Lecture 20: Optical Tools for MEMS Imaging

Transcription

1 MECH 466 Microelectromechanical Systems University of Victoria Dept. of Mechanical Engineering Lecture 20: Optical Tools for MEMS Imaging 1 Overview Optical Microscopes Video Microscopes Scanning Electron Microscopes (SEM) Scanning Probe Microscopy (SPM) 2

2 Optical Microscopes Due to the small size of MEMS, various microscopy methods are required to inspect them. The basic optical system is illustrated to the right. It consists of: Objective Lens Light Tube Eyepiece Human Eye Light Eyepiece Light Tube Objective Lens Object 3 Compound Optical Microscope Definitions: Numerical Aperture (NA) Total Magnification Field of View Resolution Working Distance φ Angle of Incidence Field of View 4

3 Optical Microscope Transmission and Bright-field Illumination Incident Illumination Light Source Transmission Illumination Condensor Lense Light Source Bright-field Illumination Incident Illumination Transmission Illumination Incident Illumination 5 Optical Microscope Co-Axial Illumination 50% Reflective Mirror Light Port Light Source Co-Axial Illumination 6

4 Optical Microscope Illumination Transmission Illumination: Pro s: Great illumination, can use phase contrast for translucent objects, can use for high magnification viewing of biological cells. Con s: Sample and holder must be transparent. Incident Illumination: Pro s: Inexpensive, easy setup, only needs natural light for low magnification. Con s: Insufficient light for high magnification. Co-Axial Illumination: Pro s: Great illumination for opaque or reflective objects, and is great for very high magnifications. Con s: Expensive. Requires special light tube, beam splitter mirror, in addition to light source. 7 CCD Cameras vs. CMOS Camera CCD: Charge Coupled Device APS - CMOS Active Pixel Sensor Complimentary Metal Oxide Semiconductor CCD: Pro s: High quantum efficiency of 60-70%, low image noise, excellent low light performance Con s: Expensive to manufacture, slow capture rates due to fundamental design CMOS: Pro s: Inexpensive to manufacture, very fast capture rate, individually addressable capture areas Con s: Lower quantum efficiency than CCD s, resulting in higher image noise, and higher illumination requirements. 8

5 CCD Camera Operation Accumulation of charge. Every photon that strikes the sensor area adds charge to the device. You can think of each sensor area like a photon well that collects incoming photons. [Wikimedia Commons, by M. Schmid] After a defined exposure time the total charge in each photon well must be tallied. However, the wells cannot be individually addressed, therefore, they must be shifted from one side to the other. [Wikimedia Commons, by M. Schmid] There are three different styles of CCD s employing different schemes to shift the charge from one well to the next. Full-Frame CCD Frame Transfer Interline Transfer [Wikimedia Commons] 9 CMOS Camera Operation Operation of the APS CMOS Imager Features: - On chip amplification and filtering - Individually addressable pixels [Wikimedia Commons] Operation of the APS CMOS Imager Must use micro-lens to concentrate light into the sensor, due to space required for amplifier or other electronics [Samsung] 10

6 Scanning Electron Microscope SEM Microscopes use tightly focused electron beams, that are scanned across substrates. The backscattered or reflected electrons are collected by a sensor, and interpreted as an image. CamScan Model CS44 Scanning Electron Microscope [ Sample of SEM image of Ant [UVic SEM facility] 11 SEM Images from Demonstration Failure of Bonding Pad [UVic SEM facility] Microassembly of Prototype Device [UVic SEM facility] 12

7 Scanning Electron Microscope The following movie describes the operation of a typical SEM system: The diagram to the right details various components of the SEM. SEM Operation Movie [Museum-of-Science, SEM Schematic Diagram [ 13 Scanning Electron Microscope The electron gun creates the supply of electrons used to form the scanning beam. The entire assembly must operate in a vacuum, otherwise the filament will burn in the presence of oxygen, or can be contaminated in the presence of other gasses. SEM electron gun assembly [ 14

8 Scanning Electron Microscope Optical glass lenses cannot be used to focus an electron beam. Only magnetic fields can deflect electrons, therefore, magnetic lenses have been designed for this purpose. SEM Cylindrical Magnetic Lens Assembly [ 15 Scanning Electron Microscope When the electron beam strikes the target surface, two main types of reflections result: Beam-Sample-Interaction [ 16

9 SEM Sample Preparation Conductivity of the sample is of paramount importance. In order to have a good backscatter of electrons, the target must not build up static charge. Gold sputtering can be used to make any target conductive. Gold Sputter Coater for SEM use [ 17 Scanning Probe Microscopy Scanning probe microscopes (SPMs) are a family of instruments used for studying surface properties of materials, with image resolutions ranging from the atomic to the micron level. All SPMs contain the components illustrated in the figure below. SPM Illustration [image from Chang Liu] 18

10 Scanning Tunneling Microscopy The scanning tunneling microscope (STM) is the ancestor of all scanning probe microscopes (SPM). The STM was invented in 1981 by Gerd Binnig and Heinrich Rohrer at IBM Zurich. Five years later they were awarded the Nobel prize in physics for its invention. The STM was the first instrument to generate real-space images of surfaces with atomic resolution. STM Operation Principle [Chang Liu] STM Tip Detail [image from Chang Liu] 19 Atomic Force Microscopy The atomic force microscope (AFM) probes the surface of a sample with a sharp tip less than 100Å in diameter. The tip is located at the free end of a cantilever that is 100 to 200µm long. Forces between the tip and the sample surface cause the cantilever to bend, or deflect. A sensor measures the cantilever deflection as the tip is scanned over the sample, or the sample is scanned under the tip. The measured cantilever deflections allow a computer to generate a map of surface topography. Several forces typically contribute to the deflection of an AFM cantilever. The force most commonly associated with atomic force microscopy is an interatomic force called the van der Waals force. The dependence of the van der Waals force upon the distance between the tip and the sample is shown in figure below. AFM Interatomic Force vs Tip to Sample Distance [Image from Chang Liu] 20

11 AFM System Diagram In non-contact mode, the system vibrates a stiff cantilever near its resonant frequency (typically from 100 to 400 khz) with an amplitude of a few tens of angstroms. Then it detects changes in the resonant frequency or vibration amplitude as the tip comes near the sample surface. The sensitivity of this detection scheme provides sub-angstrom vertical resolution in the image, as with contact AFM. In contact AFM mode, also known as repulsive mode, an AFM tip makes soft "physical contact" with the sample. AFM Operational Modes [Images from Chang Liu] 21 Magnetic Force Microscopy Magnetic force microscopy (MFM) images the spatial variation of magnetic forces on a sample surface. For MFM, the tip is coated with a ferromagnetic thin film. The system operates in non-contact mode, detecting changes in the resonant frequency of the cantilever induced by the magnetic field's dependence on tip-to-sample separation. MFM can be used to image naturally occurring and deliberately written domain structures in magnetic materials. Note: An image taken with a magnetic tip contains information about both the topography and the magnetic properties of a surface Hard Disk Surface. Field of View is 30um [Image from Chang Liu] 22

12 Many more SPM Methods There are numerous other SPM methods such as: - Scanning Hall Probe Microscopy - Lateral Force Microscopy (LFM) - Force Modulation Microscopy (FMM) - Electron Force Microscopy (EFM) - Scanning Thermal Microscopy (SThM) - Plus others... All these methods use various MEMS based micro-tips to perform the surface scan. SEM of SPM Tips [Image from Chang Liu] 23