Site Selection Considerations For
Scanning and Transmission Electron Microscopes
Wayne Vogen
Vibration Engineering Consultants, Inc.
Santa Cruz, California
  
Environmental factors must be controlled including floor vibration, acoustic level and
magnetic field when choosing a site for an electron microscope. A systematic procedure for
selecting a site from possible candidates is presented in this paper.
The best way to assure that the floor vibration is low is to locate the microscope on
the ground. The ground vibration level is rarely too high, If it is too high the vibration
sources can easily be identified and eliminated. In addition, vibration isolation tables
will not be required if the microscope is located on the ground. No single factor is as
important as finding a suitable location at ground level.
Many clean rooms have raised floors above the ground to provide room for various
utilities and airflow. For this case it is desirable to provide a platform that does not
increase the vibration level of the ground and is usually called a gravity table. These
tables can be purchased from isolation table manufacturers or built with certain simple
guidelines. A steel table with bolted, rather than welded construction is preferred. The
bolted construction provides higher damping because of the friction in the joints that
result in lower motion amplification at the table resonant frequencies. In addition a
bolted table can be assembled in pieces that eases the placement in the clean room. The
legs of the table should be located under the feet of the instrument so that the
diaphragmatic resonant modes of the tabletop do not increase the vibration level at the
feet of the instrument. Adequate cross bracing is essential to assure that the lateral
bending modes of the table and instrument combination are high enough in frequency so as
not to degrade performance. Figure I presents a design for a gravity table for a one
kilogram SEM that has proven to be quite acceptable. This table design can be scaled
easily to fit individual requirements. Leave adequate space (1 cm.) between the floor
tiles and the table to prevent foot fall vibration from affecting performance of the
microscope.
If a location on the ground is impossible to accommodate, no site should be selected
without a site survey that measures floor vibration, acoustic level and magnetic fields. A
site survey prevents expensive situations in which the electron microscope either needs to
be relocated, the site rectified, a vibration isolation table or acoustic enclosure
purchased and installed or reduced performance tolerated. In addition, most solutions to
an undesirable site are expensive, time consuming and usually less optimum then selecting
a good location originally.
When the vibration level is too high the typical reaction is to specify a vibration
isolation table.  However, isolation tables
have their own set of problems. They amplify the ambient vibration at low frequencies and
can interact with the existing isolation system of the electron microscope and the floor
resonance's and building modes of vibration to produce unwanted performance. In a clean
room the value of the additional area required for the table often exceeds the cost of the
table. The tables need to be maintained; our experience shows that over half the existing
vibration isolation tables are not working correctly because of various reasons. Some of
the reasons include 1. the table is not needed so it is not activated, 2. it is adjusted
incorrectly, 3. some miscreant has played with the air supply or leveling system, 4. or it
is broken. Usually the user is unaware that the isolation table is not working correctly.
If a vibration isolation table is being considered, expert advice should be obtained
and a floor vibration survey should be made to assure that the table will provide the
desired performance.
High acoustic levels ( >75 dB(C) ) can degrade performance of electron microscopes and
high performance steppers. Acoustic degradation of machine performance has become as
important as floor vibration. The high air flow common in modern clean rooms and the
reduced line width of semiconductors have combined to make acoustic excitation an
important consideration when selecting a site for electron microscopes and steppers.
Generally one must consider acoustic levels when the semiconductor process is producing
features smaller than 1.3 microns. Floor vibration isolation systems can increase the
sensitivity to acoustic excitation because the large mass of the instrument is no longer
anchored to the large mass of the floor because it is separated by the soft springs of the
isolation system.
How can acoustic excitation be controlled? First, when a clean room is being designed,
inlet and outlet attenuators should be installed on all HVAC fans. The attenuators can be
ordered from commercial HVAC equipment suppliers. They are relatively inexpensive, simple
and cause little pressure drop. Clean rooms fitted with this equipment should have noise
levels below 72 dB(C). If the clean room is constructed without attenuators, space should be
left to accommodate them if the desire arises later. Most existing clean rooms cannot be
retrofitted economically with attenuators. Sometimes sound absorbing material can be added
in the plenum and ducts up stream of the HEPA filters. However, the sound absorbing
material will increase the particulate load on the filters and shorten their life. our
experience has shown that a 5-6 dB(C) noise reduction is typical for this type of solution.
Another solution is a sound attenuating enclosure fitted with a separate HEPA filter
system, VEC and Industrial Acoustics Corporation have developed a system that can isolate
individual pieces of equipment in a clean room. These systems are medium priced and
somewhat inconvenient; for instance the SEM operator has to open a door to change
apertures, insert a sample, etc. However if a high
sensitivity electron microscope has to be located in a high noise room, this is a
viable solution.
If small rooms have high noise levels because of fans, low noise fans can be
substituted for existing equipment such as axial flow, plug fans. HVAC suppliers can
provide the correct solution.
Finally, active cancellation of acoustic noise in air ducts can solve selected acoustic
problems where the unwanted noise propagates in an air duct. This is usually not a
practical solution in a clean room because of the high number of fans and ducts and most
rooms are connected by a common plenum.
Electric currents cause magnetic fields that can interfere with the performance of the
electron microscope. Generally, the fields should be measured since their presence is
usually not obvious. These fields can be minimized by simple design and construction
techniques. Following these rules during construction and design usually involves very
little extra cost. in this paper we will address EMI below one kilohertz.
One of the major sources of EMI disturbances is power distribution wires that are
usually in conduits. This is almost always caused by either a fault condition or high
amounts of current in the conductors. In a normal conduit with a hot and neutral wire the
currents should be equal and opposite, and the magnetic fields from the two wires should
cancel. If high net currents from the conduit are measured with a current meter, either
the hot and neutral currents are not equal (usually caused by loss of the neutral current)
or the conduit itself is carrying significant current. In either case this represents a
fault (and possible safety) condition and should be corrected.
In order to cancel the external field totally the wires would have to be coaxial. This
is not common with current carrying wires so a net external field is present because of
the separation of the wires. This field can be minimized by twisting the current carrying
wires in the conduit. This is not standard practice but should be done in new
installations. In addition some problems have been corrected by twisting wires in existing
conduits that had external fields when the currents were balanced. It the conduit shows a
significant current when measured with a current meter and the currents in the hot and
neutral are equal, the steel conduit is probably carrying the current. Again, this is a
fault condition that should be identified and corrected.
A common EMI problem is caused by significant ground currents in water pipes, gas
pipes, rebar or metal wall studs. This situation is caused by normal grounding procedures.
The problem is extremely difficult to isolate and correct. If the current carrier is
identified sometimes it is possible to eliminate by breaking the current path with a
dielectric material, for instance by installing a dielectric union in a copper pipe. More
often the current finds a new path and remains a problem. Many times the only practical
solution is to move the pipe or electron microscope.
For currents below one kilohertz the only effective shield is a magnetically permeable
material. The most effective and expensive metal is mu metal. Iron or steel is quite
practical and less expensive. The iron conduit reduces the field from the wires roughly 3
to 5 times. If maximum shielding is desired, specify the heaviest wall conduit.
Generally, the goal is to produce a room with fields lower than 1 milligauss. It is not
effective to try to shield whole rooms because the initial permeability of mu metal or
iron is higher than a few mGauss and limit its success. If shielding is indicated it
is more practical to identify and shield the source because the field is high enough at
the source to over come the initial permeability of the material. For instance, if a
welder is a source of high fields a box of 1/8 inch thick, hot rolled steel will strongly
attenuate the fields. The larger the box, the less effective it will be.
High current carrying conduits should be routed away from EMI sensitive equipment. Care
should be taken to route the conduit supplying the power to the EMI sensitive equipment in
as direct a manner as possible. Do not route the power in a loop around the equipment.
This sometimes happens when modular power lines are installed in the laboratory.
The magnetic field decreases with distance. The reduction varies with the distance from
a line source (a straight wire), the distance squared from a dipolar source (two separated
wires with opposite currents) and at least the distance squared from a transformer or
heater. Obviously placing the sensitive equipment as far from the magnetic source as
possible is beneficial.
Florescent lights can be a source of strong EMI. The magnetic fields are produced by
some types of ballast in the light fixture. This problem can be fixed by using low
magnetic field producing ballasts or incandescent lights.
Cathode ray tubes or monitors are major sources of EMI. The horizontal scan coils are
the primary source of the EMI. The affect on the electron microscope can be detected by
measuring the spectrum of the magnetic field, or by turning the monitor off. The scan
frequency is usually different from the 60-hertz line frequency. Beware of "low
EMI" emitting monitors since most of them direct the magnetic fields out the side of
the monitor instead of the front where the personnel is usually located. Usually the
problem is corrected by moving the monitor farther away from the column of the electron
microscope. |
