Guide Nano and Micromachining (ISTE)

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Table of contents

Int J Adv Manuf Technol — Part III: influence of tool wear.

Influence of tool wear on machining forces and tool deflections during micro milling

Leach R Characterisation of areal surface texture. Altintas Y Manufacturing automation, metal cutting mechanics, machine tool vibrations and CNC design, 2nd edn. Personalised recommendations. Cite article How to cite? ENW EndNote. Buy options. The machining system 10 , including the machining capabilities, are located inside a chamber 51 , in which a partial vacuum 50 can be drawn while the machining processes are being performed.

Nano and Micromachining

Preferably, a load-lock chamber, which allows the substrate 52 to be inserted into the chamber 50 for machining and taken out without having to break vacuum inside the chamber 50 , would be used. Cassette-to-cassette loading and un-loading tools can be used in the system of FIG. The femto-second laser machining capability 12 is used for removal of substrate 52 material s at rates higher than is possible with either the PFIB or FIB capabilities, but with higher levels of precision than possible with a conventional laser machine tool capability. The plasma focused-ion beam PFIB machining tool capability 18 is used to remove substrate 52 material s at lower rates, but with higher levels of resolution and precision than either the femto-second or conventional laser machine tool capabilities.

The conventional laser machining tool capability 30 is used to remove substrate 52 material s at higher rates, but with limited resolution and precision as compared to the femto-second laser 12 , PFIB 18 , or FIB The femto-second laser machine tool 12 may include a focusing lens element 14 that allows the laser's radiation beam 16 and resultant spot size on the substrate 52 surface to be reduced in diameter and the intensity of the laser's radiation beam 16 to be increased to thereby accelerate the removal rates.

Similarly, the conventional laser machine tool 30 may also include a focusing lens element 32 that allows the laser's beam 34 and resultant spot size on the substrate 52 surface to be reduced in diameter and also the laser radiation intensity to be increased, to thereby accelerate the removal rates.

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Preferably, both the femto-second tool 12 and conventional laser machine tool 30 employ lens elements 14 and 32 that have automatic adjustment that is connected in the close-loop control loop system 20 , in which the laser beams 16 and 34 and resultant spot sizes are automatically adjusted in real-time to optimize the removal rates and precision of the machining processes for a specific device.

These lens elements also allow the minimum resolution possible, that is, the smallest feature that can be machined into the substrate, with these capabilities to be reduced.

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Therefore, when combined with the automatic closed-loop control, the feature sizes of the machining processes can be adjusted for optimal results based on a given design and material type being machined. Preferably, the PFIB machine tool 18 employs a close-loop control loop system, in which the ion beam size 22 and resultant ion beam spot size on the substrate 52 surface are automatically adjusted using electronic lens elements in real-time to optimize the removal rates, minimum feature size and precision of the machining process for a specific device design.

Preferably, the PFIB and FIB machine tool capabilities both employ a close-loop control loop system 20 to automatically optimize the removal rates and precision of the machining process for a specific device s. The closed-loop control system 20 is connected to a computerized control system , in which a computer-aided design CAD file of a device design is inputted to control the tools 12 , 18 and 30 to implement the device design. The inputted device design to be machined in the substrate 52 may include various patterns, including horizontal lines 60 and 88 , as well as circular, curvilinear and irregularly-shaped features The control system allows the direct writing, that is the machining, of features by the tools 12 , 18 and 30 without having to perform photolithography on the substrate 52 for the purposes of pattern transfer, and therefore is a more efficient fabrication process than the current conventional means of fabrication of micro- and nano-devices and structures.

The femto-second laser 12 allows reasonably high machining rates for features down to the submicron level, while the PFIB 18 would be used to machine at much higher precision levels, but at lower machining rates. These machining capabilities may be combined with a conventional laser machine tool 30 for high-rate machining but with lower machining precision and a Focused-Ion Beam FIB 18 for extremely precise machining but at very low machining rates. Conventional laser micromachining 30 is a relatively mature process technology that uses a focused optical beam 34 of light to selectively remove materials from a substrate 52 , thereby creating features in the surface of the substrate 60 , 88 and The advantages of conventional laser micromachining include: it is a direct-write process and requires no mask or photolithography; it can be used to machine several different material types; and the machining rates are relatively high, thereby allowing reasonable wafer through-puts to be achieved.

However, there are some attributes of conventional laser micromachining that limit its usefulness for the machining of features bellow a few ten's of micron-sized level. Additionally, conventional laser machining generates a heat-affected-zone HAZ with a large temperature gradient that can lead to micro-cracking, damage to the surrounding material, and re-deposition of machined material near the machining site, that limit it usefulness for the machining of features at the ten's of micron-size level.

Femto-second laser micromachining 12 is a relatively new technology that utilizes the properties of ultra-short laser pulses to achieve an unprecedented degree of control and precision in machining microstructures in a wide class of different material types without heating or damaging the surrounding material. The use of femto-second laser 12 allows the photon energy to be deposited into small volumes by a multi-photon nonlinear optical absorption, which directly leads to avalanche ionization. Because the typical heat diffusion time is in the order of a nanosecond to a microsecond time frame, and the electron-phonon coupling time of most materials are in the picosecond to nanosecond time frame, the femto-second laser energy is deposited on a time scale much shorter than either the heat transport or the electron-phonon coupling.

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Consequently, the area of material where the laser pulse is deposited is directly converted from a solid to vapor phase and to plasma formation nearly instantaneously—that is, it is an ablation process. As compared to conventional laser micromachining 30 , femto-second laser 12 machining greatly reduces the damage to the substrate 52 in locations in close proximity to the machining site 60 , 88 and Moreover, since the machining process is not dependent on the linear absorption at the laser wavelength, virtually any dielectric, metals, and mechanically hard materials can be machined using the same laser system Also, the breakdown threshold of femto-second lasers 12 can be determined with great accuracy, thereby making it a deterministic machining technology.

Some of the differences between conventional laser micromachining 30 and femto-second laser micromachining 12 are illustrated in FIG. The conventional laser beam 34 may be focused using a lens element 32 into a smaller diameter beam 34 having higher laser radiation intensity. The conventional laser machine tool 30 is limited to machining features above the ten's of microns dimensional scale for reasons explained below. Conventional laser machining 30 results in heating of the substrate 52 that creates a melt zone at the impingement site and the molten material is ejected away from the impingement site This process causes localized heating in a heat transfer zone of the surrounding material around the impingement site resulting in a melt zone area in immediate proximity of where the laser beam impinges A large thermal gradient is established between the impingement site and the substrate 52 material away from the impingement site This may result in the formation of microcracks near the impingement site where the machining is being performed.

Also, the molten material ejected from the impingement site can condense on the nearby surfaces creating a debris field Some of the molten material also creeps upward from the machining or impingement site and cools to create a recast layer This recast material is highly porous and usually undesirable for any device application. The underlying surface layer of the melt zone near the impingement site may also have ripples in it due to the thermal shock wave resulting in plastic deformation of the substrate material 52 near the impingement site Since the melt zone is larger than the beam spot size 34 , it is very difficult for a conventional laser machine tool 30 to be used to machine features below a few microns in diameter.

The conventional laser machining tool 30 works best on substrates 52 materials having ample free electrons, such as metals and semiconductors, but does not work very well on substrate 52 materials with a limited number or no free electrons, such as dielectrics. These free electrons are necessary to initiate the avalanche ionization of the material of the substrate In short, conventional laser machining 30 results in a large debris field and damages the substrate 52 material around the impingement site where the machining is performed conventional laser machine tools 30 do not work well on some commonly used substrate 52 materials commonly used in micro- and nano-device and structure fabrication, such as glasses, dielectrics and some ceramics.

It cannot machine features below a few microns. Nevertheless, the conventional laser machine tool 30 does have a high removal rate depending on laser intensity, wavelength and substrate material 52 type. In comparison, a femto-second micromachining tool 12 FIG. Since the laser pulse length is shorter than either the heat diffusion time scale or the time scale for electron-phonon coupling, the substrate material 52 surrounding the impingement site does not heat up and no thermal gradient is established.

Therefore, no thermal shocks or microcracks are created in the process. The femto-second laser beam 16 can be focused onto the substrate surface 52 using a lens element The intensity levels of femto-second lasers 12 are very high, reaching TWatts per cm 2. Therefore, the material temperature at the impingement area instantaneously goes into the plasma regime.

The intensity levels of femto-second lasers 12 are so high that no materials, even those with very high melting temperatures, such as Molybdenum, can withstand these energy levels. Importantly, femto-second machining by laser 12 does not result in the debris field near the impingement site found in conventional laser machining by laser This is because the plasma created by the femto-second laser 12 energy results in a pressure gradient that forces the ablated material away from the plasma plume.

Additionally, there are electrical forces that propel the ablated material from the impingement site The plasma contains ionized species that have a positive charge, which repel one another. These ions can be pumped from the site similar to how a RIE etch process works. As a result, there is none of the surface debris found in conventional laser machining by laser Furthermore, the extremely high intensity of the femto-second laser 12 results in bound electrons becoming free electrons and therefore can initiate the avalanche ionization process immediately.

Therefore, femto-second lasers 12 can be used effectively on any material type, including metals, semiconductors, dielectrics, polymers and ceramics. Importantly, the femto-second laser 12 is capable of machining sub-micron features. In fact, a femto-second laser 12 can be used to machine features less than the wavelength of the laser radiation.

The reason for this is that femto-second laser 12 machining is a threshold process. That is, the beam impingement area intensity has a Gaussian profile with the peak intensity at the center and falling to lower values toward the beam edge.

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Therefore, if the beam intensity is adjusted so that only a portion of the beam is above the threshold for ionization then the machining only occurs in this location. With this method, features below 1 micron can be made in a material substrate, and possibly smaller, depending on the wavelength of the laser source.

This is illustrated in FIG. Any intensity below the threshold intensity will not ablate the substrate material 52 and any intensity above the threshold intensity will result in ablation.

Two different laser intensities as shown, one with a higher intensity and another with a lower intensity Only the intensity levels of each beam profile above the threshold intensity results in ablation, and therefore, as shown in FIG. Similarly, the laser profile with the lower intensity results in a smaller machined feature in the substrate Therefore, depending on the specific radiation wavelength of the femto-second laser 12 , features as small as about to nanometers can be machined into the substrate 52 surface.

The femto-second laser 12 technology allows the machining of relatively small features, even down to about to hundred nanometers. Conventional Focused Ion Beam FIB milling is a mature technology that uses a liquid-metal ion source, usually a Gallium ion source, whereby the Gallium wets a tungsten needle that is heated and has a large electrical potential to cause ionization and field emission of the Gallium ions. The source ions are accelerated to an energy usually between 5 and 50 keV and focused to a small spot size using a special electrostatic lens.

This sputtered material leaves the surface as either secondary ions or neutral atoms. Secondary electrons are also produced from this process. The sputtered ions or secondary electrons can be collected to form a high-resolution image of the surface similar to the way a Scanning Electron Microscope [SEM] works. Plasma FIB milling works by creating a magnetically-induced plasma ion source above the sample and using the ions from the source to augment the machining rate. The plasma converts the material on the surface where the ion impingement is occurring to a volatile gas species, which can be removed through low pressure pumping.

This allows the material to be removed at an increased rate without the problem of material re-deposition on the surface. As can be seen in the chart of of FIG. The removal rates shown in are estimated since the maximum rates possible with these technologies are continuously increasing as these technologies improve. It is important to note that the maximum spot size for each machining capability does not mean that this is the largest feature that can be made with the specific tool since each of these machining systems raster scan the surface of the substrate and can be used to make nearly any sized feature.

For example, conventional laser machining has a spot size of above 10 microns, but can be used to make nearly any sized micro- and millimeter sized device. As can be seen, the machining technologies proposed herein can deliver machining rates comparable to current technologies, such as DRIE, but at far superior levels of precision. This is important since the maximum removal rates will determine the substrate through-put, that is, the number of substrates that can be machined per unit time and any machining technology must have substrate through-put rates comparable to current production rates.

Importantly, the type and number of machining capabilities to be included in a machining system 10 configuration can be varied from one or more depending on the specific dynamic range of removal rates and level of precision in the machining process desired for a given application. Therefore, a femto-second laser combined with a PFIB machining capability may be desirable for a certain application whereas a conventional laser combined with a femto-second laser may be desirable for another application.

The type and number of machining capabilities in a specific machining system configuration would be determined based on the desired dynamic range of machining rates and the precision of the machining process desired using the plot of FIG. However, if the needed machining precision level extended down to a few tens of nanometers, then the femto-second laser combined with a conventional laser would need to be combined with the PFIB and FIB. In general, the major factor determining the machining capability to be selected in a given system configuration is the minimum feature dimension.

The secondary factor determining the machining capability to be selected in a given system configuration is the maximum machining rate desired. It may also be desirable to combine several of the same machining systems, such as a multiplicity of PFIBs or femto-second lasers in the same tool system to speed up the rates for a given dynamic range of machining resolutions, For example, if two of each of the femto-second lasers and conventional lasers were configured in a single machining system, the machining precision would be set by the minimum feature dimension possible with those capabilities, that is, between a few hundred nm's to more than 20 microns.

However, if two of each of the femto-second lasers and conventional laser were configured in a single machining system, the machining rates would be twice that of a system configuration with only one of each of these machining capabilities. All configurations of these machining capabilities are part of the present invention.

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The precision machining tool 10 of the present invention in one preferred embodiment includes the conventional laser 30 , femto-second laser 12 , plasma FIB 18 , FIB 18 and a Scanning Electron Microscope SEM 82 imaging capability in one machining tool system The machining stage 40 onto which the device substrate would be positioned during the machining operation would have 6-Degrees of Freedom 70 , high-precision positioning accuracy, and would be located within a vacuum chamber 50 enclosure where the machining would occur Depending on the resolution of the machining processes on a machine system configuration, the stage movement may have sub-nanometer positional accuracy enabled using piezoelectric actuation mechanisms or similar means.

Each of the machining capabilities are under computer control—that is the machining process and stage position and rotation angle, would be determined by the closed loop control system 20 and computerized software system so as to machine the device's dimensions and design, similar to the way modem numerical controlled machines CNC machines operate. The machining process can be viewed in real-time using the electron beam gun 82 and lens element 84 that creates a scanning electron beam 86 that is under computer control The secondary electrons from the substrate are captured in the secondary electron detector 15 and formed into an image 17 digitally.

The data is also fed back into the computer controller and the closed-loop control system 20 for controlling the machining tools 12 , 18 and 30 , as well as the 6-degree of freedom stage 70 that precisely moves and positions the substrate 52 during the machining process.

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The imaging system 15 and 17 is essentially a Scanning Electron Microscope SEM capability that would be included in the machine system 10 for the purposes of real-time imaging and control of the machining process. Elemental analysis could be enabled by incorporating energy-dispersive X-ray spectroscopy EDAX capability This allows the feedback loop on the machining process to have information based on the makeup of the material being machined which may be useful for determining exactly where to stop the machining process.

Other spectroscopic detection capabilities may also be included in the system for similar purposes of determining material types at a certain machining position. Also, the SEM 15 can be used to measure frequency of the device during the machining process. The electron beam is focused on a feature having a strong contrast in the image and any movement such as from movement of the device of the sample causes a modulation in the imaging signal that can be processed through a fast-fourier transform FFT to extract the frequency spectrum of the sample movement.

From this spectrum the drive frequency and mechanical resonant frequencies can be identified.

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This technique can be used to detect movement amplitudes of about 1 nm in any direction. This technique can be used to enable direct measurement of the actual device resonance frequency as the feedback signal for closed loop process control. The machine tool of the present invention that incorporates each of these capabilities functions as follows: the user loads into the tool the CAD design of the micro- or nano-device or structure to he made; a substrate of the desired material type 52 or having the desired material layers would be loaded into the machine tool by placing it onto the precision stage 40 ; the conventional laser 30 performs the coarse machining of the device, removing large sections of material very quickly at a rate similar to DRIE in silicon.

The conventional laser 30 is then turned off once the machined features had reached a level equivalent to the resolution of the conventional laser 30 or moved to another position on the substrate 52 for the purposes of machining other locations of the substrate The femto-second laser 12 is then turned-on and machines the sample at a finer scale.