The Caltech Nanofabrication Group uses the world-class cleanroom facilities at Caltech's Kavli Nanoscience Institute, a 1500 square foot cleanroom comprising three separate laboratories for optical lithography, electron beam lithography, and thin film processing. We also maintain a full line of additional fabrication, characterization and testing equipment in our own laboratories on campus. Below, you can find information about some of our equipment, although the list is far from comprehensive.
The KNI maintains four scanning electron microscopes (SEM), including three ultra-high resolution systems, two dual-beam focused ion beam systems and an environmental SEM. Additional features include EDX (energy dispersive x-ray analysis, for materials characterization), in situ metal/insulator gas injection needles, TEM grid preparation using the Omniprobe manipulation needle, in situ electrical probing, a cold/hot stage, and electron beam lithography capabilities.
In addition, the KNI provides a 200kV (twin) TEM with STEM, EFTEM, EDX and EELS capabilities, and a 300kV (ultra twin) TEM with HAADF and EDX.
FEI Quanta 200F
An environmental SEM useful for analysis of biological samples and the evaluation of devices under vacuum. One of the features of this system is its ability to observe samples without completely dehydrating them, permitting the imaging of cells and bacteria with a minimum of specimen preparation. As part of the configuration, a heater and cold stage are available, along with a probe station that enables in situ electrical probing of samples
An ultra high-resolution field emission SEM which enables imaging of nanostructures on samples. This instrument, with a point-to-point resolution of 1.5nm, is optimized for the characterization of microfabricated structures as well as chemical analysis through energy-dispersive x-ray analysis. The Sirion column provides high electron beam brightness along with excellent resolution at acceleration voltages ranging from 500V to 30kV, and therefore can be used to image the surface morphologies of semiconductors, metals, polymers, and resist samples.
FEI Nova-600 Nanolab
A dual-beam (focused ion-beam/electron-beam) system that has been configured to enable the mask-less patterning and modification of samples with a liquid gallium ion source. The major features of this system include a high-resolution ion beam (7 nm minimum diameter) that enables excellent placement accuracy for three-dimensional nanofabrication, and an integrated field emission SEM column. The Nova-600 has a reactive gas introduction system for enhanced etch rates as well as deposition of material. The focused ion beam enables the high-resolution milling of samples as well as the preparation and cleaning of specimen surfaces prior to chemical analysis. The Nova-600 has an Autoprobe 200 tool for the in situ modification of samples for transmission electron microscopy (TEM), which gives the user precise control over sample preparation and extraction. With a 6" wafer capability and a high-resolution inch-worm stage, this state-of-the-art FIB system can also deposit metals (such as platinum) and insulators (such as SiO2) for in situ circuit modification, and is ideally suited for post-processing of nanomechanical and nanophotonic devices.
FEI Nova-200 Nanolab
A dual-beam (focused ion-beam/electron-beam) system configured to perform ultra-clean surface chemical analysis and high-resolution ion modification of samples. This system includes a wavelength-dispersive x-ray spectrometer, an energy-dispersive x-ray analysis tool, as well as a time-of-flight mass spectrometer. These spectroscopic tools provide high-resolution analysis capabilities without the risk of atmospheric surface contamination after specimen preparation.
This field emission SEM provides high-resolution imaging of various metallic and/or semiconductor samples. One of the features of this system is its ability to perform high-resolution electron beam writing with a custom-developed control system and software. We have demonstrated features with lateral dimensions below 10nm, and various photonic nanostructures including photonic crystal patterns with lattice constants as small as 150nm. Via RS-232 serial communication ports, the functions of this SEM can be externally programmed and controlled.
FEI Tecnai F-20
A scanning transmission electron microscope (STEM) with a point-to-point resolution of 0.27nm and a 200kV field emission source. This system can be used for analytical electron microscopy on traditional thin-sections, as well as reflection electron microscopy for the evaluation of surface morphologies with nanometer resolution. This system is optimized to integrate chemical analysis tools such as energy dispersive x-ray analysis, energy loss spectroscopy, and energy filtered imaging for enhanced contrast.
FEI Tecnai F-30UT
A scanning transmission electron microscope (STEM) with a point-to-point resolution of 0.17nm and a 300kV field emission source. This system has an HAADF for STEM imaging and an energy dispersive x-ray detector. The TF30UT can be used for lattice imaging at high resolution and for analytical microscopy statisfying the requirement for high-resolution electron microscopy and for nanostructure metrology and analysis.
The KNI provides state-of-the-art electron-beam, ion-beam and optical lithography equipment, including an SEM with electron-beam lithography capabilities at 30kV, two dual-beam FIB/SEMs capable of automated ion-beam masking/electron-beam lithography and 3D patterning, a 200kV TEM for ultrahigh resolution electron-beam lithography, a cutting edge direct-write 100kV electron-beam lithography tool capable of defining 10nm features over 6" wafers, and a direct-write laser system for optical mask preparation or grayscale lithography. Complementing these pattern generation facilities, there are two UV front/back-side mask aligners and a 10X i-line stepper for optical mask exposure. These tools are augmented by an additional SEM with lithographic capabilities and an additional optical mask aligner in our external labs.
A state-of-the-art direct-writing electron-beam lithography tool with a 100kV electron source and an interferometric stage. We have demonstrated fabricated nanostructures with lateral dimensions as small as 10nm, and can define such nanostructures over large areas (up to 6" wafers). The laser interferometer stage assures high placement accuracy and multi-level alignment of microfabricated structures to define complex devices. This instrument has been used for the fabrication of photonic, fluidic, mechanical, magnetic, and electronic devices and represents the foundation of the nanolithography effort at the KNI.
FEI Nova-200 and Nova-600 Nanolab
These dual-beam focused ion-beam/electron-beam systems are capable of mask-less patterning and modification of samples with a liquid gallium ion source, while the pattern generation system may also control the electron beam for traditional resist processing. We have demonstrated gray-scale ion masking of features down to 30nm, with effective mask thickness controllable by implantation parameters. Reactive gas introduction systems provide in situ insulator deposition and electron- or ion-beam activated local etching. 6" wafer handling and high-angle, high-resolution inch-worm stages provide true 3D lithography capabilities.
FEI Tecnai F-20
This scanning transmission electron microscope (STEM) has a point-to-point resolution of 0.27nm and a 200kV field emission source. By transferring control of the scan coils to an external pattern generation system, this system provides near-atomic-resolution electron-beam lithography with the in situ analysis capabilities inherent in the TEM. We have demonstrated electron-beam lithography of features as small as 1nm.
This field emission SEM provides high-resolution electron beam writing with a custom-developed control system and software. We have demonstrated features with lateral dimensions below 10nm, and various photonic nanostructures including photonic crystal patterns with lattice constants as small as 150nm. Via RS-232 serial communication ports, the functions of this SEM can be externally programmed and controlled.
The Heidelberg Instruments DWL-66 is a tool for mask making and for direct patterning of wafers by the use of a HeCd laser. Precise control of the laser head and alignment produces 800-nm resolution lithography. The DWL 66 is an extremely high-resolution imaging system where over a million dpi is achieved using a 20nm writeable address grid for exposing chrome plates or wafers. The ability to perform automatic dose variation enables grayscale lithography.
GCA i-Line Stepper
The 6000 Series DSW Wafer Stepper wafer exposure system is fully automatic and capable of exposing an array of images directly on photoresist-coated wafers. Image field size is dependent on the lens selected for the user's particular application. The lens in our system is a Zeiss 10X with a maximum field size of 10 mm x 10 mm. This reduces the pattern from the reticle by a factor of 10 onto the substrate. The KNI stepper has paddles for wafer handling to accommodate 2, 3, 4, 6, and 8 inch wafers as well as pieces. The stepper is located inside of an environmental chamber set to maintain 0.1°C temperature control.
Software allows conversational input dialogue to reduce errors and simplify the specification of complex operating parameters, a part of which permits selection of either circular of rectangular arrays on the wafer. A laser position transducer with automatic compensation for atmospheric conditions and work piece temperature is employed to meter X and Y coordinate stage positioning over a 150 mm x 150 mm (6 in x 6 in) square exposable area. Maximum throughput is assured through use of X and Y coordinate stage speeds of up to 50 mm (2 in) per second and exposures in both directions of travel (boustrophedonic stepping).
Karl Suss MA6/BA6
The Karl Suss MA6/BA6 mask aligner is a tool that enables front- and back-side alignment of photomasks to create 500nm structures on sample sizes up to 6". This system is ideally suited for rapid definition of sub-micron devices through contact printing and enables the high-resolution alignment of several lithographic layers to define complex devices. It is a state-of-the-art mask aligner and is typically used to define contacts and connections to the nanostructures that are defined with our other fabrication tools. Additionally, the back-side alignment feature may be used to align wafers for bonding in the SBL6. A second MA6 is available for front-side alignment.
The KNI provides three top-of-the-line Oxford ICP-RIE systems for deep reactive ion etching, optimized for silicon etching including cryogenic and Bosch processing, dielectric etching, and III-V/metal etching. A dual-chamber RIE contains a wide array of process gases and is available for many other applications. In our own laboratories, we maintain an additional Oxford ICP-RIE, primarily for critical III-V etching, a chemically-assisted ion beam etcher (CAIBE), and several ion mills.
The KNI's Oxford ICP380s are load-locked, inductively coupled plasma-reactive ion etchers (ICP-RIEs) allowing selective processing of samples which have been defined by our other fabrication tools, capable of processing samples ranging from small pieces to 8" wafers. All three systems provide a 5000W ICP source and a 300W RIE supply operating at 13.56MHz, an actively controlled, liquid nitrogen-cooled, helium-backed sample stage for temperature control from -150ºC to 400ºC, adjustable table height, alumina chamber dome, and high-throughput magnetic levitation turbos for exquisite control over process quality, uniformity, speed, sidewall angle and other etch characteristics. The three systems are further optimized for different applications:
- Silicon deep reactive ion etching, including cryogenic etching and Bosch processing, this tool is dedicated exclusively to ultra-clean silicon processing and is suitable for both deep MEMS applications and ultra-high aspect ratio nanoscale etches. The machine supports higher process gas flows and has the MFCs directly above the sample chamber for maximum control over rapidly switched process conditions. Available gases include SF6, C4F8, O2, Ar and N2.
- Dielectric etching and ICP-CVD, including SiO2 and SiNx etching. This tool contains a separate gas injection ring immediately over the sample stage for high-density, low-temperature ICP-CVD of amorphous silicon, SiO2 and SiNx. This tool is also suitable for cryogenic silicon processing and nanoscale silicon etches. Available gases include SF6, C4F8, O2, SiH4, NH3, N2O, Ar and N2.
- III-V and metal etching, including GaAs, GaP, InGaAsP, InP, GaN, InGaN, W, Ti, Al, Mo, and Nb. Configured to enable etching of compound semiconductors, the heated stage provides process temperatures up to 400ºC, and the system is dedicated to halogen chemistries, containing a wider selection of process gases. The tool is also capable of dielectric and silicon etching, including cryogenic processes. Available gases include Cl2, SiCl4, CH4, H2, SF6, C4F8, O2, Ar and N2.
This load-locked, dual-chamber RIE contains a 13.56MHz and a 40MHz capacitively coupled plasma source, capable of delivering up to 500W RF. The system contains a laser end point monitor for accurate and repeatable etch depth control and a large array of gases enabling a wide variety of processes, including SF6, CF4, Cl2, BCl3, CHF3, H2, O2, Ar, and N2.
This custom-built system uses XeF2 vapor for selective, isotropic silicon etching. The etch is almost infinitely selective to SiO2, Si3N4, and photoresist. The chemical reaction between XeF2 and silicon occurs spontaneously and does not require a plasma, and is thus ideal for gentle releasing or undercutting of NEMS/MEMS devices where stiction is a concern or careful stopping on suspended membranes. Etches occur in timed pulses with configurable pressure, duration, and purge cycles.
Outside of the KNI, we maintain an additional load-locked, inductively coupled plasma - reactive ion etching (ICP-RIE) system dedicated for III-V semiconductors. This tool has a temperature controlled stage cooled with chilled water, a 1000W ICP source and 300W RIE supply, and is capable of processing 4" samples. The system is configured with halogen chemistries (chloride and iodide) optimized for the processing of III-V compound semiconductors, including GaAs, InGaAsP, InP, GaN, AlGaInP, and GaAlInAs. Available gases include Cl2, HI, C4F8, Ar, O2 and N2.
Our chemically-assisted ion beam etcher (CAIBE) is an argon-based ion-beam etching system with a 3cm Kaufman ion-source enabling deep, vertical etching for various III-V material systems including InP, GaAs, and InGaAsP. An added-on heated stage together with chlorine gas flow can boost chemical reactions in those material systems by forming InClx or GaClx, resulting in highly anisotropic etching profiles in sub-micron sized pillars and holes.
The group has access to a wide variety of deposition methods including chemical vapor deposition (CVD), thermal and electron beam evaporation, and DC or RF magnetron sputtering. The Oxford systems provided by KNI allow us to create high quality silicon layers via ICP-CVD or PECVD. The many other systems offer different capabilities in available film materials, sample operating environments, and film properties.
Oxford HD ICP-CVD
A plasma-enhanced chemical-vapor-deposition (PECVD) system optimized for, and exclusively dedicated to, dielectric etching and deposition using an ICP chamber. It offers PECVD capability for the deposition of silicon nitride, oxide, and oxy-nitride, with relevant gases. It operates with a heated stage (-150 to 400 °C) to provide exquisite control over the sidewall angles of etched dielectric materials. ICP-PECVD enables the low-temperature deposition of dielectric films, a capability that enables novel patterning of dielectrics, since photoresist-coated samples can be coated with pinhole-free dielectrics. It can also be used in atomic-layer-deposition (ALD) mode, which provides very precise thicknesses of dielectrics grown using sequential deposition of the precursors to generate very high-quality films. Applications of this capability include the fabrication of microfluidic channels with dielectric walls.
A plasma-enhanced chemical-vapor-deposition (PECVD) system configured as a conventional PECVD system with a high-temperature (700 °C) stage. Both liquid delivery (TEOS) and gas delivery (silane) of the silicon source are provided. It can also be used in atomic-layer-deposition (ALD) mode. This system provides the traditional high-temperature dielectric deposition capabilities for generating etch masks or alignment marks, but can also be operated to provide pinhole-free films for MEMS and biochemical analysis applications, as well as to form stress-free nitride films.
TES Electron Beam Evaporator
The electron beam evaporator is a completely rebuilt Temescal BJD-1800 six-pocket electron beam deposition system with a load-lock for rapid sample exchange. This system, equipped with an ion gun for surface cleaning as well as ion-beam assisted deposition, enables the precise deposition of metals for lift-off and mask deposition purposes. The system is fully software-controlled and includes a CTI cryo-pump and compressor.
A Tescal BJD-1800 3 target thin film deposition sputter system. This system can perform RF and DC co-sputtering and table substrate etching. The sample stage can accommodate a 6" wafer and has automatic rotation. Evacuation is performed by a direct drive mechanical pump and a turbopump. 2" diameter magnetron cathode guns can sputter various metals, semiconductors and dielectrics in argon, nitrogen, and/or oxygen environments. Deposition process steps are software-controlled.
Kurt Lesker Sputterer
A Kurt Lesker-built 4 target thin film deposition sputter system. This system can perform RF and DC co-sputtering, magnetic material sputtering, and substrate etching. The loadlock also feeds into an ion beam etching chamber which can be used for substrate cleaning. Evacuation is perfomed by a direct drive mechanical pump, a diffusion pump, and a cryopump. 2" diameter magnetron cathode guns can sputter various metals, semiconductors and compounds in argon, nitrogen, oxygen, and/or methane evironments.
Torr Thermal Evaporator
The Torr Thermal Evaporator System is a 4 source water-cooled 2kW resistive thermal metal evaporator dedicated to aluminum deposition. The system runs software-controlled codeposition in a large box chamber equipped with shutters, thickness monitors, a turbopump and cryopump.
Edwards 306 Thermal Evaporator
The BOC Edwards Auto 306 Thermal Evaporator is used for the deposition of thin films of precious metals including: Au, Ag, Au/Ge, Au/Zn, etc. The system is evacuated by a direct drive mechanical pump and a diffusion pump which can reach a base pressure in the high 10-7 torr range. Evaporation is performed by resistively heating a tungsten or tungsten/alumina boat using a manually controlled high amperage supply and is monitored by a quartz crystal deposition monitor.
The Veeco VE-400 dual-pocket thermal evaporator is used for the deposition of thin films of various metals and unusual materials such as salts. The system is evacuated by a direct drive mechanical pump and a turbo pump which can reach a base pressure in the high 10-6 torr range. Evaporation or co-evaporation is performed by resistively heating a tungsten pocket using a manually controlled high amperage supply and is monitored by a quartz crystal deposition monitor.
Edwards 306A Copper Evaporator
The Edwards E306A Coating System is used for the deposition of thin films of non-precious metals including: Ni, Fe, Cu, Cr, etc. The system is evacuated with a belt drive mechanical pump and a turbomolecular pump which can reach a base pressure in the high 10-7 torr range. Evaporation is performed by resistively heating a tungsten or tungsten/alumina boat using a manually controlled high amperage supply and is monitored by a quartz crystal deposition monitor. Vacuum feedthroughs are available for low temperatue substrate deposition.
Electron Beam Evaporator
This custom-built electron beam evaporator is built around an MDC 6kW electron beam supply and e-Vap 4000 multi-pocket source. The system is evacuated by a Varian ion pump in addition to a turbo pump for the sample loadlock which allows for very clean operation in the low 10-7 torr range. Deposition is measured by a quartz crystal thickness monitor.
In both the KNI and our own laboratories, a wide variety of processing tools are available which enable us to perform heat treatments, cleaning treatments, or other tasks as required for wafer, membrane, or microfluidic device fabrication.
Critical Point Dryer
For the preparation of nano-electromechanical structures as well as for the preparation of biological specimens, we operate a critical point dryer that prevents surface tension damage in nanostructures when removing these from etching or sample preparation solutions. Critical point drying is a very common technique for the preparation of cells and bacteria before electron microscope imaging.
Jetfirst Rapid Thermal Annealer
A 6-inch rapid thermal annealer (RTA) for the annealing of contacts and doping into semiconductor nanostructures. This system, with an accurate pyrometer surface emission measurement and a sample ramp of 500 °C/second enables the reproducible fabrication of p-n junctions as well as the controlled oxidation of up to 6-inch wafers. The RTA is fully automated and has been widely used for the exploration of nanophotonic devices.
Tystar Tytan Oxidation Furnaces
The Tystar Tytan 4600 is a large diameter multi-wafer furnace dedicated to wet and dry oxidation processes. Our system has two tubes and can handle up to 100 6" wafers per tube. This system gives us the capability of high-throughput high precision automated oxidation of silicon as needed for our wafer process requirements.
Karl Suss SB6L Wafer Bonder
The Karl Suss MitroTec SB6l Substrate Bonder is a semi-automatic, software controlled, wafer bonder accommodating up to 6" wafers used to integrate heterogeneous materials systems onto chips. The system provides high post-bond alignment accuracy, independent upper and lower substrate heaters and precise programmable force control during the wafer bonding process. The system includes an automated loading arm positions for positioning the wafer stack. Capable of anodic, glass frit, eutectic, adhesive, fusion and metal diffusion wafer bonding. The bond chamber atmosphere (pressure and/or vacuum) can be controlled to tune device performance. An additional plasma surface activation system that permits bonding of III-V materials to silicon, or for the definition of complex MEMS structures.
Oxygen Plasma System
The group has access to multiple oxygen plasma etchers/ashers which are typically used for removing photoresist and organic matter from sample surfaces. The systems use a 13.56 MHz RF generator, with manual impedance tuning, a pyrex chamber and a mechanical vacuum pump for removing byproducts.
Microfluidics Fabrication Tools
The group owns and has access to all the tools necessary to build precision aligned multi-layer soft elastomer microfluidic devices. This includes mixers, ovens, alignment microscopes, vacuums, and other valuable tools/chemicals enabling us to make a wide variety of devices.
Oxidation and Annealing Furnaces
Our lab maintains several quartz tube furnaces for sample oxidation and annealing. Two 6" furnaces each have three, PID-controlled heating zones for maximum temperature uniformity, digital temperature adjustment and programmable soak times, ramp rates and temperature profiles. Both of these furnaces can accommodate samples up to 4" in diameter. One furnace is configured primarily for oxidation, and can perform both dry and wet oxidation using a steam bubbler and ultra-high purity O2, as well as nitrogen and forming gas (a mixture of N2 and H2) for annealing. The second 6" furnace is primarily intended for sample annealing in an inert atmosphere, and is currently configured with nitrogen and forming gas. Two additional 2" furnaces are used for oxidation and annealing of III-V, metal and silicon piece samples.
In addition to the tools listed below, many of our Imaging and Testing instruments are also utilized to characterize our fabricated devices.
The UHV Omicron Auger/XPS/Cathodoluminescence/STM Surface Analysis and Measurement tool is an advanced system for nanostructure/device/material characterization. It is equipped with a 4-point contact probe for transport/electrical measurements, high resolution <4nm SEM, high resolution <10nm SAM for chemical mapping, piezoelectric stage-positioning, and capable of non-destructive and full STM operation. Chamber pressures better than 10^-10 torr are attainable.
The Filmetrics F40 thin film-analyzer is mounted on an optical microscope and measures the reflectivity spectrum of a sample stack to determine the material thicknesses. Depending on the index of refraction of the materials and the wavelength fraction of the thickness, the boundary conditions can be used to simulate the expected reflection spectrum and compare to collected data. The spectrometer range is 400-800nm and can measurely accurately down to 20nm thickness.
The Dektak 3 ST surface profilomer is a high precision surface measurement tool. A software controlled contact stylus measures vertical features such as roughness, waviness, step height and geometry ranging in heigh from 100nm to 100microns. The stylus is scanned along the surface by a precision sample stage and sensor data is stored by the instrument software.
High Performance Computing Systems
The group maintains servers and a Beowulf cluster dedicated to electromagnetic field simulations. Potential and realized devices are characterized by Finite Difference Time Domain software that can model Maxwell's equation inside complicated material geometries. Performance of waveguides, lasers, and other optical/electroptical devices are determined in a relatively short time span.
We maintain a wide variety of optical and electrical testing equipment to verify our fabricated devices. Stations with micropositioning probes and optical microscopes allow us to characterize electronic semiconductor or optoelectronic devices. Optical tables with a multitude of optomechanical components and spectrometers let us measure UV-Vis-IR light. And multiple filtered and inverted microscopes allow us to observe performance of microfluidic devices.
Optical probe station
The group maintains a couple optical probe stations which are typically built on microscope stages set on optical tables and typically feature multiaxis stage and probe control, laser and broadband sources, current and voltage sources, and digital cameras and spectrometers. These stations allow us to quickly determine device performance and tune parameters to characterize our devices immediately after fabrication.
Electrical Probe Station
The group maintains a couple electrical probe stations which are typically built around microscope stages and feature multi-axis stage and probe control, various current and voltage sources, and digital cameras. Chacterization of electrical performance may be performed by a current/voltage readers or one of our semiconductor parameter analyzers. Chacterization of luminescence of LEDs and lasers may be performed by an optical spectrum analyzer to perform spectral analyses.
Semiconductor Parameter Analyzer
The 4155C Semiconductor Parameter Analyzer can be used to measure the current-voltage or capacitive characteristics of a wide variety of semiconductor devices. The system has 10fA/2microV measurement resolution and 20microsecond time resolution. Measurement data can be displayed graphically, can be mathematically manipulated, and can be saved as a numerical data file.
The Acton SpectraPro 2300i is a flat-field imaging spectrograph optimized for CCD spectroscopy. The system has a triple grating monochromater turrent that can be quickly changed to optimize for desired wavelength collection. The spectrograph has 300mm focal length, can scan from 0-1400nm and has .09nm CCD resolution. The high-performance optics when combined with the other equipment on our optical benches and optical probe station allows for excellent device characterization in the UV-Vis-NIR range. The system is software-controlled and provides a PC graphical interface for handling and saving data.
The Triax 320 imaging monochromator/spectrograph is optimized for high resolution multichannel spectroscopy. It features a motorized three grating turret, toroidal mirrors, The TRIAX320 has a 320mm focal length, .06nm resolution and 2 automated entrance/exit ports. One output path has a flat optical focal plane for imaging spectroscopy. The other output path is designed for conventional scanning spectroscopy. The system may be used for reflection, fluorescence and dark-field spectrscopy and can be automated via computer control.
We have several optical and fluorescent microscopes, which are equipped with digital cameras, NIR/optical/UV filters, and pressure sources for running microfluidics chips making them ideal for characterizing biomedical device structure and performance. These include an Olympus IX50, an IX71, a Nikon AZ100, and various other models. The Nikon Multizoom AZ100 optical microscope captures high-resolution, high-contrast images with on-axis viewing, which is in contrast to the conventional optical microscopes where stereoscopic column designs always capture images in a diagonal direction.