a. Overall Plan On the Construction
We are in the process of constructing a versatile ultrahigh vacuum (UHV) system that enables
the investigation of soft-contact formation and the development of novel contact strategies
for technologically important material systems. This instrument,
as schematically shown in Fig. 5, has several stations
that accept samples: (1) metal
deposition/surface-analysis station, (2) stamping/electrical-characterization station
(3) sample load-lock station, (4) sample flipping station, (5) organic material deposition
station, (6) glove box, and (7) scanning tunneling microscopy (STM) station.
Several of these stations are described in detail below.
Samples, of nominal size 3.5 cm x 2.1 cm, are carried in a “sample transporter”
and transferred between various stations by a wobble stick, which is also used to
perform simple mechanical maneuvers in UHV. A major part of the system, not shown
in the top view of Fig. 5, is an evaporation facility, which sits underneath the metal
deposition station. Additionally, there are several surface analysis capabilities on
the proposed instrument, including a double-pass cylindrical mirror analyzer (CMA),
a reverse-view low energy electron diffraction (LEED), and an oscillating Kelvin probe.
Several pieces of the overall system are existing equipment and involve no construction,
acquisition, and expenses. Designs of several new pieces of the instrument are also
finished or near completion. These will be described in detail below. The complete system
has a rather modest footprint (~8’ x 6’) and will be housed, along with three racks of
electronics, in one of several available lab spaces, owned by the physics department,
in the Ingersoll Building of the Brooklyn College campus. Standard service utilities,
including power, chilled water, compressed gas, and vacuum, are available in these lab spaces.
Various parts for the instrument will be made in the machine shop by two skilled machinists,
provided for by the university.
The instrument is to be constructed in two stages. In Phase 1, we plan to design and
construct stations (1) through (4), as outlined above. An existing Si-based molecular
beam epitaxy chamber will be connected to these stations at this stage, and will provide
the metal deposition capability needed for the initial assessment of the instrument.
By the completion of Phase 1, the system will be fully operational to conduct the kind
of in-situ stamping and characterization experiments as shown in Fig. 3. It will also
have full surface analysis capability by then. Note that station (7) STM is included in
Phase 1, as it is existing equipment. At this stage, we plan to make good use of our
website, and also through other means of communication, to announce the concept of the new
instrument, to attract potential users, and to provide updates on the status of this instrument.
In Phase 2, which covers the next 9 months of the project, we plan to have students
design and construct a dedicated metal-evaporation chamber and also stations (5) and
(6), as outlined above. Additions in Phase 2 enhance our ability to study more materials
and interfaces, and they add the capabilities to deposit organic material in situ and to
attach self-assembled monolayer (SAM) molecules on the surface of the sample. Phase 2 is
also used to test, modify, and calibrate the operational parts of the instrument, and to
train interested users of the instrument. New developments/results will be announced on
the website. Finally, in the remaining 3 months of the project, performance of the added
parts of the instrument will be evaluated. The final three months is also used to document
the operation procedures/instructions on the website and to establish a web-based sign-up
program for the instrument. It is also time to publish the design and the principles
behind the operation of this instrument in a scientific journal.
b. The Metal Deposition/Surface Analysis Station
The basic housing for the sample manipulator, shown in Fig. 6,
is assembled from an 8” x-y stage, a differentially pumped 4.5” rotatable flange, a
4.5” welded-bellows linear translator, a 2.75” linear translator, a rotary feedthrough,
a hollow rotary feedthrough, and other vacuum hardware. The temperature of the sample
can be controlled in the range 100-1300 K, by resistive heating and by contact to a LN2
reservoir. From its normal, downward position, the sample can receive deposition, through
shadow mask, from sources in the evaporation chamber below. In-house designed and machined
parts are used to construct a compact sample holder, attached through three 1/4” rods to
a double-sided 2.75” conflat flange, and a liquid nitrogen reservoir, attached through
thermal isolation and three 1/4” rods to the larger 4.5” conflat flange. As shown
in Fig. 7, the design of the sample holder is near
completion. The inner rotation feedthrough provides, through a bevel gear set,
sample tilt of up to ~90o, and the outer rotation from the hollow rotary feedthrough
is used to unlock/lock the sample through loaded springs. A receptacle for the sample
transporter is constructed on the LN2 reservoir. The relative motion between the sample
holder and the LN2 fixture, provided by the 2.75” linear translator, allows the height of
the sample transporter to be adjusted to the loading or unloading configuration. The
same motion, with the sample properly secured on the sample holder (and with the sample
transporter detached), could provide cooling to the sample if desired. Samples are held
on both ends by flexible tantalum clamps (~0.010” thick), which do not put excess stress
on the sample while pressed against the milled and tantalum-coated bottom of the LN2
reservoir. The two sides of the sample holder are electrically isolated and can supply
resistive heating to semiconducting and conducting samples and substrates. Sample
temperature can be measured by a thermocouple bead spot-welded to the tantalum clamp
and also by hand-held pyrometer through a nearby quartz viewport. With the sample holder
tilted 90o, a rotation of the 4.5” flange can bring the sample to face low-energy electron
diffraction (LEED), Auger electron spectroscopy (AES), and an oscillating Kelvin probe.
The x-y movement on the 8” flange provides fine adjustment of the sample position for
surface analyses. Sample tilting is facilitated by counter-weight (not shown) to minimize
stress on the mechanical linkage. Copper bushings are used on stainless steel axle to
minimize galling, with “catchers” to prevent the metal powder from entering the sample
area. In addition, linear recirculating ball-bearings are used to guide the motion and
maintain the rigidity between the sample holder and the liquid nitrogen assembly.
Deposition can be performed through an aperture, with or without shadow masks. A sliding
adjustment on the shadow mask holder allows two different patterns to be accessed.
Combined with sample rotation, a variety of patterns can be deposited on the sample
through masking.
c. The Stamping/Electrical-Characterization Station
This station is made up of an upper sample manipulator and a lower stamping/characterization
mechanism. The upper manipulator, with rotational and x-y-z translational movements,
controls the precision placement of sample for the stamping experiment. It also provides
sample heating and holds the weights used in the stamping experiments. Samples can be placed
in two positions on the manipulator: one with clamping and heating capabilities; the other,
with the sample freestanding, is used to drop the sample onto the lower jig. The rotational
motion from the rotary feedthrough is used to adjust the height of the “sample transporter”
to either the loading or the unloading position. The main features of the lower clamping
station are a small platform with temperature control, from LN2 to elevated temperature,
and an electrical probe with x-y-z adjustment. A port is available to add a second probe,
if the need arises. A mirror wedged on the chamber ceiling provides a top-down view of
the probe position with respect to the pattern on the sample. Weights, as opposed to
mechanical drives, have been chosen as a simple means to provide the stamping pressure,
without precise alignment. They are hung from a support rod with small separations between
them, as shown in the left hand of Fig. 8. As the weight assembly is continuously lowered,
the weights of the individual blocks are applied to the sample, one-by-one from the bottom up.
The under side of the bottom block is gold coated, connected electrically, and can be used as
an electrode for the electrical measurement. For weights ranging from 50 g to 1.00 kg, the
nominal pressure exerted on the sample is 1.2 - 24 kPa, which should be adequate for contact
stamping. The actual pressure will depend on the pattern used.
d. Evaporation Chamber
There is a Si-based molecular beam epitaxy (Si-MBE) system in the PI’s laboratory,
which is presently employed for NSF-sponsored research on nano-particle assembly in
liquid thin films. This is the chamber that will provide the evaporation of metal
during Phase 1 of the development of our instrument. As shown in the red shaded area
of Fig. 9, the present Si-MBE chamber
has LN2 shrouding and partitioning to separate a 40 c.c. e-gun and a dual (Temescal-GEMINI)
16 c.c. e-gun in two compartments. In addition, two ionized Knudsen cells, originally
designed for impurity doping of Si, are available for depositions. There are two INFICON
electron impact emission spectroscopy (EIES) sensors to optically detect the flux intensities
and two independently controlled shutters. The Si-MBE chamber is pumped by a large cryo-pump.
The Si-MBE chamber has large volume e-guns and was designed for sustained use and co-evaporations.
It is adequate for initial evaluation and testing of the UHV stamping mechanism, but is
cumbersome (and costly) to use for the exploring of new materials. Therefore in Phase 2,
a less elaborate deposition chamber will be constructed, as shown in the green-shaded area
of Fig. 9, to take over the responsibility
of metal evaporation. Material evaporation can be provided by a small “button” e-gun,
capable of handling four different materials, and a series of evaporation boats. Each
boat is individually powered and shielded from the other boats. A linear translator
allows a particular boat to be centered for the deposition. In addition, ports are set
aside for inclusion of Knudsen cells in the new evaporation chamber. Deposition is
monitored by both quartz crystal monitor and EIES sensor. LN2 shrouding and shuttering
serve to minimize the contamination of the main chamber.
e. Organic Deposition Chamber
To minimize cross contamination, the organic evaporation chamber is pumped by an
ion pump and normally valved off from the main chamber. It has an extendable arm to
receive sample from the main chamber and retract to a fixed sample stage inside the organic
evaporation chamber. Evaporation boats are used to evaporate organic molecules through a
shielding. A crystal oscillator is used to monitor the thickness of the deposition.
f. Scanning Tunneling Microscope
An existing Omicron UHV-STM1 is attached to the main chamber.
Special cassettes are made to receive samples for STM scan and transfer to the
evaporation station via the usual “sample transporter” assembly. Cassettes are
also used to replace scanning tips used in the STM.
g. Vacuum Vessel and Pumps
The main stainless steel chamber, which houses the metal evaporation
and stamping stations has already been bought and received.
Selected drawings of the chamber are shown in Fig. 10.
An ion getter pump will
provide pumping for the main chamber.
h. Miscellaneous
The sample transporter is constructed from stainless steel and tantalum sheet metal
to minimize its weight. An Omicron style platen allows it to be picked up by the wobble stick.
The wobble stick is also used to slide an arm on the sample transporter to move the sample.
The sample flipper receives and rotates the sample without touching the center portion of
the sample. The glove box is a standard commercial product with nitrogen capability.