Microscopy Extends Inspection to the Nanometer Level

The theoretical resolving limit of the conventional light microscope is 0.2 µm and the numerical aperture (NA), the light-gathering capacity of the objective or the light-providing capability of the condenser, is limited to 1,400 NA. Although today’s microscopes offer magnifications from 2,000X to 5,000X, it is an empty magnification because no more detail is revealed beyond 1,400X magnification.1

Optical microscopy is essential for inspecting blank wafers for gross defects, scratches and contamination. It also helps inspect, classify and pinpoint defect locations on wafers, and enables wafer manufacturers to log yield data and analyze the effectiveness of each production step.

To extend the reach of the microscope and increase its value, manufacturers offer complete inspection systems like Nikon’s Microstation. It has a wafer loader, a microscope, a programmable stage, a shuttle assembly to take the wafer from the loader to the stage, and image software.

Many optical microscope companies supply modular units that allow you to add features. These components include optical tube units, illumination types and stage variations.

To improve resolution, microscope manufacturers have introduced a variety of analytical instruments, including the focused ion beam (FIB), the scanning electron microscope (SEM), the transmission electron microscope (TEM), the scanning probe microscope (SPM) and its derivatives, the scanning tunneling microscope (STM) and scanning force microscopes (SFM).

Scanning Probe Microscopes

SPMs are unmatched in their ability to measure surface angles nondestructively, said Yale Strausser, Applications Scientist at Digital Instruments. For example, SPMs can monitor planarization effectiveness without destroying the test wafer.

However, the SPM is not a product for searching large areas for isolated small defects. It is best suited for extreme detail after an optical instrument discovers the defect.

One of the most important applications of the SPM is quantitative measurements of surface roughness at the angstrom level in a clean room on an 8(“) wafer, said Tony Abbis, Vice President of TopoMetrix. For example, oxide integrity is directly linked to surface roughness at the nanometer level. To be useful to wafer fabs, the measurements must be performed in a clean room and include all the vibration and acoustic noise on the intact wafers.

Scanning Tunneling Microscopes

The STM is an offshoot of the SPM and produces 3-D topographic images of a surface. Most solid surfaces, including semiconductors and conductors, can be studied with an STM. Surfaces are examined in air, liquid or vacuum with a field of view from the atomic level to 250 µm x 250 µm and a vertical range from subangstrom to 15 µm.

STMs use an atomically sharp tip, usually made of tungsten or platinum iridium. The quality of STM images depends on the mechanical and electronic structure of the tip.

In the most common mode of operation, the STM uses a piezoelectric transducer (PZT) to scan across the sample (Figure 1).(1) A feedback loop operates on the scanner to maintain a constant separation between the tip and the sample. The position of the scanner is monitored, providing a 3-D measurement of the tip’s position. The STM is even capable of imaging individual atoms.

Scanning Force Microscopes

The SFM uses a sharp tip mounted on a flexible cantilever. When the tip comes within a few angstrom of the sample’s surface, van der Waal forces between the atoms on the tip and on the sample cause the cantilever to deflect. Normally, the lateral resolution of the SFM is about 1 nm.

The SFM, like the STM, employs a PZT to scan the tip across the sample and a feedback loop to operate the scanner and maintain a constant separation between the tip and the sample (Figure 2)(1). In this scheme, light from a laser diode is reflected from the back of the cantilever into a position-sensitive photodiode (PSPD). The image is generated by monitoring the position of the scanner in three dimensions.

STMs and SFMs are capable of in-depth statistical analysis of surfaces 10 nm or smaller. They provide topographic information that can be viewed from any altitude or azimuth. They also operate in an ambient environment with little or no sample preparation. Applications include analysis of optical components, magnetic disks, semiconductors and polymers.

Scanning Electron Microscopes

The SEM provides a highly magnified, but simplified, image of material surfaces. It operates at magnifications from 10X to 300,000X with a resolution approaching a few nanometers.

Within a vacuum, the SEM focuses a beam of electrons into a probe which is scanned over the surface of the specimen (Figure 3).(2) As the beam interacts with the sample, it creates electrons, internal currents and photon emissions. A portion of the electrons is collected by detectors and used to modulate the brightness of the CRT and produce an image.

The energy of the primary electron beam in most SEMs ranges from a few hundred electron volts (eV) to 30 keV. Although the unit produces excellent topographical contrast, spatial resolution depends upon the accelerating voltage and the mode of analysis.

Used in a noncontact mode, SEMs measure voltages on the surface of semiconductor devices. The measurements are made rapidly and can be used to determine circuit waveforms at internal circuit nodes.

Add-ons to the SEM, such as the auger electron spectrometer (AES), aid defect review and detection of extremely small particles with a depth resolution of 20 Å, said Charles Bryson, President of Surface/Interface. AES units help identify the elemental composition and the chemical bonding of atoms in the surface region of solid samples.

The auger process, first described by Pierre Auger in 1923, is used to identify the elemental composition and the chemical bonding of atoms on the surface of samples. It is the most frequently used surface, thin-film or compositional analysis technique because it can specify sampling depth and has a lateral spatial resolution to 300 Å.

Transmission Electron Microscopes

The TEM has become the mainstay of scientists needing to characterize materials. It offers a lateral resolution better than 0.2 nm and provides image and diffraction information from a single sample.

The TEM uses a focused electron beam on a sample less than 200-nm wide. It obtains information from undeflected and deflected electrons that penetrate the sample thickness. A series of magnetic lenses at and below the sample delivers the signal to a detector, which is usually a fluorescent screen, a film plate or a video camera. The spatial information in the signal is magnified from 50 to 1 million times.

The higher the operating voltage of a TEM, the greater its lateral spatial resolution. Some commercially available high-voltage instruments operating from 300 keV to 400 keV have point-to-point resolutions greater than 0.2 nm. High-voltage TEMs also have greater electron penetration and can work with thicker samples than lower-voltage models.(3)

Focused Ion Beam Technology

FIB technology helps inspect defects in the micron to submicron range. It uses a set of high-resolution optical images with coordinates to direct the FIB to a subsurface location.

It removes material, exposes defects and provides a cross-sectional area for direct observation and analysis, said Dave Laken of FEI. It then prepares the material for characterization by SEMs and other microscopy techniques.

An FIB and SEM workstation is a popular combination in wafer fabrication. It provides the micromachining and microdeposition capabilities of an FIB with the high-resolution field-emission techniques of an SEM.

Software Helps Navigation

For most microscopy systems today, software is a must. It helps guide the equipment to a particular feature, defect or even a particle on an IC, and directs the ion beam of the FIB or the electrons of the SEM.

Software packages and device drivers such as Merlin’s Framework from Knights Technology access IC design data bases and allow you to direct the beam or probe on the sample stage of a schematic, netlist or cell hierarchy. The company also provides networking capabilities to connect various inspection instruments together for sharing information.

From the simple identification of solids, microscopy has expanded into inspection using secondary electron beams, internal currents and other technologies to provide the atomic resolution needed for today’s microelectronics.

References

1. Howland, R. and Kirk, M., “Scanning Tunneling and Scanning Force Microscopy,” Encyclopedia of Materials Characterization, Butterworth-Heinemann, 1992, pp. 85-98.

2. Bindell, J., “Scanning Electron Microscopy,” Encyclopedia of Materials Characterization, Butterworth-Heinemann, 1992, pp. 70-84.

3. Sickafus, K., “Transmission Electron Microscope,” Encyclopedia of Materials Characterization, Butterworth-Heinemann, 1992, pp. 99-115.

Microscopy Products

TEM Offers Continuous Zoom

From 40X to 600,000X

The EM 906 Transmission Electron Microscope offers continuous magnification zoom from 40X to 600,000X with no change in image orientation. The unit provides constant image brightness throughout the magnification range and a continuous diffraction camera length zoom from 180 mm to 1,600 mm. Electromagnetic image orientation is supported in all magnification ranges. The microscope stores instrument data, operating conditions and specimen positions. $230,000. Electron Optics Division, Carl Zeiss, Inc., (800) 356-1090.

SPM Performs 3-D Measurements

On 14(“) Samples

The Dimension 5000 Scanning Probe Microscope offers 3-D measurements on 14(“) dia samples. It requires no sample preparation and uses nondestructive inspection techniques. The microscope performs roughness measurements to 0.3 Å rms. Other measurements include planarization angles, grain size, via profiling, bearing analysis, and vertical and horizontal measurements with subnanometer resolution on areas to 90 µm2. $173,000 to $180,000. Digital Instruments, (800) 873-9750.

FIB Handles Wafers

With 0.1-um Position Accuracy

The FIB 800 Focused Ion Beam Workstation handles 8(“) wafers with 0.1-micron position accuracy. The automated station provides micromachining, metal deposition and scanning ion microscopy functions for cross-section cutting and viewing, device modification and failure analysis. The gallium liquid metal ion source is guaranteed for 1,500 h. The beam voltage ranges from 5 keV to 30 keV. The system accommodates 10 programmed beam sizes. It has a five-axis stage with optical encoders. $800,000. FEI Co., (503) 640-7500.

Emission Microscope

Docks to Face of Test Head

The Docking Emission Microscope (DEMI) connects to the face of ATE test heads to detect dynamic functional failures in socketed devices and wafers. Users can introduce test vectors that switch the DUT to a functional state to reveal chip failures. DEMI replaces long cable sets and allows chips to run at full clock speeds. It places the DUT on the test head and within the optical plane of the emission microscope. Commercial docking tools allow users to attach DEMI to any of 17 families of commercial test heads. $230,000 to $300,000. Hypervision, Inc., (510) 651-7768.

CAD Navigation Software

Offers New Capabilities

The Merlin’s Framework high-performance CAD navigation software is used to locate particular features, defects or particles on an IC containing up to 16 million transistors. The upgrade features faster display time of IC designs, 400% reduction in memory swap space, and wafer or defect navigation capability. Response time to signal highlighting between the netlist and the layout views has been improved. A schematic option for displaying an EDIF schematic file also allows signal highlighting between the schematic and the layout view. The software is available for more than 40 systems, including scanning electron microscopes. $85,000. Knights Technology, Inc., (408) 988-0600.

Microscope Has Constant Focus

And 1:8 Magnification

The WILD MZ8 Stereomicroscope provides a 1:8 zoom and retains focus when the magnification is changed. The modular design accommodates various accessories. It offers a choice of binocular tubes, eyepieces and stands, and interchangeable objectives. The microscope provides coarse and fine focus drives. Interchangeable planachromat and achromat objectives are available. Starting at $3,200. Leica Inc., (708) 405-0123.

Wafer Inspection Station

Provides Automated Routines

The MicrostationTM is a single-cassette wafer inspection and review system that allows routines to be automated. It consists of a wafer loader, a microscope, a motorized programmable stage and a wafer shuttle assembly. The company’s proprietary software is available for storing or printing video images and interfacing to Tencor or KLA inspection systems. $70,000 to $140,000. Nikon Inc., (516) 547-8531.

Inverted Microscope Provides

12.5X Eyepiece Magnification

The XJP Series Inverted Metallurgical Microscope is available in four models, and includes the microscope body and choice of a tube, stage, stage plate, eyepiece, objective and coaxial illuminator. The eyepiece magnification ranges from 5X to 12.5X and the objective from 10X to 100X. A 6-V/15-W halogen lamp coaxial illuminator is provided. Starting at $620. Optro-Mechanics (USA) Corp., (800) 890-3333.

System Performs Contact

And Noncontact Measurements

The AutoProbeR VP is an atomic resolution scanning probe microscope that performs scanning tunneling and atomic force microscopy (AFM) in ultra-high vacuum. The scanning probe touches the sample surface when used in the contact AFM mode and also can perform noncontact measurements. It is used to conduct atomic resolution surface studies on semiconductor samples, data storage products and composites without applying a conductive coating. The VP system can be used alone or incorporated into other equipment. $240,000. Park Scientific Instruments, (408) 747-1600.

Spectrometer Combines With SEM

To Detect Wafer Defects

The Pierre Model 10 Auger Electron Energy Spectrometer (AES), when added to a scanning electron microscope, performs wafer defect review and detection. The AES provides spatial resolution to characterize particles to 20 Å. The lens-to-sample working distance is 25 mm. The energy ranges from 50 eV to 2,100 eV. The data acquisition software for the system is available for a workstation or a PC platform. Additions to the software include constant retardation analysis for improving the resolution of the spectra during particle detection. $98,870. Surface/Interface Inc., (415) 965-8205.

Measuring Microscope Adapts

To Video Screening Tasks

The Model XAM-1 Measuring Microscope uses the reticle for a reference point and performs measurements as the frame moves along a 1.1(“) dovetail slide. The frame is actuated by a control knob on a digital micrometer, which is programmable in inches or millimeters with divisions of 0.00005(“) and 0.001 mm. The basic model measures at 40X with additional objectives. Eyepieces can vary magnifications from 10X to 800X. The portable microscope rotates 360° around the support post. $2,100. Titan Tool Supply Co., Inc., (716) 873-9907.

Scanning Microscope Measures

Thermal Features of Structures

The Scanning Thermal Microscopy (SThM) probe of the TMX 2000 Atomic Force Microscope measures the thermal characteristics and features of structures. A control unit sends thermal image signals to the electronic control unit via an analog-to-digital converter. The force between the probe and the sample provides thermal and topographic information. It is used in material sciences and electronic-component failure analysis. $16,000. TopoMetrix Corp., (408) 982-9700.

Surface Profiler Is Suited

For Production and QC Tasks

The Maxim·GP is a noncontact surface-profiling microscope suited for research and development, production and quality control. It measures polymers, metals, glass, ceramics and silicon materials. Lateral resolution of images is <0.5 µm with measurement areas of 4.5 mm2. Vertical surface-profile resolution is <1Å, and the effective magnification ranges from 0.7X to 100X. The unit also has a graphical user interface with software to analyze surface profiles and provide datalogging capabilities. Zygo Corp., (203) 347-8506.

Copyright 1995 Nelson Publishing Inc.

April 1995

Sponsored Recommendations

Comments

To join the conversation, and become an exclusive member of Electronic Design, create an account today!