fig 5. Thermal image of the diamond tip and adjacent area in the last 5 seconds: stage-1
Fig 6. Thermal image of the diamond tip and adjacent area in the last 5 seconds: stage-2
Fie7. Thermal image of the diamond tip and adjacent area in the last 5 seconds: stage-3
Fig 8. Thermal image of the diamond tip and adjacent area in the last 5 seconds: stage-4
Fig 9. Thermal image of the diamond tip and adjacent area in the last 5 seconds: stage-5
Fig 10. Thermal image of the diamond tip and adjacent area in the last 5 seconds: stage-6
The IR thermal camera images presented here demonstrate the effectiveness of preferential heating of the high pressure metallic phase of silicon using an IR diode laser. The observed time dependent or delayed heating effect between the laser beam and the high pressure silicon phases has not been quantified and thus warrants further investigation. The results are in agreement with the AFM groove depth measurements and analysis of the hardness in the previous sections and the micro Raman spectra results presented elsewhere .
It has been demonstrated that it is feasible to preferentially heat the high pressure metallic phase of silicon during a material deformation process using IR laser heating. The thermal softening effect resulting from the localized heating enhances the ductility, and thus improves the material’s machinability. Different scratching speeds are reported and the resultant thermal softening effect is found only when the speeds are lower than a threshold value, which suggests the following: (1) The total heating time has to exceed a minimum threshold or (2) The laser power needs to be sufficient enough to provide the requisite heating effect, i.e. the requisite laser energy is a function of time (speed) and power available.
Heat absorption has been detected with an IR thermal camera image when the high pressure metallic phase of silicon is preferentially heated by an IR laser. This successful demonstration of the IR heating process will directly lead to future advances in utilizing this technology to augment the ductile regime machining of semiconductors and ceramics. This provides additional confirmation that the high pressure phase of silicon can be utilized and acted upon separately from the covalent phase to potentially improve manufacturing processes, such as ductile regime precision machining.
The work presented herein provides critical information for designing the future micro-laser-assisted-machining (-LAM) system, in which the laser heating system is incorporated into the single-point-diamond-turning (SPDT) process. The heated and thermally softened high pressure metallic phase of silicon helps to reduce the cutting force and potentially reduce tool wear as well in precision machining processes.
A technical difficulty that is yet to be resolved is that the emissivity of the high pressure metallic phase of silicon is not known. To determine the temperature from an IR camera measurement, the emissivity must be known, along with its change with temperature. Due to the complex nature of the instrument configuration, no temperature measurement was extracted from the measured photon energy, i.e. the thermal images were not converted to actual temperatures. Additionally, it would be very difficult to separate the radiation resulting from the heating effect and the radiation from the laser beam, as these IR wavelengths partially overlap (as evident from the laser energy within the diamond detected by the thermal camera), which makes the temperature estimation nearly impossible. These issues are not critical to the success of the research project, as the thermal images are primarily pursued to establish the preferential heating effect, rather than the absolute temperature measurement.
The authors would like to acknowledge the National Science Foundation (grant numbers DMR-0203552 and DMR-0403650) for funding this work. The authors would also like to thank Jay Mathews (Digital Optics Corporation, Charlotte, NC, 28262) for technical help; April Cooke and Dr. Davies for assistance on IR camera imaging.
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Lei Dong is a recent PhD graduate from the Center of Precision Metrology in University of North Carolina at Charlotte, United States. Her research interests are materials characterization of materials under extreme conditions including high pressures and high temperatures, thermal-assisted machining of brittle materials.
Dr. Patten’s machining research has always coupled experimental, theoretical, and modeling components. For the past 20 years, Dr. Patten has concentrated his machining research on nominally brittle materials, e.g. ceramics and semiconductors. These materials have included germanium, silicon, silicon nitride, and silicon carbide. His research is concentrated on the anomaly behavior of these materials to possess a ductile regime machining mode under certain process conditions. Dr. Patten and his colleagues were the first to report on the origins of this ductile regime, i.e. the high pressure phase transformation, of these materials. 5>