Key Microscope Specs Guide Buying Decisions
Understanding the science behind optical systems makes it easier to know what specs to ask about when buying a microscope.
C.G. Masi, Contributing Technical Editor -- Test & Measurement World, 10/1/2000
| Basic Microscopy Series
• Part 1: Microscopes Rely on Basic Optical Components • Part 2: Basic Optical Effects Limit Image Quality • Part 3: Light Characteristics Limit Optical Quality • Part 4: Proper Lighting Gets the Most from Microscope Images • Part 5: Key Microscope Specs Guide Buying Decisions • Part 6: Test Drive a Microscope |
Previous articles in the Test & Measurement World Basic Microscopy Series1 dealt with the optical engineering concepts that microscope designers use to create products. Many of the optical parameters used in design correspond to useful specifications. Knowing how these specifications relate to microscope use will help you buy the type of instrument best suited to your needs.
Users are likely to quote different specifications as critical because their needs vary depending on what they’ll use a microscope to examine. Semiconductor wafer inspectors constantly push the limits of resolution, so they think first about numerical aperture. Inspectors of bare circuit boards, however, care more about magnification, which determines how easy it is to spot what they want to find.
Table 1 lists the most useful common optical specifications for a microscope, along with a concise description of what each specifies. Many of these specifications, such as magnification and numerical aperture, are taken directly from the optical concepts introduced in previous articles. Three, however, arise from considerations that are beyond the scope of this series: coarse-focus resolution, fine-focus resolution, and zoom ratio.
Magnification is the first thing most people think about when they consider buying a microscope. After all, magnification lets them see a sample as it would look if it were much larger than it really is.
What users really want, however, is better visibility. They want to examine something and see what it is. If you need to read the label on the top of an IC package, you must see the characters well enough to identify them. You don’t necessarily need to perfectly resolve the vertical legs of a lower-case m, as long as you can tell the m from an n.
Focus on Key Specs
The trick, therefore, is to understand what characteristics of a microscope define the visibility necessary for your particular application. Then, you can choose the specifications in Table 1 that will best express—and quantify—them.
For near-macroscopic electronic subassemblies, such as surface-mount PCBs, you need a modest magnification of the sample to make the inspection task easier. Most human inspectors can see SMT components and even their solder joints (providing the joints aren’t hidden under components) with their unaided eyes. But these features are near the eye’s limit of resolution. A magnification of only a few tens of diameters and adequate shadow-free illumination will provide good visibility.
For semiconductor wafer inspection, on the other hand, limiting resolution is the key spec, because inspectors want to distinguish between small surface features. Each microscope has a limiting resolution that represents the smallest separation you can clearly see. If you try to examine features beyond the microscope’s limiting resolution, all you see is a blur. Of course, you need enough magnification to make the separation visible to your eye, but magnification beyond that is “empty” magnification because it simply makes the blurs bigger.
A microscope’s numerical aperture (NA) relates resolution to the wavelengths of light used to illuminate a sample:2
where:
Y = the minimum spacing that can be resolved between two objects
l = the wavelength of light forming the image
NA = the numerical aperture of the lens.
In practice, virtually all manufacturers of high-quality microscopes make objectives having numerical apertures approaching 0.95. That’s about as close as they can get to the theoretical limit of NA =1 for lenses in air. These lenses are generally called diffraction-limited lenses. That means the manufacturers have corrected Seidel aberrations, coma, and distortion so they do not affect the resolution limit imposed by the lens’ numerical aperture and the wavelength band used by the optics. And it means you can’t increase resolution by buying a “better” lens. It doesn’t exist.
The relationship between the numerical aperture and wavelength shows that for a given NA value, as a wavelength decreases, resolution improves. Thus, moving to shorter wavelengths for sample illumination can improve resolution.
Until recently, microscopes for inspecting electronic components all used wavelengths in the visible spectrum, roughly from 500 nm to 800 nm. The short-wavelength end of the band provides a theoretical limiting resolution of 263 nm—about a quarter of a micron.
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| Figure 1. Because UV radiation is invisible to human eyes, a UV microscope requires a video camera sensitive to UV light as well as optics made of special materials that pass UV light. (Courtesy of Nikon.) |
Within the past decade, however, the need for higher resolution to inspect submicron semiconductor features has forced microscope manufacturers to produce instruments that produce images using radiation at 393.4 nm (I-line) and 365 nm (K-line) ultraviolet (UV) wavelengths. Because the limiting resolution becomes smaller at these shorter wavelengths, UV microscopes, such as the one in Figure 1, can resolve submicron features.
Magnification Reaches a Limit
Modern microscopes can reach beyond the theoretical maximum resolution, but not with optics alone. They accomplish this feat by acquiring images from high-resolution electronic cameras and using image-processing software to sharpen the images. In this way, they can resolve features as small as 100 nm.
For microscopes operating near the resolution limit, magnification is no longer an issue. It’s possible to reach magnifications of nearly 10,000X through optical means alone. Combining that magnification with the magnification available by using a CCD camera can boost the overall magnification to 400,000X or more. Although a microscope-camera combination can produce such astronomical magnifications, they generally have no use. The microscope still has a resolution limit of 100 nm.
Too much magnification also can compromise the system’s field of view (FOV), which is that part of the surface (measured in microns) you can see through the microscope. In a well-designed microscope, the ocular is the element that limits the FOV.
The ocular has its own FOV, which is the angular field of view, or angular size, of the image you see through the ocular. It is fixed by the ocular’s optical design. Wide-field oculars can have fields of view in the range of 40 degrees or more.
The higher the magnification, however, the less actual surface you can cram into the ocular’s fixed angular FOV. If you need to see more surface in one image, you can either get a wider-field eyepiece or reduce the magnification.
Think in Three Dimensions
In addition to a field of view, you also need to think about a microscope’s depth of focus, a specification that describes how deeply you can see into a sample. If you plan to examine surface features on semiconductor wafers, you don’t need much depth of focus to clearly see the features you want to view. On the other hand, if you expect to view PCBs, you may need a depth of focus of several millimeters. This larger depth lets you clearly see things that exist between the tops of tall components and the PCB’s surface. Luckily, depth of focus tends to increase as magnification decreases.
Generally, you want as long a working distance—the distance from the objective to the sample—as you can get. Most microscopists have embarrassed themselves by turning the focus knob too far the wrong way, thus driving an objective into, and possibly through, a fragile sample. Inspecting populated PCBs calls for long working distances that let you avoid catching the objective on protruding components when you move samples under the lens. A long working distance also tends to increase depth of focus.
Zoom ranges and ratios specify an optical system’s flexibility. Zoom oculars allow the user to smoothly vary the overall system magnification. The zoom range gives the ocular’s maximum and minimum magnifications. The ocular’s zoom ratio is its maximum magnification divided by its minimum magnification.
Change Lenses Easily
In applications such as failure analysis, you may need to change the microscope’s overall magnification as you examine a sample. Changing objectives interrupts your view of the sample and may require you to refocus the image. Instead of changing the objective lens, you can use a zoom ocular that changes magnification from, say, 20X to 60X. You simply twist a knurled ring around the eyepiece to adjust the zoom magnification. A zoom ocular lets you change magnification without interrupting your view or refocusing the image. Remember, though, these characteristics are all related. So as magnification increases, the depth of focus decreases, the image appears more blurry, the FOV decreases, and so on.
Just as several specs help you determine the optical characteristics of a microscope, several other specs tell you about the ergonomic characteristics. The ergonomic specifications of ocular tilt range and optical axis offset specify the amount the eyepiece tilts away from the optical axis and its distance from the optical axis, respectively. Depending on microscope models, the amount of tilt and offset may be fixed or variable. A microscope purchaser can get just about any tilt or offset called for by an application.
A tilted ocular lets an operator position the lenses for comfortable viewing. The offset places the eyepieces closer to a user. Without sufficient offset, an operator might have to lean over the near edge of a large sample, perhaps a semiconductor wafer, to examine a site on the far edge.
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| Figure 2. Most binocular microscopes provide for adjustable interpupilary distance to accommodate the eye spacing of many users. (Courtesy of Leica Microsystems.) |
To accommodate different users with different face sizes and shapes, a binocular microscope (Fig. 2) must let you adjust its interpupilary distance. Because the oculars can move back and forth, the interpupilary distance spec is usually a range, rather than a single value.
The final specifications listed in Table 1 are contrast and illumination. These important considerations were covered in the previous installment of this series (September 2000).
Think Before You Buy
Microscope optical technology has reached a level where you can find an instrument to fit just about any set of specifications you could wish for. When approaching the purchase of an optical microscope, first look closely at your application. Saying, “I want an inspection microscope,” won’t do. You have to know what you need to inspect, what you must look for, who will use the microscope, and where the microscope will be located—lab, production floor, or other location.
Knowing what you are going to inspect will probably provide most of the information about what to specify. For example, the size of the features for a wafer inspection microscope will determine the resolution you need. The heights of the components will determine the minimum working distance for a microscope used to inspect populated PCBs. Inspecting 300-mm wafers will call for a larger optical-axis offset than inspecting 50-mm wafers.
The details you’re looking for affect most specifications in some way. For example, the optical system’s resolution partly determines the accuracy a measuring microscope can achieve. To see strain increase in transparent samples may require polarized-light illumination. Scanning thousands of bare boards every day for flawed traces will probably lead you to specify a video microscope with the widest possible field of view consistent with sufficient resolution on a video monitor. Looking down through an ATE test head to make sure probes touch their targets for overhead wafer sorting will call for a much longer working distance than measuring critical dimensions on the same wafer.
The characteristics of the person or people who will use a microscope governs what ergonomic specifications you’ll look for. Most binocular microscopes, for example, can accommodate a wide range of interpupilary distances. But, in some cases, you may have to special order a microscope for someone with wide-set or narrow-set eyes. Remember to consider how much the users know about microscopes and how often they will use the microscope you want to buy. Their familiarity—or lack of it—may determine whether you buy a manual or an automated microscope.
The environment around a microscope also makes a difference. If there’s a lot of dust around, you must protect the optics and clean them often. For operation in a semiconductor fab’s clean room, you’ll need to find a microscope with a mechanical system that minimizes the number of small particles sloughed off as the mechanism wears.
I suggest you write a narrative description covering the points above (what you are looking at, why you are looking at it, who is doing the looking, and where the looking is done). Handing this document to a microscope sales engineer can save hours of back-and forth communications and relieve the gnawing feeling you’ve forgotten key information.
After you have a clear idea of what you’ll do with an instrument, you can work with microscope vendors’ representatives to identify the most important specifications. Working in concert, you can determine the actual numerical specification for the instrument you need to buy. T&MW
FOOTNOTES
1. The previous installments can be found on our Web site: www.tmworld.com/articles/2000/microscopes.htm.
2. Masi, C.G., “Light Characteristics Limit Optical Quality,” Test & Measurement World, June 2000. p. 82. www.tmworld.com/articles/2000/06_microscopes3.htm.
C.G. Masi works as a freelance technical journalist. He is the former chief editor of Test & Measurement World. E-mail: tmw@cahners.com.
