Microcolumn assembly using laser spot welding

Details Microcolumn assembly using laser spot welding Microcolumn assembly using laser spot welding

1999-07-30

2001-02-27

Mack, Ricky (Department: 2873)

Optical: systems and elements

Lens

With support

C250S310000

Reexamination Certificate

active

06195214

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to electron beam microcolumns, and in particular to a microcolumn assembly technique.
BACKGROUND OF THE INVENTION
Electron beam microcolumns based on microfabricated electron optical components and field emission sources operating under the scanning tunneling microscope (STM) aided alignment principle were first introduced in the late 1980s. Electron beam microcolumns are used to form a finely focused electron beam. See Chang, T. et al., “Electron-Beam Microcolumns for Lithography and Related Applications”
J. Vac. Sci. Technology
, B 14(6), pp. 3774-3781, November/December 1996, and Lee, K. et al, “High Aspect Ratio Aligned Multilayer Microstructure Fabrication”
J. Vac. Sci. Technology
, B 12(6), pp. 3425-3430, November/December 1994, incorporated by reference herein. These columns offer the advantages of extremely high resolution with improved beam current, small physical size, and low cost, and can be used in a wide variety of applications, such as electron beam lithography.
Microcolumns are high-aspect-ratio micromechanical structures comprised of microlenses and deflectors. The microlenses are multilayers of silicon chips (with membrane windows for the lens electrodes) or silicon membranes spaced apart by thick, 100-500 &mgr;m, insulating layers. The lenses have bore diameters that vary from a few to several hundred micrometers. For optimum performance, the roundness and edge acuity of the bores are required to be in the nanometer regime and alignment accuracy between components on the order of less than 1 &mgr;m.
Electrodes of the microlenses can be made from 1 to 2.5 &mgr;m thick silicon membranes by electron-beam lithography and reactive-ion etching (RIE). The starting material is a 4 inch diameter and 500-&mgr;m-thick double-sided polished wafer containing arrays of 7 mm×7 mm chips. At the center of each chip is a 1 mm×1 mm membrane formed by wet isotropic etching using in preferred form either a highly boron doped or a reverse-biased p
junction etch stop.
Assembly of the lenses and the column typically involves stacking together silicon components and Pyrex spacers and using anodic bonding.
FIG. 1
shows a cross-sectional view of a 1 kV microcolumn based on the well-known STM aligned field emission (SAFE) concept, showing source section
1
and Einzel lens section
3
. Scanning tunneling microscope (STM) scanner
5
emits an electron beam
6
in the direction of sample plane
25
. The beam
6
first passes through the source
1
, composed of silicon microlenses, 5 &mgr;m diameter extractor
7
, 100 &mgr;m diameter anode
11
, and 2.5 &mgr;m diameter limiting aperture
13
. The three microlenses are separated by two insulating spacers
9
. The insulating spacers
9
are preferably formed of Pyrex, but could be made of any other suitable insulator, such as SD-2 glass made by Hoya. The source
1
is mounted on aluminum mounting base
15
, which contains an octupole scanner/stigmator
17
. The electron beam
6
then passes through the Einzel lens
3
, which is composed of two 100-200 &mgr;m diameter silicon microlenses
19
and
23
with a 1-1.5 &mgr;m thick free-standing silicon membrane
21
disposed therebetween. Each silicon layer is again separated by insulating spacers
9
. The electron beam
6
then passes on to sample plane
25
and channeltron detector
27
.
The source
1
and Einzel lens
3
are shown expanded and in greater detail in FIGS.
2
(
a
)-(
b
) with similar reference numbers identifying the same structures.
The conventional approach to bonding the insulating and microlens layers of the microcolumn involves the use of anodic bonding. Anodic bonding is an electrochemical process for heat sealing of glass to metal and semiconductors, as shown in FIGS.
3
(
a
) and (
b
). At elevated temperatures (300-600° C.), Na
2
O in the Pyrex or other glass dissociates to form sodium and oxygen ions. By applying a potential with voltage source
52
between a first silicon layer
53
and a glass insulation layer
55
, sodium ions in the glass migrate from the interface in a direction indicated by arrow
63
, while uncompensated oxygen anions
61
move toward the induced positive charge
59
of the silicon anode to form chemical bonds.
This process, previously used for single sided bonding only, has been extended to multilayer bonding. After the first silicon-to-glass bond, another silicon chip or membrane can be bonded to the free surface of the glass by reversing the applied potential, as shown in FIG.
3
(
b
). In this case, second silicon layer
57
is placed atop glass insulation layer
55
and an opposite potential is applied by voltage source
52
. Here, the induced positive charge
59
causes the sodium ions to migrate downward in the direction of arrow
63
, causing the oxygen anions
61
to form chemical bonds with the second silicon layer
57
. To achieve satisfactory multilayer bonding, special attention has to be given to the control of temperature, the applied voltage, the bonding time, and, in particular, the surface condition of the layers.
One disadvantage of the anodic bonding process is that it must be conducted at elevated temperatures, which typically requires several hours of heat-up (to approximately 400° C.) and cool-down time, as well as a physical connection of a high voltage probe, during which time drift, bond-induced shift, and expansion can cause the alignment to degrade. This process must then be repeated for each additional layer.
Accordingly, it is clear that there is a need for a method of forming microcolumn structures that avoids the burden of anodically bonding each layer of glass to silicon.
SUMMARY
In accordance with the present invention, a method for forming microcolumns is provided in which laser spot welding is utilized to bond the multiple layers of an electron beam microcolumn.
In one embodiment, a first microlens is welded to a glass insulation layer by focusing a laser through the insulation layer onto the silicon microlens. The glass is transparent to the laser, allowing all of the energy to be absorbed by the silicon. This causes the silicon to heat, which, in turn, heats the adjacent surface of the glass insulation layer creating a micro-weld between the silicon and glass.
The insulation layer includes a portion which protrudes beyond the edge of the first microlens so that when a second microlens is attached to the opposite side of the insulation layer, the second microlens can be laser spot welded to the portion of the insulation layer which protrudes beyond the edge of the first microlens.
In a second embodiment, in place of the protruding portion of the insulation layer, the first microlens includes a window so that a laser may be shone through the window and through the insulation layer to irradiate the second microlens disposed opposite the first microlens.
In another embodiment, the present method is used for batch fabrication of microcolumns. Using the second embodiment of this invention, wafer-scale microlens and insulative layers are bonded together.


REFERENCES:
patent: 4822167 (1989-04-01), Lobazov et al.
patent: 5122663 (1992-06-01), Chang et al.
patent: 5155412 (1992-10-01), Chang et al.
patent: 5789748 (1998-08-01), Liu et al.
patent: 6077417 (2000-06-01), Lee et al.
Chang, T.H.P. et al, “Electron beam microcolumn technology and applications” Proceedings Reprint, reprinted from Electron-Beam Sources and Charged-Particle Optics by theSociety of Photo-Optical Instrumentation Engineers, vol. 2522, pp. 4-12 (1995).
Chang, T.H.P. et al., “Electron-beam microcolumns for lithography and related applications”J. Vac. Sci. Technol. B. 14(6) pp. 3774-3781 (1996).
Chang, T.H.P. et al., “Electron beam technology — SEM to microcolumn”Microelectronic Engineering 32pp. 113-130 (1996).
Kim, H.S. et al., “Miniature Schottky electron source”J. Vac. Sci. TechnologyB 13(6), pp. 2468-2472 (1995).
Kratschmer, E. et al., “Experimental evaluation of a 20×20 mm footprint microcolumn”J. Vac. Sci. TechnologyB 14(6), pp. 3792-3796 (1996).
Lee, K. Y. et al., “High a

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