Three-dimensional fabrication of miniature electron optics

ADVANCES IN IMAGING AND ELECTRON PHYSICS VOL. 121

Three-Dimensional

Fabrication of Miniature Electron Optics

A. D. FEINERMAN A N D D. A. CREWE Microfabrication Applications Laboratory, Department of Electrical and Computer Engineering, University of Illinois at Chicago, Chicago, Illinois 60607-7053

I. I n t r o d u c t i o n . . . . . . . . . . . . . II. S c a l i n g L a w s for Electrostatic L e n s e s . . . III. F a b r i c a t i o n of M i n i a t u r e Electrostatic L e n s e s A. R e v i e w . . . . . . . . . . . . . . B. S t a c k i n g . . . . . . . . . . . . . . 1. D e s c r i p t i o n of Silicon Die P r o c e s s i n g 2. P y r e x F i b e r P r o c e s s i n g . . . . . . 3. S t a c k e d M S E M A s s e m b l y . . . . . 4. S t a c k e d M S E M Electrostatic Deflector and S t i g m a t o r

IV. V.

VI. VII. VIII.

IX.

C. Slicing . . . . . . . . . . . . . . . . 1. Slicing P r o c e s s i n g . . . . . . . . . . D. L I G A L a t h e . . . . . . . . . . . . . . 1. L I G A L a t h e P r o c e s s i n g . . . . . . . 2. L I G A L a t h e D o s e C a l c u l a t i o n . . . . F a b r i c a t i o n of M i n i a t u r e M a g n e t o s t a t i c L e n s e s Electron Source . . . . . . . . A. Spindt S o u r c e . . . . . . . B. Silicon S o u r c e . . . . . Detector . . . . . . . . . Electron-Optical Calculations A. A Tilted M S E M .... P e r f o r m a n c e of a S t a c k e d E i n z e l L e n s A. M S E M C o n s t r u c t i o n . . . . . . B. M S E M O p e r a t i o n and I m a g e F o r m a t i o n S u m m a r y and F u t u r e Prospects . . . . . . References . . . . . . . . . . . . . .

9

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91 93 94 94 95 98 100 100 102 104 106 108 109 lll ll8 119 119 121 124 126 130 132 132 136 140 141

I. I N T R O D U C T I O N

The term miniature electron optics is used in this article to refer to electrostatic lenses that are smaller than 10 cm. The technology to reduce the size of the lens is being used to reduce the beam voltage and miniaturize the scanning electron microscope (SEM). There are several applications for a miniature SEM (MSEM). An MSEM can be brought to the sample instead of bringing the sample to a standard SEM. This would be convenient when access to the sample is limited, for example, when the researcher is inspecting the hull of a

Volume 121 ISBN 0-12-014763-7

91 ADVANCES IN IMAGING AND ELECTRON PHYSICS Copyright 9 2002 by Academic Press All rights of reproduction in any form reserved. ISSN 1076-5670/02 $35.00

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A.D. FEINERMAN AND D. A. CREWE

spacecraft or inside a fusion reactor, or when it is desirable to inspect objects in situ instead of bringing them to the analytical laboratory. In semiconductor processing there is a need for a low-voltage, high-resolution SEM that could observe integrated circuits in situ during each deposition and etching process. In biology the same instrument could observe specimens immediately after they were sliced with a microtome to minimize sample degradation. MSEMs can complement other analytical instruments like the scanning tunnel microscope (STM) or the atomic-force microscope (AFM). When the STM and AFM are operated at atomic resolution, their field of view is limited to a few tens of nanometers and the researcher can spend hours trying to determine if the atoms under view are the atoms of interest. An MSEM observing those instruments would allow the researcher to quickly locate the interesting areas of the sample. Miniaturization will speed up the stereo observation of threedimensional samples, which at present proceeds in three steps: observation, rotation, and observation. Two or more MSEMs mounted at 10 ~ with respect to each other can directly acquire a stereo image. Three-dimensional samples of interest range from the evaluation of the pore size and permeability of minerals in the petroleum industry (Huggett, 1990) to the submicron linewidth on an integrated circuit. The technology to make one MSEM could make an array of MSEMs, which would be useful for electron beam lithography and wafer inspection: The present state of the art dynamic random access memory (DRAM) technology is 256 Mbit with a minimum feature size of 0.4 # m (Adler et al., 1994). In general, the size of a memory chip doubles and the smallest feature is reduced 70% every 3 years, which quadruples the amount of information that can be stored on a chip (Sematech, 1994). DRAMs are often developed with electron beam lithography and then manufactured with optical steppers (Larrabee and Chatterjee, 1991). The reason for switching technologies is the order of magnitude increase in throughput in the number of wafers an optical stepper can process in 1 h. Optical steppers are faster because all the pixels are exposed in parallel, whereas an electron beam machine exposes pixels in a serial fashion. An array of N beams would reduce the total writing time by a factor of N and would make electron beam lithography economically competitive. In semiconductor processing the minimum feature size will soon be less than 0.1 # m across an 8-in.-diameter wafer. Determining the most economic method of patterning wafers is an active area of research, with X-ray lithography (Fleming et al., 1992), deep ultraviolet (UV) steppers with phase shifting (Lin, 1991), and arrays of electron beam columns (Feinerman, Crewe, Perng, Shoaf et al., 1992a) or STMs (Marrian et al., 1992) under consideration. Regardless of the lithography method chosen, a method will be required to rapidly inspect large wafers with a resolution of one tenth the minimum feature size or 10 nm. This indicates that an inexpensive array of STMs, AFMs, or SEMs will be

3D FABRICATION OF MINIATURE ELECTRON OPTICS

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essential for the continued growth of this industry. The inspection problem will not be insignificant, however, and it may be simpler to fabricate an array of high-resolution SEMs with the methods discussed in this review than to process the data they will generate. For example, if we examine an 8-in. wafer consisting of 250 identical 1-cm 2 die with 250 parallel beams 100 x 100 nm, there will be 2.5 x 1012 pixels/wafer and 10 l~ pixels/die. A 10-nm or larger foreign particle will vary the backscattered or secondary electron signal just enough so that when 250 channels are being compared simultaneously the equipment can determine which areas might have particles or defects and must be examined at a higher magnification to resolve a 10-nm particle. If we assumed that the data can be processed as fast as it comes in on the 250 channels and 0.1 #s to examine each pixel, it would take at least 1000 s to observe the entire wafer. Following, three techniques are described that can miniaturize electrostatic lenses operating in different voltage regimes. The integration of an electron source, a deflector, and a detector into the electrostatic lens in order to make an MSEM and a method to miniaturize a pancake magnetic lens are also discussed.

II. SCALING LAWS FOR ELECTROSTATIC LENSES

There are two common types of scaling: constant potential, in which all the lengths are reduced by a factor k, where k is less than 1, and constant electric field, in which both the lengths and the voltages are reduced by the factor k. The effect of scaling is shown in Table 1 where ot is the maximum angle of emission of an electron that travels down the electrostatic column

TABLE 1 EFFECT OF SCALING

Constant potential Lengths Potentials Fields Spherical aberration des = 0.5Cs ly3 Chromatic aberration

de = Cco~(A V~ V) Interactions de "" L~ V 1"5

Stray magnetic field deflection

Constant field

k 1 k -1

k k 1

k

k

k

1

k

k -5

k 5/2

k 3/2

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A.D. FEINERMAN AND D. A. CREWE

(Change et al., 1990). Constant potential scaling provides the largest improvement in resolution. The electric field increases in this case as 1/k until a maximum electric field for a given gap size is reached. In our research we have held off 2.5 kV with 138-#m gaps or 18 kV/mm at 8 • 10 -9 torr.

Ill. FABRICATION OF MINIATURE ELECTROSTATIC LENSES A. R e v ie w

This section briefly reviews the SEM miniaturization methods developed by other research groups and the three methods developed at the University of Illinois at Chicago (UIC): stacking, slicing, and using the LIGA lathe (LIGA is a German acronym for lithography and galvo-forming or electroplating) (Feinerman, Crewe, and Crewe, 1994; Feinerman, Crewe, Perng, Shoaf et al., 1992a; Feinerman, Lajos et al., 1996). Miniaturizing the SEM involves miniaturizing each component: electron source, deflector, detector, and one or more lenses to focus the beam. The lenses and deflector can be electrostatic or magnetic but electrostatic devices are more easily micromachined and do not dissipate power in vacuum (Trimmer and Gabriel, 1987). There are two main approaches to miniaturizing an electrostatic lens: either assemble layers and then make apertures (method 1) or make apertures in the individual components and then assemble the components (method 2). The drawback of the first approach is the limited flexibility to vary the aperture size along the column. As discussed in Section VII, einzel lenses perform better if one can make the second or focusing electrode aperture larger than the first and third apertures. The drawback of the second approach is the aperture alignment error during assembly of the components. An example of the first miniaturization method is the proposed lithography wand that would be fabricated by thin-film deposition of several layers followed by reactive ion etching (RIE) of the apertures (Jones et al., 1989). The maximum column length with this method is ~ 10 # m and is determined by the thickness that can be reliably anisotropically etched to form the apertures and the maximum thickness of the conductor and insulator thin films. A standard vacuum electrostatic design guideline is to restrict the maximum field between electrodes in a column to ~ 10 kV/mm (Chang et al., 1990) and a 10-#m-long column can accelerate and deflect a 100-V beam, which would be capable of exposing only a very thin resist layer. Another problem with a very short column is that since the working distance is approximately half the column length, it would be difficult to mount two columns at 10~ with respect to each other for stereo microscopy. An example of the second type of miniaturization method has been developed at IBM. Layers with pre-etched apertures are optically aligned to

3D FABRICATION OF MINIATURE ELECTRON OPTICS

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assemble a 2- to 3-mm-long column with a scanning tunneling microscope tip as the electron source (Muray et al., 1991). The disadvantages of this approach are the elaborate column fabrication method in which a sophisticated optical inspection system allows the operator to manually align and then epoxy individual layers and the use of a large but well-characterized electron source. The maximum length of a column fabricated with this techniques is ~ 10 mm and is determined by the accuracy of the optical inspection system as it examines layers at different heights. In another example of the second method, an electrostatic lens is made from a perforated carbon film mounted to a transmission electron microscope (TEM) grid placed over a second TEM grid containing several hundred 20-#mdiameter holes where an 8-#m-thick insulating sheet of polyimide separates the two electrodes (Shedd et al., 1993). This technique relies on the random alignment of one of the several thousand perforations in the carbon film with one of the 20-#m holes and there is a small but finite chance of creating a well-aligned column. Our research program has developed three simple methods for manufacturing extremely accurate and inexpensive electron beam columns: stacking, slicing, and using the LIGA lathe. All three methods can vary the aperture size and location along the optical column, the electrode thickness and spacing, and the position of the deflector within the column. Stacking and the miniaturization methods discussed previously approximate the SEM as a series of infinite planes with circular apertures separated by thin insulating layers (Feinerman, Crewe, Perng, Shoaf et al., 1992a). In the slicing method the electrodes are not apertures in planes but are cylinders that are bonded to an insulating substrate where the common cylinder axis defines the electron-optical axis (Feinerman et al., 1994). The maximum length of the column is limited by the size of the substrate and can be 300 mm or longer. Several sliced columns can be fabricated in parallel. As is shown later, the LIGA lathe method is capable of fabricating electrodes with the widest variety of shapes (Feinerman, Lajos et al., 1996).

B. Stacking*

In stacking, a (100) silicon wafer is anisotropically etched to create an array of die as shown in Figures 1 and 2. On each die there is an aperture etched through a membrane and four v-grooves on the top and bottom surfaces of the die. Precision Pyrex fibers align and bond the v-grooves on both surfaces of the die. The structure can be designed to have the fibers rest either on the

*Portions of this section are reprinted, with permission, from Journal of Vacuum Science and Technology A, 10(4), 611-616, July 1992. Copyright 1992 American Vacuum Society.

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FIGURE 1. (a) Silicon die (D1-D4) are stacked with Pyrex fibers that align and bond to the dies' v-grooves. The v-grooves are staggered and truncated to increase the die strength. The top and bottom surfaces of each die are optically aligned during fabrication. The first silicon die contains a micromachined field-emission lectron source and a gate electrode to generate the emitting field. The next three silicon die form an einzel lens. The last die, D4, has an electron detector on the surface facing the sample. The MSEM is on a Pyrex die to provide electrical insulation between the electron source and the vacuum chamber. (b) The stacked design approximates the SEM as a series of infinite planes with circular apertures separated by thin insulating layers. A design guideline is to make the membrane surrounding the aperture 10 times larger than the aperture diameter. (c) One of the die is diced into eight electrically insulated sections (V l-V8) to generate a transverse electric field in the center of the die to deflect the electron beam. This die can also correct for astigmatism. The Pyrex washer holds the die together. The die are rectangular instead of square to facilitate electrical contact to the stack. As indicated in Figure 1a, the contact region is to the right on D 1, out of the page on D2, to the left on D3, and into the page on D4.

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97

FIGURE 2. Silicon wafers are anisotropically etched to create four v-grooves on the top and bottom surfaces of each die (only three grooves are shown), and an aperture to allow the electron beam to pass through the die. One 4-in.-diameter wafer contains a hundred 7 • 9-mm die. Rectangular die are used to facilitate electrical contact to the column. Precision Pyrex fibers are diced to the proper length and placed in the v-grooves. The Pyrex fibers provide electrical insulation between the die, align the die in three directions, and are bonded to both die.

etched groove surface or on the groove's edges (Fig. 3). The relationship among groove width (W), fiber diameter (D), and gap b e t w e e n silicon die is given by the following equations (Mentzer, 1990), where 0 = c o s - 1 ( 4 ~ - / 3 ) = 35.26 ~ This is the angle b e t w e e n the normal to the (100) surface and a (111) plane. W If D < cos(O )'

W If D > - cos(O)'

D Gap -- sin(O) Gap-

v/(D 2-

W tan(O )

( 1)

W 2)

(2)

If the v-grooves are allowed to etch to completion their depth will be W/~. We have found that for structural integrity the wafer thickness should be at least W and that a large gap can be obtained by choosing D -- W/cos(O), which makes the Gap - W/V'2. The length of the column shown in Figure 1 with these choices is then 5 W + 3 W/~/2, or 7.1W. Adhering to a m a x i m u m electric field design guideline of 10 k V / m m , a 15-kV c o l u m n would require 1.5-mm gaps and 2.1-mm-thick wafers, and it would be 15 m m long. A 1-kV column would require 0. l - r a m gaps and 0.14-mm-thick wafers, and it would be 1 m m long. The stacked design can be scaled to a wide range of voltages since silicon wafers and Pyrex fibers of almost any dimension can be commercially manufactured, processed, and assembled.

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A . D . F E I N E R M A N AND D. A. CREWE

,,,....-

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FIGURE 3. The gap between silicon die is determined by the v-groove width (W) and the fiber diameter (D -- 2R). The half angle of the v-groove is 0 = 35.26 ~ and the depth is h - W/v/-2. (a) The center of a 308-#m fiber is positioned 76 # m above a 270-#m v-groove. The fiber contacts the silicon within the v-groove, 13 # m below the silicon wafer surface. (b) The center of a 450-#m fiber is positioned 180 # m above a 270-#m v-groove. The fiber contacts the silicon at the silicon wafer surface and rests on the groove's edges.

1. Description of Silicon Die Processing (Fig. 4) A silicon wafer was cleaned and then oxidized in steam at 900~ to grow 40 nm of SiO2. A 200-nm Si3N4 layer was then deposited over the SiO2 in a low-pressure chemical vapor deposition (LPCVD) reactor. Both sides of the wafer were coated with photoresist, and rectangular and square windows were opened in the photoresist on the bottom of the wafer after alignment of the pattern to the wafer flat. The flat indicates the silicon (110> direction. The rectangular and square windows were processed to produce v-grooves and apertures, respectively. The Si3N4 w a s etched in a plasma etcher, then the

3D FABRICATION OF MINIATURE ELECTRON OPTICS

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FIGURE 4. The process sequence for the silicon die used in the MSEM. The starting point is a silicon wafer covered with a dielectric layer consisting of Si3N4 on SIO2. (1) Rectangular windows (381 • 5000 #m 2) are opened in the bottom dielectric layer. (2) Rectangular and square windows (381 • 3400 #m 2 and 1500 • 1500 #m 2) are opened in the top dielectric layer. (3) A 30- to 50-#m-deep circular aperture is etched into the silicon by using aluminum as the etch mask. (4) The aluminum is etched away and 2 #m of SiO2 is grown on the exposed silicon. The silicon below the Si3N4 is not oxidized. (5) The oxide protecting the silicon on top of the aperture is removed and the wafer is placed in an anisotropic etchant. The anisotropic etch is interrupted when the etch is about half the thickness of the wafer. (6) The oxide protecting the v-grooves is removed and the wafer is placed into the anisotropic etchant until the silicon above the aperture is removed. (7) The Si3N4 and SiO2 layers are removed and the wafer is cut into individual die.

photoresist was removed. The top surface of the wafer was aligned to the etched features on the b o t t o m of the wafer with an infrared aligner, then plasma etched. An a l u m i n u m film was deposited on the bottom of the wafer and circular holes were etched into this film. The patterned a l u m i n u m film serves as a m a s k for a vertical plasma etch ~ 3 0 - 6 0 # m into the silicon. The metal m a s k is r e m o v e d and a thick SiO2 layer ~ 2 # m is grown on any exposed silicon. The oxide protecting the silicon on top of the aperture (on the opposite side of the wafer) is r e m o v e d and the wafer is placed in an anisotropic etchant (44% by weight KOH in H20 at 82~ This solution etches the silicon (100) direction 400 times faster than the {111) direction (Petersen, 1982). The solution has a slight etch rate for SiO2 and a negligible etch rate for Si3N4 (Bean, 1978). The wafers were kept in this solution until the KOH solution etched about halfway through the wafer. The SiO2 protecting the v-grooves is r e m o v e d and the wafer is placed into the anisotropic etchant until the silicon above the aperture is removed. The wafer is cut into individual 7 x 9 - m m 2 die and the Si3N4 and SiO2 layers are r e m o v e d with a 10-min i m m e r s i o n in 50% HF acid followed by a 5-min deionized H 2 0 rinse.

100

A.D. FEINERMAN AND D. A. CREWE

2. Pyrex Fiber Processing Precision Pyrex fibers were drawn on a laser micrometer-controlled fiberoptic tower. Duran and Pyrex were chosen because their thermal expansion coefficients of 3.2 • 10-6/~ closely match that of silicon at 2.6 x 10-6/0C. Both glasses have nearly identical chemical composition and are trademarks of Schott and Coming, respectively. The Pyrex fibers were waxed to a silicon wafer and cut to the desired length on a MicroAutomation 1006A dicing saw. The fibers were then solvent cleaned before being used in the MSEM assembly.

3. Stacked M S E M Assembly The die were aligned and anodically bonded with 308-#m Pyrex fibers as shown in Figures 5 and 6 (Feinerman, Crewe, Pemg, Shoaf et al., 1992a). Pyrex can be bonded to silicon at 250~ with a bond strength of 350psi (Wallis and Pomerantz, 1969). The bond is strong enough (1.0 + 0.5 lb) to allow the die to be wire bonded. The glass deforms up to 1.6 # m during anodic bonding to silicon (Carlson, 1974; Carlson et al., 1974). This deformation will increase the fiber/silicon contact area and the increase will be larger if the contact point is below the silicon wafer surface (Feinerman, Shoaf et al., 1991; Fig. 3a). The bond strength between the fiber and the silicon will increase as the contact area increases. Die have been stacked with 308- and 450-#m-diameter fibers in 270-/zm-wide grooves yielding 152- and 360-/zm gaps between the silicon die, respectively. Attempts to bond 510-#m fibers into the 270-#m grooves have not been successful, possibly because of the small fiber/silicon contact area.

FIGURE5. Two silicon die are aligned and anodically bonded to a 308-#m-diameter Duran fiber. The die are aligned to withinthe accuracyof the optical micrograph-~q-2#m. The separation between the die is 152 #m.

3D FABRICATION OF MINIATURE ELECTRON OPTICS

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FIGURE 6. (a) Optical micrograph of three 381-#m-thick silicon die stacked with glass fibers. V-grooves on the right are to check infrared alignment, while the rest are for fibers. At present the overall structure is limited by a • infrared alignment of the die's top surface to its bottom surface. (b) Three silicon die will form an einzellens. The 0.16-in. vacuum pickup tool is visible in the micrograph, showing that the stack is self-supporting. The overhang of the die is rotated 90~ between layers to facilitate electrical connections.

The accuracy of the stacking technique is limited by the precision of the glass fibers, silicon die, and v-groove etching. Optical fibers have a diameter tolerance of i 0 . 1 % / k m of fiber (Gowar, 1984) or 4-0.3 # m / k m for a 308-/zm fiber. A kilometer of fiber would provide enough material for several thousand microscopes. The total indicated runout (TIR), which is defined as the maxim u m surface deviation, on a 7 x 9 - m m 2 double-polished silicon die is much less than 1 tzm. The etched v-groove (111) surfaces also have less than 1 k~m of TIR (Feinerman, Shoaf et al., 1991). At present, the overall accuracy of the

102

A.D. FEINERMAN AND D. A. CREWE

column is limited by the + 5 - # m infrared alignment of etched features in the top and bottom surfaces of the silicon die. This accuracy can be improved by exposing the bottom surface of the wafer with X-rays through a metal mask on the top surface of the wafer. The stacking technique should achieve submicron accuracy.

4. Stacked MSEM Electrostatic Deflector and Stigmator A compact MSEM requires a micromachined electrostatic or magnetostatic deflector and stigmator integrated in the column (Figs. 1 and 7). Electrostatic deflector/stigmators can be implemented by generating a transverse electric field with a single die (Fig. 1) or with two die (Figs. 7 and 8). The first design generates the field within a single die, which minimizes the column length. In the second approach a transverse field is generated between two successive die. This design has an advantage when one is building an array of MSEMs,

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FIGURE7. (a) Deflecting the electron beam inside a decelerating einzel lens increases the field of view and working distance at the expense of increased circuit complexity. (b) The beam is focused by a three-electrode einzel lens and then deflected. The deflector operates near ground potential.

3D F A B R I C A T I O N OF M I N I A T U R E E L E C T R O N OPTICS

103

FIGURE 8. (a) A silicon die at a single potential will have a uniform coating of metal on its top and bottom surfaces. (b) If a pair of silicon surfaces are used to deflect the electron beam and correct for astigmatism, one surface of each die will have eight independently controlled metal electrodes insulated from the silicon with a thick high-quality SiO2 layer. (c) Cross-sectional view of deflector indicating the transverse electric field between the pair of die. (d) The deflectors for an array of MSEMs can be operated in parallel with integrated circuit interconnection technology. The interrupted lines indicate where a second level of metallization is required to avoid shorts between potentials. The contacts at the edge of the array (V l - V 8 ) have been repeated for visual clarity. Only eight contacts are needed to drive an N • N array of deflectors in parallel.

because integrated circuit technology can be used to fabricate the multilevel interconnects that can drive all the electrodes in parallel (Fig. 8d). If a single die generated the transverse electric field then wire bonding or a similar technique would be required to drive all the electrodes. The beam deflection angle is given by tan y = LEtr/2Vb, where L is the axial length of the deflector (thickness of D3 in Fig. 1 or the gap between D3 and D4 in Fig. 7a), Vb is the beam energy as it enters the deflector, and Etr is the uniform transverse electric field created between the deflector plates. If the transverse field is 30 V/mm and the beam travels 220 # m through the gap between D3 and D4 in Figure 7a, then to a target 500 #rn beyond D5, a 5-#m beam deflection would be obtained with a 3-milliradian (mrad) deflection angle (Vtrans/l)beam).Deflecting the beam more than the 4-mrad convergence angle would introduce higher-order aberrations.

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A.D. FEINERMAN AND D. A. CREWE

FIGURE8. (Continued)

As shown in Figure 7, the minimum working distance for an MSEM is obtained when the deflector is inside the einzel lens. A practical problem with this choice is that the deflector's electronics operates at the einzel electrode potential rather than operating at ground potential.

C. Slicing*

As discussed earlier the electrodes fabricated in the slicing method are not apertures in planes but are conducting cylinders bonded to an insulating substrate where the common cylinder axis defines the electron-optical axis (Fig. 9) *Portions of Section III.C are reprinted, with permission, from the Journal of Vacuum Science and Technology B, 12(6), 3182-3186, November 1994. Copyright 1994 American Vacuum Society.

3D F A B R I C A T I O N O F M I N I A T U R E E L E C T R O N O P T I C S

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FIGURE 9. Sliced MSEM. (a) A (100) silicon wafer with a patterned silicon nitride layer is anodically bonded to a Pyrex wafer and anisotropically etched. The nitride is removed with buffered hydrofluoric acid. A dicing saw separates the silicon into electrically isolated electrode sections. (b) Precision GE772 capillary tubes are anodically bonded into the v-grooves. The glass has a thermal expansion coefficient of 3.6 x 10-6/~ and contains 2% PbO. (c) The capillary tubes are separated into electrodes with a dicing saw and a micromachined field-emission source is added to the column. (d) A three-dimensional view of a sliced electrostatic column. Electrodes E 1, E2, E3, and E4 are 1.5, 1, 1.5, and 1 mm long, respectively. A 1-mm gap separates electrodes El, E2, and E3, and a 1.5-mm gap separates E3 and E4. Electrodes E 2 - E 4 have a 300-/zm inner diameter and a 5 0 0 - # m outer diameter.

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FIGURE 10. Sliced deflector. (a) A (100) silicon wafer with a patterned dielectric layer is anodically bonded to a Pyrex wafer and anisotropically etched. The dielectric is removed with buffered hydrofluoric acid. A second layer is "stacked" over the first layer. (b) A preform consisting of GE772 and Pyrex is drawn and anodically bonded into the v-grooves. The structure is then diced as in the previous figure, with the blade electrically isolating the tube sections. (c) A second double-layer substrate is bonded to the top of the composite fibers. Electrical contact can now be made to each section of GE772 glass.

(Feinerman, Crewe, and Crewe, 1994). The maximum length of the column is limited by the size of the substrate and can be 300 mm or longer. The electrode inner and outer diameter, length, and aperture size can all be varied in the design. The slicing method can also produce an integrated electrostatic deflector (Fig. 10).

1. Slicing Processing A (100) silicon wafer with a patterned Si3N4 film is bonded to a Pyrex (Corning 7740) wafer and anisotropically etched (Fig. 9). The anisotropic etchant removes silicon faster in the (100) direction than in the (111) direction and has a negligible ctch rate for Pyrex and the nitride film. This etch creates v-grooves in a silicon wafer whose normal is parallel to a (100) direction and the depth of the v-groove is W/x/~, where W is the opening in the nitride film.

3D FABRICATION OF MINIATURE ELECTRON OPTICS

107

If the opening in the nitride is larger than ~/~; where t is the wafer thickness, the etch will terminate on the Pyrex. The nitride etch mask is designed to create v-grooves in silicon islands on the Pyrex wafer. A Corning 7720 glass capillary is anodically bonded to the silicon and a dicing saw is used to create the required gaps in the capillary. Anodic bonding is a technique in which glass is bonded to silicon at elevated temperatures by passing a current from the silicon into the glass (Wallis and Pomerantz, 1969). As discussed later in this section, the anodic bond is sufficiently strong that solid fibers and capillaries can be diced without any organic "potting" compound. In Section VII the resolution of the proposed sliced column is calculated when each electrode has 300- and 500-#m inner and outer diameters. The electrode aperture size can be varied along the column by bonding capillaries with different inside diameters into the v-grooves holding electrodes E2-E4 (Fig. 9d). After the structure is fabricated the glass surfaces that will be exposed to the electron beam must be made sufficiently conductive to form an electrostatic column. Electrical contact to the conductive glass can be made by attaching leads to the silicon sections. A crucial question for the slicing method is the minimum conductive coating required for each electrode in the column. As is well known, insulators exposed to an electron beam will charge and the resulting electrostatic fields will have a deleterious effect on the electron beam itself. A starting point to determine the minimum conductive layer is to assume that after the first beam-limiting aperture no more than 1% of the beam will strike any surface. The stray current striking the middle of the electrode's walls should not raise the potential of the wall by more than one tenth of a volt, which is the variation in beam voltage expected from a cold field-emitter scurce. If the glass surface has a coating of Rsq ~/square, the resistance of the electrode Rel is given by the following formula, where DI and Do are the inner and outer diameters and L is the axial length of the electrode:

Re,=

N+ln

(Oot

(3)

The preceding formula assumes that the length of the v-groove is half that of the electrode and ignores current bunching where the electrode makes contact to the v-groove. If L, Do, and DI are 1.5, 0.5, and 0.3 mm, respectively, then Re~ -- 0.56Rsq. If there is a 1-nA beam, then Rsq must be less than 1.8 x 10 ~~ ~/square to avoid a 0.1-V variation in the electrode's potential. This is an approximation and the assumptions will have to be confirmed by experiment. We have not yet made the glass conductive but we have three proposed solutions. Our first solution is to use a glass containing PbO wherever the glass electrode surface will be exposed to the electron beam. This PbO could be reduced in a hydrogen ambient with the process used to create microchannel

108

A.D. FEINERMAN AND D. A. CREWE

plates. A typical microchannel plate produced at Galileo Electro-Optics in Massachusetts is 400 # m thick with an active area that contains 3.4 x 106 capillaries, 10 # m in diameter. The resistance of each capillary is approximately 13Rsq. The minimum microchannel plate resistance reached is 10100 kf2 when Corning 8161 (which contains 51% PbO) or Galileo MCP- 10 glass is used (Feller, 1990; Laprade, 1989). This translates into a conductive layer of 3 x 109 to 3 x 1010 S2/square, which is sufficient for the low beam currents used in imaging. A second solution would be to metallize the glass by chemical vapor deposition. For example, a thin coating of polycrystalline silicon deposited on glass could be exposed to tungsten hexafluoride to form a tungsten film with a sheet resistivity of 2-100 f2/square (Busta et al., 1985). An alternative procedure is to electroplate a thin layer of gold onto the reduced glass surface on the short capillary sections. The deflector shown in Figure 10 will require that complex glass cross sections be drawn (Jansen and Ulrich, 1991) and selectively made conductive. Any metallic coating will lower the sheet resistance of the glass to a point at which it can be used in an electron-optical column. The overall accuracy of a sliced column depends on the accuracy with which a v-groove can be etched and a capillary can be drawn and diced. The total indicated runout, or maximum surface deviation, on an etched v-groove (111) surface is less than 1 # m (Feinerman, Shoaf et al., 1991). Fibers can be purchased that are drawn with laser micrometer control and have a 1-#m or better tolerance on their diameter. The largest error is in the control of the length of each capillary section which is ~ 3 / z m . Most commercial electron-optical columns have a dimensional tolerance of ~0.1%. The errors in a sliced column are on the order of 0.3% (100 x 3 # m / 1 0 0 0 / z m ) and are expected to slightly degrade the performance of an electron-optical column. The impact each error will have on the column's resolution will have to be directly measured.

D. L I G A Lathe*

As shown in Figure 11, the LIGA lathe is capable of patterning the widest variety of electrode shapes on a micron scale, including shapes impossible to achieve with a conventional lathe (Feinerman, Lajos et al. 1996). The electrode spacing and aperture size within an electron-optical column can also be varied. The maximum length of the column is limited by the size of the X-ray exposure at a synchrotron, which is 100 mm at Argonne's Advanced Photon Source (APS). However, successive exposures can be stitched together. *Portions of Section III.D are reprinted, with permission, from the IEEE Journal of Microelectro-mechanical Systems 5(4), 250-255, December 1996.

3D FABRICATION OF MINIATURE ELECTRON OPTICS

109

FIGURE 11. Three-dimensional views of electrostatic columns that can be produced on a LIGA lathe. The technique can create the widest variety of electrode shapes and can vary the aperture diameter along the length of the column. The technique used to create these structures requires that a cylindrical layer of X-ray resist be exposed and developed. After resist development, metal can be electroplated into the regions where the resist was removed or a conformal metal coating can be deposited around the structure.

1. LIGA Lathe Processing In the standard LIGA process (as mentioned before, LIGA is a German acronym for lithography and galvo-forming or electroplating), a planar substrate is covered with an X-ray-sensitive resist and exposed with a collimated X-ray source (Guckel et al., 1990). A typical X-ray resist is poly(methyl methacrylate), or P M M A (also known as Lucite). The exposed resist is removed in a developer (positive resist) and this process is the analog of a binary mill operating on a micron scale capable of creating two-dimensional structures that are as thick as the PMMA. Metal is electroplated into the exposed and developed voids formed in the resist. The modifications developed in our laboratory extend LIGA into a variety of three-dimensional structures. A cylindrical core coated with an X-ray-sensitive resist is schematically illustrated in Figure 12. Nylon filament 460 # m in diameter has been coated with P M M A as has 125-#m gold-plated copper wire. The P M M A is built up to the desired thickness with multiple layers. This core is mounted with slight tension between the headstock and tailstock of a custom-built glassblower's lathe shown in Figure 13. The two chucks on the lathe rotate simultaneously to avoid twisting the core during exposure. The lathe rotates at 1 rpm during 30-min and longer exposures.

110

A.D. FEINERMAN AND D. A. CREWE

I FIGURE 12. A blank substrate ready for use on the X-ray lathe. An X-ray-sensitive resist surrounds an opaque core. A solid rod of X-ray-sensitive resist could also be used as substrate. A two-level (binary) surface possessing cylindrical symmetry was fabricated by exposing the substrate with a mask consisting of opaque bars (Fig. 14). Micrographs taken after P M M A development are shown in Figure 15. The current cylindrical resist layers are not as uniform as planar resist coatings. If the coating technology cannot be significantly improved, a uniform layer could be achieved by exposing the resist through a mask that absorbs all X-rays below the desired radius and removing the excess resist in a developer. The starting material can also be solid P M M A rod. A cylindrically symmetric pattern with a variable radius was fabricated as shown in Figures 16 and 17. The radial penetration of the X-rays is determined by the shape of the X-ray absorber. If the mask extends beyond the outer radius of the resist, no resist is exposed. Conversely, if the mask does not block the exposure all the resist will be exposed. The X-ray mask becomes the analog of the cutting tool of a conventional lathe.

FIGURE 13. LIGA lathe prototype. Both ends of the substrate shown in Figure 12 rotate at the same rate. Antibacklash gears are used to prevent the substrate from twisting during the exposure.

3D FABRICATION OF MINIATURE ELECTRON OPTICS

111

Fmt0RE 14. X-ray mask used to create a two-level cylindrically symmetric surface. The X-ray resist exposed below the transparent regions of the X-ray mask is subsequently removed in the developer.

There are other possible modifications of LIGA technology in which the X-ray exposure is modulated in time. As indicated in Figure 18, octupoles for an electrostatic deflector/stigmator can be created if the substrate is exposed through an aperture and the exposure is chopped synchronously with the rotation. The shutter motion in this case would have to be much faster than the time needed to make one complete rotation, which is 1 min with our current fixture. Solid rods of PMMA were machined and exposed at Argonne's Advanced Photon Source (APS) with the mask design in Figure 19. The rods are shown in Figure 20 after the exposed PMMA has been removed in a developer. The more energetic X-rays available at the APS allows for the micromachining of macroscopic electrostatic lenses. 2. LIGA Lathe Dose Calculation

The binary exposure doses (Fig. 14) are compared with that of a planar slab with the same resist thickness. The exposure time calculation for the binary radius cylinder structure assumes an opaque core with radius Ri covered with resist to a radius Ro. The variables are defined in Figure 21a. This structure rotates with an angular speed of co while illuminated with collimated X-rays. The X-ray path length h at a particular radius r and angle 0 is given by the following formulas: fi

-

s i n - ' ( r x sin 0 ) Ro

h = Ro • c o s ( f l ) - r x cosO

(4) (5)

112

A . D . F E I N E R M A N AND D. A. CREWE

FIGURE 15. (a) An --~55-#m-thick PMMA coating on a 125-#m-diameter Au-plated Cu wire. The PMMA cross-section thickness is not uniform with thick coatings. (b) An ~ 15-#mthick PMMA coating on a 125-#m Au-plated Cu wire.

3D FABRICATION OF MINIATURE ELECTRON OPTICS

113

FIGURE 16. X-ray mask used to create a variable-level cylindrically symmetric surface. The separation of the transparent region of the mask and the rotating substrate axis determines the final radius of the resist. The exposure time can be reduced by 50% with a mask that exposes both sides of the substrate simultaneously.

FmURE 17. Micrograph of a variable PMMA surface. A 460-/~m nylon fiber was coated with ~-125 #m of PMMA. The substrate was intentionally overexposed and the nylon was damaged by the X-rays.

114

A.D. FEINERMAN AND D. A. CREWE

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(a)

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The exposure at a particular radius takes place between 4-0c, where Oc - ~ + cos-

(6)

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]cosO[ exp

-

dO

(7)

where Iinc is the incident power/area, and ~ is the X-ray absorption length at a particular wavelength. If the core is not opaque and has the same ~ as the resist,

3D FABRICATION OF MINIATURE ELECTRON OPTICS

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FIGURE 19. (a) Solid rods of PMMA (Lucite) were machined to make a two-conductor corrugated filter. A two-conductor design simplifiesradiofrequency (RF) testing. (b) Brass mask 0.8 mm thick used to pattern 0.06-nm X-rays. These X-rays are blocked by 50 #m of Au, 250 #m of Cu, or 7.4 mm of Si.

the integral extends between 0 and Jr. The IcosOI factor in the integral takes into account the angle between the resist and the radiation. This calculation neglects refraction because the index of refraction for P M M A differs from 1 by less than 10 .4 across the range of X-rays used in the LIGA lathe exposures (Cerrina et al., 1993). In one revolution of the substrate, the bottom of a planar resist layer t # m thick would receive the following exposure: E _ p l a n a r ( t ) - /inc •

• exp

--

(8)

O9

The ratio of the cylinder to planar exposure times for a comparable resist thickness varies slightly with the exact value of the X-ray absorption length, Ro and Ri. The longer exposure time is a consequence of the cylindrical geometry (longer path length and opaque core blocking the X-rays); however, there is more P M M A per unit mask opening in the cylindrical case 0.5jr(Ro + Ri)/Ro or 2.4 with a 125-#m-diameter core covered with 50 # m of resist. As shown in Table 2, the exposure ratios range from 3 to 4 for the values of R~, resist thickness, and wavelength used at the CXrL (Center for X-ray Lithography, Stoughton, WI, USA). The calculations in Table 2 assume an X-ray absorption length of 100 # m , which corresponds to 0.36-nm X-rays. At the CXrL facility with a 1-GeV beam and a 2 5 - # m beryllium window, the X-rays range from 0.25 to 0.5 nm with absorption lengths ranging from 310 to 40 # m (Cerrina et al., 1993). A planar PMMA sheet 50 # m thick is exposed in 6 min with a

116

A . D . F E I N E R M A N AND D. A. CREWE

FIGURE 20. (a) Top and (b) side views of a 3-mm outside diameter PMMA rod that has been exposed and developed to create 0.15-mm-wide slots that are ~0.9 mm deep.

3D F A B R I C A T I O N OF M I N I A T U R E ELECTRON OPTICS

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FIGURE 21. (a) The X-ray dose at radius r with the mask shown in Figure 14 depends on the X-ray path length h(O) and the radius of the opaque core. The exposure takes place from - 0 c to +0c. The hatched area represents the resist, which is not exposed, since the core blocks the X-rays. (b) The X-ray dose at radius r with the mask shown in Figure 16 depends on the X-ray path length h(O) and the amount of the incident radiation the mask intercepts. The exposure takes place between Omi and Omf on both sides of the substrate. The hatched area represents the resist, which is not exposed, since the mask blocks the X-rays.

storage ring current of 150 mA, and 21 min were required to expose the 50-#mthick layer of PMMA surrounding a 125-#m-diameter core. In a planar resist geometry the ratio of the exposure on the top and bottom surfaces is exp(t/~). In a cylindrical geometry this ratio increases by a factor of ~ 1.2 because the core shadows the inner surface more than the outer surface during each revolution. Consequently, planar resist can be slightly thicker than cylindrical resist for any given X-ray energy.

118

A . D . FEINERMAN AND D. A. CREWE TABLE 2 EXPOSURE GEOMETRY Core diameter (#m)

Resist thickness (#m)

Cylinder/planar exposure ratio

125 125 125 125 460 460 460 460

5 10 50 124 5 10 50 124

3.20 3.23 3.37 3.45 3.21 3.26 3.54 3.81

The exposure time for the variable radius cylinder shown in Figure 16 is calculated next. In this structure, incident radiation is blocked by the X-ray mask at radii less than Rm from the substrate's axis of rotation, as shown in Figure 2 lb. The exposure at a radius greater than Rm takes place on either side of the core between tgmi and Omf, where Omi = 2

COS

(9)

L~mf = -~- -Jr-COS-

(10)

The exposure in one revolution is then given by E_variable(r) -- Iinc • -]cosOI exp (_Z),./tgmi 7

dO

(11)

There is no exposure at radius Rm since L~mi --" Lgmf. The radius at which the resist is sufficiently exposed (Rc) is determined by the actual exposure time. If the ratio of the exposure at the outer and inner surfaces was 5, then with a 125-#m-diameter core covered with 50 # m of resist and Rm -- Ri, Rc -- 1.16 Ri. The mask must be undersized to achieve the desired radius.

IV. FABRICATION OF MINIATURE MAGNETOSTATIC LENSES

Researchers have developed impressive techniques to fabricate electromagnetic components on silicon wafers (Ahn and Allen, 1993). Such techniques include winding conductors around electroplated magnetic material or winding magnetic material around conductors (Ahn and Allen, 1994). These techniques are compatible with the stacked MSEM fabrication approach described in

3D FABRICATION OF MINIATURE ELECTRON OPTICS

119

Section III.B and could be used to create a magnetic pancake lens (Mulvey, 1982). As was already discussed, magnetostatic lenses have a disadvantage with respect to electrostatic lenses of power dissipation in vacuum but they have the benefit of lower aberration coefficients. It is highly unlikely that magnetic pancake lenses can be micromachined with the same tolerances as electrostatic lenses.

V. ELECTRON SOURCE

A micromachined field-emission electron source is an essential component for an MSEM. As discussed in Section VIII, initial images in our laboratory were obtained with a commercial thermal field-emission electron source that was an order of magnitude larger than the micromachined einzel lens. A macroscopic source is also very difficult to align to the lens. The majority of the research on micromachined electron sources concerns stable current versus voltage characteristics. A good review of existing work on micromachined sources can be found in an earlier volume in this series (Brodie and Spindt, 1992). An ideal source for an MSEM will produce at least I nA of electrons, which will travel down the electron-optical axis. The electrons will appear to originate from a small source on the order of 1 nm in diameter and will have an energy spread no larger than 0.1 eV. The current from field-emission sources depends exponentially on the tip work function 4~and the proportionality factor /3 between applied voltage and the electric field at the tip's surface. A common source of variation of these parameters for microfabricated field emitters is a carbonaceous contamination and oxidation of the emitter surface (Somorjai, 1981) and field-enhancing protuberances on the emitter surface. The micromachined electron sources discussed in this section can be easily incorporated into a stacked MSEM and are rugged enough to withstand processes that can remove these deposits from the emitting surface.

A. Spindt Source The fabrication of an array of Spindt sources is schematically shown in Figure 22 (Spindt, 1968; Spindt et al., 1991). A 0.5-#m-thick thermal silicon dioxide (SiO2) layer is grown on a high-conductivity silicon wafer ~0.01 ~2-cm. The SiO2 is then coated with a 0.25-#m-thick film of molybdenum. Submicron holes ~0.8 # m in diameter are etched in the Mo film and electropolished to smooth off any deviations from a circular opening. The Mo film serves as an etch mask when holes are being isotropically etched in the SiO2 layer with buffered hydrofluoric acid. A second mask opens rectangular patterns in the Mo and SiO2 films and the wafer is etched in KOH to form

120

A.D. FEINERMAN AND D. A. CREWE

FIGURE 22. Schematic of a thin-film field-emission array fabricated by using anisotropic etching. The base is single-crystal silicon and the field-emitter cathodes and gate film are vapordeposited molybdenum. The insulating layer is thermally grown SiO2. (a) Schematic of a Spindt cathode array. (b) Scanning electron micrograph of Spindt cathode array. (c) Scanning electron micrograph of Spindt cathode.

3D FABRICATION OF MINIATURE ELECTRON OPTICS

121

v-grooves for aligning the source to the electron-optical axis. A third mask patterns the Mo film into a gate electrode. The wafer is then cut into individual die. Aluminum oxide (A1203) is deposited on a die at 60 ~ with respect to the normal of the surface. A shallow deposition is used so that the A1203 does not reach the bottom of the submicron hole. A second Mo layer is deposited at normal incidence forming a cone in the hole. An immersion in KOH removes the A1203 and the second Mo layer everywhere except where it formed a cone in the hole. Micrographs of an array of Mo cones are shown in Figures 22b and 22c. The maximum temperature of this source is approximately 550~ and is limited by reaction between the molybdenum and silicon. The effects of a pure H2 and a 9/1 mixture of H2 and Ne plasma glow discharges on the I - V characteristics and emission uniformity of a single emitter tip have been investigated (Schwoebel and Spindt, 1993). The discharges were operated with a current-regulated direct current supply. During glow discharge processing the emitter tip and gate were electrically connected and served as the cathode in the discharge; thus, the emitter was bombarded with only positive ions. The anode of the glow discharge was ~1 cm from the cathode. At this distance and with a pressure of ~ 1 torr, operating voltages of 275-450 V were required to sustain a glow discharge in the gases employed. Total ion current densities at the emitter array were on the order of 1016 ions/cm 2 s -1. Gases used were of Matheson research-grade purity admitted to the system from l-liter glass flasks. Following the glow discharge treatment the system was evacuated to ultrahigh vacuum (UHV) conditions prior to array operation. An in situ hydrogen plasma treatment to doses of 10 ~8 to 1019/cm2 has been shown to reduce the work function for a single tip from 0.5 to 1.5 eV and to increase its emission uniformity. The emission patterns before and after the H2 plasma are shown in Figures 23a and 23b, respectively. The hydrogen cleaning allowed the tips to be immediately operated at 5 # A without the usual 50- to 100-h seasoning time. The hydrogen cleaning was followed by a Hz/Ne plasma treatment, which further improved the emission uniformity (Fig. 23c). The two plasma treatments lower the voltage required to achieve a given emission current compared with that of samples that have not been plasma cleaned, as shown in Figure 24.

B. Silicon Source

Conventional field-emission sources for electron microscopes can either "flash" a cold tip (heat briefly to 1600 K) to clean and reform its surface or operate a zirconium/tungsten tip at 1800 K for thermally assisted field emission. Our laboratory is investigating a micromachined field-emission source on a single-crystal silicon microbridge that can be flashed or operated continuously

122

A . D . FEINERMAN AND D. A. CREWE

FIGURE 23. (a) Field electron micrograph of a single tip prior to hydrogen plasma treatment (V= 175 V, I = 1 #A). (b) Field electron micrograph of the single tip following a H2 plasma treatment (V-- 133 V, I-- 1 #A, total dose ~10 is ions/cm2). (c) Field electron micrograph of the single tip after 9/1 Hz/Ne plasma treatment (V-- 130 V, I = 1 #A, total dose-~7 x 1018 ions/cm2).

at 1000-1200 K. The thermally assisted sources are not as bright as cold fieldemission sources but have more stable emission characteristics. The energy spread of these tips is approximately twice that of a cold field-emission tip. Even operating a cold field-emission tip at 500 K improves its stability. In the past, micromachined sources were developed for high-density displays or for vacuum integrated circuits, and designers could not afford the space or heat

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dissipation required with this approach. However, as shown in Figure 25, a stacked MSEM has the space to accommodate a miniature heater. In addition, a heat source would be valuable because silicon micromachined tips (Ravi and Marcus, 1991) are often contaminated with a thin SiO2layer and there are data showing that heating them to 1000 K in 1.5 x 10 -8 torr would desorb this layer (Yamazaki et al., 1992). A 15-#m-high silicon tip without a gate has been fabricated in the center of a micromachined microbridge with a cross-sectional area of 1.1 x 10 -2 m m 2 and 5 m m long. The center of the bridge has been heated continuously

124

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A.D. FEINERMAN AND D. A. CREWE

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FIGURE25. Fabricationof a field emitter on bridge center. (a) Anisotropic etching of silicon to form an "~100-#m-thick membrane. (b) Isotropic etch of silicon using a 30-#m-diameter SiO2 circle island to form a 15-#m-high silicon tip on the membrane center. (c) Anisotropic etching of the membrane to form the bridge and v-grooves. (d) Oxidation sharpening of tip. (e) Device is bonded to Pyrex die. (f) Dicing saw electrically isolates bridge from the rest of die. up to 935~ The field-emission characteristics of this tip at room temperature, 395~ and 935~ have been investigated. The current fluctuations during 20 min of operation were reduced from 150% at room temperature to 24% at 935~ SEM micrographs in Figure 26 indicate that after the tip operated at 935~ it became rounded and some protrusions and concentric tings developed around the tip. Further investigation is needed before this tip can be used in an MSEM.

VI. DETECTOR A drawback of the MSEM design is that the short working distance reduces the number of choices for detecting secondary and backscattered electrons and X-rays. In the basic MSEM design shown in Figure 1, the working distance is only 0.5 m m and the beam energy is 2.7 keV. The simplest electron detector is a Faraday cup, which collects electrons with a unity gain, which will limit the pixel acquisition time. An approximation to a Faraday cup can be achieved by grounding the last electrode (D4) shown in Figure l a. A metallic layer can be patterned and electrically isolated from the last silicon chip in the column in order to operate the detector at an accelerating potential with respect to the last die in the column to improve the electron collection efficiency.

3D FABRICATION OF MINIATURE ELECTRON OPTICS

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FIGURE 26. SEM micrographs of the 15-#m-high silicon tips after operation at (a) 25~ (b) 395~ and (c) 935~

A standard silicon detector for secondary electrons is a reverse-biased p - n or Schottky junction. A problem with implementing this in an MSEM is the shallow penetration of low-energy electrons into silicon, with 1000-, 100-, and 10-eV electrons penetrating up to 20, 0.5, and 0.02 nm into silicon, respectively. Our laboratory is investigating a surface p - n junction (Fig. 27), which should be able to detect low-energy secondary electrons. The energy of the electrons

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A . D . FEINERMAN AND D. A. CREWE

FIGURE 26. (Continued)

can be increased with the electrode arrangement shown in Figure 28 (Crewe, 1994). A microchannel plate is another secondary electron detector under consideration for an MSEM. The typical microchannel plate consists of an array of glass capillaries that are 400 # m long, with a 10-#m diameter, which would not fit in the 0.5-mm working distance. Researchers at Cornell's Nanofabrication facility have fabricated a microchannel plate into a silicon chip and achieved a gain of 10 (Tasker, 1990).

V I I . ELECTRON-OPTICAL CALCULATIONS

The resolution of an ideal electrostatic column is determined by the diffraction, spherical, and chromatic aberration limited probe size: dd, des, and de. These quantities are given by the following formulas where ot is the final convergence angle, Vb and A V are the beam voltage and spread, and Cs and Cc are the spherical and chromatic aberration coefficients: dd --

0.61~.

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--

7.5 x 10 - s

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0 . 2 5 C s o t 3 (cm)

AV de -- Ccot--Tz-.(cm) Vb

(cm)

(12) (13) (14)

3D FABRICATION OF MINIATURE ELECTRON OPTICS

127

FIGURE27. Surfacep-n junction detector. Top and bottom square contacts are for the vertical n-t- fingers. Left and right contacts are to the p-type bulk. Low energy incident electrons on the detector surface will be collected by the n+ fingers. Guard rings have been omitted for visual clarity. The method used to calculate the aberration coefficients of a stacked lens was reported previously and is also used to calculate the properties of a sliced column (Crewe, Perng et al., 1992). The ultimate resolution is frequently reported as the root-mean-square of dd, dcs, dc, and source size, but this has been shown to be incorrect (Born and Wolf, 1980; Crewe, 1987). A more accurate determination is obtained by graphing dd, dcs, and de versus o~, the final convergence angle. Two resolution plots for a stacked and a sliced lens are shown in Figure 29. The resolution of the column with this method is the maximum value of dd when it crosses ds, dc, or the source size. In general, electrostatic einzel lenses above 5 kV are spherical-aberration limited, while below 5 kV, chromatic aberration dominates their performance. The most accurate determination of the resolution happens to be the intersection of the diffraction curve with the line 0.15Csoe 3 when the system is spherical-aberration limited (Crewe, 1987). A stacked column 3.2 m m long with a 0.5-mm working distance has spherical and chromatic aberration coefficients of 18.2 and 2.2 cm, respectively. The calculations for a stacked lens assume a column constructed with 381-/xm wafers separated by 2 2 0 - # m gaps with the two-layer deflector, and is

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A.D. FEINERMAN AND D. A. CREWE

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FIGURE28. Interdigitated fingers on the detector surface at + 1000 and - 1000 V can boost the incident electron energy from 0-10 eV to several hundred eV. A circularly symmetric interdigitated pattern like two spirals rotated 180~ would be preferred since it would introduce less astigmatism. The n+ fingers shown in Figure 25 have been omitted to simplify the figure.

shown in Figure 7a. The structure would have - 1-, - 2 . 2 5 - , - 2.25-, and 0-kV potentials applied to electrodes D 2 - D 5 , respectively, which would make the m a x i m u m electric field between electrodes ~ 10 kV/mm. This column would be chromatic-aberration limited and should have a resolution of 3.8 nm when operated at 2.5 kV with a 4-mrad convergence angle. The stacked electronoptical calculations assume a point electron source at - 2 . 5 kV and 220 # m below D2. As mentioned in Section III.A, there is an advantage to etching the apertures separately and then assembling the electrodes rather than assembling the electrodes, then making the apertures. The three-electrode lens shown in Figure 7b with all apertures 100 # m in diameter operating at a beam energy of 2.5 keV and a working distance of 0.5 m m will have an optimal resolution of 5.25 nm when the final angle of convergence is constricted to be 3.25 mrad. If the focusing electrode aperture (D3 in Fig. 7b) is 200 # m in diameter the same lens will have an optimal resolution of 3.8 nm at the same working distance and beam energy at a final angle of 4 mrad.

3D F A B R I C A T I O N OF M I N I A T U R E E L E C T R O N OPTICS

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(b) FIGURE 29. (a) A stacked lens 3.2 mm long at 2.5 kV is chromatic-aberration limited to a resolution of 3.8 nm at a working distance of 0.5 mm. (b) A sliced lens 8.5 mm long at 15 kV is spherical-aberration limited to a resolution of 2.2 nm at a working distance of 1 mm.

The need to use apertures of different diameters in the column is also apparent when one considers the above restriction on the final angle of convergence being limited to something on the order of a few milliradians. Restricting the beam to such a small angle requires a beam-limiting aperture on the order of 10 # m or less to be placed somewhere in the optical column. If the apertures in the column are fabricated simultaneously, then all the apertures would have to be ~ 10 #m, which would result in high axial field gradients near the apertures and large aberration coefficients. With the ability to vary the aperture diameters

130

A.D. FEINERMAN AND D. A. CREWE

the final electrode could be used to limit the beam angle and the preceding resolution calculations would not be affected, since that electrode is held at ground potential. A sliced column 8.5 mm long with a 1-mm working distance has spherical and chromatic aberration coefficients of 30.98 and 2.57 cm. The inner and outer diameteres of all electrodes are 300 and 500 # m (see Fig. 9 for other column dimensions). The electron-optical calculations for the sliced column assume a point electron source at - 1 5 kV with a 0.1-eV energy spread 1 mm below E2. The structure would require - 5 - , - 13.7-, and 0-kV potentials applied to electrodes E2-E4, respectively, to focus the beam at the 1-mm working distance. This column should have a resolution of 2.2 nm when operated at 15 kV with a 3-mrad convergence angle. Since the column operates with approximately unity magnification, the source size becomes important only when it exceeds the minimum attainable resolution of 2.2 nm shown in Figure 29. A 125-#mdiameter tungsten wire at 25~ oriented along the (111) or (310) direction whose tip has been electrochemically etched to a 0.1-#m radius will have a 1- to 2-nm source size and is therefore acceptable. The sliced electrode fabrication method proposed in Section III.C makes the electrode's surface highly resistive, on the order of 10 ~~ f2/square. If stray current strikes the electrode's surface the electrode's potential will no longer be constant. Simulation of a fraction of the beam symmetrically striking the electrode's inside surface was accomplished by choosing a voltage perturbation that linearly increased from zero at the edge of the electrode to a maximum at its center. If a 100-V perturbation occurred on electrodes E2 and E4 (Fig. 9), the effect on Cc and Cs for the column was less than 1%. Ten- and 100-V perturbations of electrode E3 had a negligible effect on Cc and increased Cs by 5 and 11%, respectively.

A. A Tilted MSEM

The stacked design can be easily modified to create an array of high-resolution MSEMs that can be tilted 60 ~ or more with respect to the sample. Very large tilt angles are required for defect review and inspection. As shown in Figure 30 the working distance with a 2-kV MSEM exceeds 6 mm when the sample is tilted 60 ~ and the smallest probe size will be several microns. This hypothetical stack would be constructed from 7 x 9-mm 2 silicon die that are 381 # m thick with 269-#m gaps, and the aperture radii in D2, D3, and D4 (Fig. 1) are 100,100, and 5 #m, respectively. A linear array of MSEMs could perform high throughput inspection of wafers. As shown in Figure 31 the working distance can be reduced substantially to 1.082 mm by using anisotropic etching to reduce the width of D4. A dicing saw

3D FABRICATION OF MINIATURE ELECTRON OPTICS

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would trim D3 so that it was 1 m m from the electron-optical axis or 10 times the 1 0 0 - # m aperture radius. The smallest probe size with a 2-kV b e a m is --'5 n m in this case, ignoring any astigmatism introduced by the stack. The working distance can be reduced further to 649 # m by reducing the aperture radius to 75 # m and using a dicing saw to trim D3 so that it c o m e s within 0.75 m m from the electron-optical axis. The smallest probe size with a 2-kV b e a m in this case is ~ 4 nm, again ignoring any astigmatism introduced by the stack. A 20-kV M S E M could also be created along these lines that could achieve a 1.5-nm probe at a working distance of 2 m m and a 60 ~ sample tilt. The

132

A.D. FEINERMAN AND D. A. CREWE

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higher voltage would allow the MSEM to also perform chemical analysis on the particles detected.

VIII. PERFORMANCE OF A STACKED EINZEL LENS* A. M S E M Construction

The focusing properties of a stacked electrostatic electron lens have been evaluated within a macroscopic assembly shown in Figure 32 (Crewe, Ruffin et al., 1996). The entire MSEM test structure is a cylinder 7.5 cm in diameter and 10 cm tall. This assembly consists of a 2.5-kV einzel lens, an electron source, parallel plate deflectors, and a Faraday cup as an electron detector. The test assembly positions the electron source over the silicon lens. The beam will be electrostatically scanned over the sample and an image can be formed from a current signal taken either from the sample itself or from the detector below the sample. The apparatus can be easily modified to incorporate the other micromachined components (deflector/stigmator, detector, and electron source) in the column as they are developed. The electron source is a macroscopic zirconiated tungsten thermally assisted Schottky field-emitter operating at 1800 K. The thermally assisted ZrO2 field-emission source available from FEI Inc. was chosen because it can provide highly stable field emission in a desirable *Portions of Section VIII are reprinted, with permission, from the Journal of Vacuum Science and Technology A, 14(6), 3808-3812, November 1996. Copyright 1996 American Vacuum Society.

3D F A B R I C A T I O N OF M I N I A T U R E E L E C T R O N OPTICS

133

FIGURE 32. (a) A commercial thermal field-emission (TFE) source can be aligned to a micromachined einzel lens with this experimental arrangement. There are two Macor push rods that move the TFE source. Each rod is driven by a linear-motion feedthrough and works against a UHV spring. Below the einzel lens are electron beam deflectors; a 3-min TEM grid, which serves as the sample: and a Faraday cup to detect the transmitted beam. The entire arrangement is surrounded by a mu-metal can to shield the electrons from the earth's magnetic field. (b) Thermally assisted field-emission source positioned over micromachined silicon lens demonstrating electron beam focusing to a point on the wire grid sample.

134

A.D. FEINERMAN AND D. A. CREWE

current range (1-25 #A) at readily achieved vacuum levels (10 -9 torr). The chief drawbacks to this source are its large physical size (a cylinder 2 cm in diameter and 2 cm tall) relative to the micromachined lens, the need for a mechanism to align the emitted electrons to the optical axis of the micromachined lens, and the relatively high extraction voltage required to achieve field emission (>3 keV). The design of the test structure was dictated by the need to secure, align and electrically insulate the source from an extractor electrode. As recommended by the manufacturer, the source is placed 500 # m from an extractor electrode containing a commercially available 500-#m-diameter Pt-Ir aperture. We chose to machine a bulky stainless-steel extractor electrode fit with a commercially available aperture for the purpose of absorbing most of the emitted electrons from the source. The source and extractor are placed 1 cm before the silicon lens. The test assembly consists of alternating stainless steel and Macor (a machinable glass ceramic made by Corning Inc.) tings. From the bottom up the structure consists of a Faraday cup to collect electrons; a sample holder designed to house a commercial 3-ram gold grid; a parallel plate deflector assembly, which must electrically isolate the deflectors from each other as well as the elements above and below; the micromachined electrostatic lens, which is mounted to a 16-pin Airpax header; an extractor electrode; and the FEI source. The assembly is stacked one ring above another and is held together under compression in a mu-metal exterior can, which provides both the structural integrity of the assembly and magnetic shielding of the optical column (Fig. 32). The critical alignment necessary in the structure is the alignment of the lens electrode apertures to one another and the alignment of the electron source to the lens apertures. The electrode-to-electrode alignment is accomplished through our micromachining technique and the electron source alignment is accomplished by means of two insulated linear-motion feedthroughs, which push on the FEI source at 90 ~ with a return spring. This allowed the majority of the pieces in the assembly to be machined to fairly low tolerances (tolerances were specified to ~ i 5 0 #m), which kept the machining cost low. The entire assembly is inserted into a commercially available 6-in. UHV vacuum chamber containing a 30-1iter/s nonevaporable getter pump that is mounted to a 120-1iter/s ion pump. The motion feedthroughs are attached, electrical connections are made, and the system is evacuated. A base pressure of 1 x 10 -9 torr is achieved in 48 h. The silicon lens was fabricated from 380-#m-thick silicon chips separated with 250-#m gaps. The performance of a three-element lens using these physical parameters has been calculated and the results are shown in Figure 33. These calculations indicate that the lens can produce a high-quality focus from a position near the exit aperture of the lens to a working distance of up to a

3D FABRICATION OF MINIATURE ELECTRON OPTICS ,

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few centimeters with potentials on the focusing electrode(s) that are allowed by the die-to-die gaps. The extractor aperture is optically aligned to the silicon apertures in the micro-machined lens by placing the assembly under a microscope, using bottom illumination to view the bright circular spot formed by the apertures in the silicon, centering the 5 0 0 - # m extractor aperture over that spot, and securing the extractor in place. Typical operating potential differences between the source and the extractor electrode are in the range of 2.5-3.75 kV for an emission current of 1-25 # A . The FEI source also contains a suppressor electrode, which is biased negative with respect to the tip to prevent thermally generated electron emission from escaping the source. Initially our micromachined silicon apertures were only 3.5 # m thick, which was probably not thick enough to take the b o m b a r d m e n t of ~ 3 0 # A of 3-keV emission. However, we subsequently improved the silicon process to give 1 0 0 - # m - t h i c k apertures and will later remove the extractor from the assembly. With the stainless-steel electrode in the system, the first two silicon electrodes can be operated in parallel as one optically long focusing electrode. This has been calculated to produce a higher-quality probe as well as to provide more flexibility in operation (Feinerman, Crewe, Perng, Spindt et al., 1994). Calculations indicate that a stacked lens with 1 5 0 - # m - d i a m e t e r apertures will produce a 4 2 5 - n m focus with a 2.5-kV b e a m at a working distance of 4 m m and a field-emission source 1 cm above the lens (Fig. 33). If the final angle of convergence is reduced from 10 to 2.6 mrad, the focus improves to 6.2 nm. If the working distance is reduced to 0.5 mm, a 2.3-nm

136

A . D . F E I N E R M A N A N D D. A. C R E W E

resolution can be achieved at a final angle of convergence equal to 6.5 mrad. The efficiency of the electron detector will have to be increased, however, since the probe current is inversely proportional to the square of the convergence angle. Images of a 200- and 1000-mesh gold TEM wire grid at a working distance of 4 mm have been obtained in transmission. The beam is scanned over the sample by using parallel plate deflectors. The silicon lens is 1.64 mm long and consists of three silicon die separated by Pyrex optical fibers as shown in Figure 2. Images of the grid at magnifications above 7000x are now being obtained.

B. MSEM Operation and Image Formation The potentials applied to the source and lens electrodes and the filament heating current are supplied by a computer-controlled set of electronics (Fig. 34). Three high-voltage power supplies and a constant current supply are floated with their

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3D FABRICATION OF MINIATURE ELECTRON OPTICS

137

virtual ground at the beam potential. Isolation from earth ground is achieved through optical couplers. The suppressor and focus potentials, and the filament heating current are controlled through an RS232 serial connection to a personal computer (PC). The beam potential is manually set on an externally regulated high-voltage power supply. After initial conditioning of the extractor electrode to allow for electronstimulated desorption of gas ions, the total emission is increased to ~3 #A and the source-to-silicon aperture alignment is performed. Once a beam is brought through the lens, the focus electrode potential is optimized by comparing successive line scans over the gold grid. The optimal focusing potential agrees well with calculated values, differing by less than 10%. The deflection potential signals and the image data are generated and received by data-acquisition boards in a PC. The low-voltage deflection ramps are the input to a high-speed, high-voltage amplifier capable of generating - 5 0 0 - to +500-V signals at a rate of 10 kHz. The faces of the deflectors that are perpendicular to the electron beam measure 1.5 • 1.5 mm and are spaced 1.25 mm apart. A simple time-of-flight deflection calculation predicted a beam deflection of 0.5 # m / V of applied deflection signal. Experimentally, we have observed that one volt of deflection potential yields approximately 0.4 # m of beam deflection. Typical deflection signals are staircase ramps in the range - 1 5 0 to + 150 V (for a field of view 120 by 120/zm) generated at a line rate of 10 Hz. Imageacquisition time for a 512 x 512-pixel image is then 51.2 s. The image data consist of the Faraday cup current (for a dark-field image) or the sample current (for a bright-field image) that has been put through a current-to-voltage amplifier with a gain of approximately 10 j~ and a maximum pixel rate of 100 kHz. This 0- to 1-V signal is the input to a 12-bit analog-to-digital converter that acquires the image data synchronously with the deflection ramp generation. The raw image data are then normalized and imported into a commercially available image-processing software package for viewing. The initial and final voltages of the X and Y deflection ramps can be software selected, and the magnitude of the amplified deflection signal can be varied, which allows the user to perform a direct current offset high-magnification scan of a region of interest that is not in the center of a low-magnification image. Low- and high-magnification images of a 1000-mesh gold wire grid are shown in Figures 35-38. The 10-90% rise time of the line scan shown in Figure 39 covers a lateral distance of 2.1 #m. This indicates that if the probe is Gaussian, it has a sigma of 0.75 #m. This is a worst case estimation of the beam probe size, since the grid wires in reality have a finite slope, but does give a value that agrees well with calculations.

138

A . D . F E I N E R M A N AND D. A. CREWE

FIGURE 35. This image obtained with test apparatus demonstrates the ability of the micromachined silicon electron lens to focus on a 1000-mesh gold TEM grid. The grid wires are 6 # m wide and are spaced 19 # m apart, and the signal is from the Faraday cup current.

FIGURE 36. Magnification is ~2000 of 6 # m grid, and the signal is from the Faraday cup current. Defect in center of image is from screen saver turning on.

FIGURE 37. Magnification is ~3500 of 6 - # m grid, and the signal is from the Faraday cup current.

FIGURE 38. High-magnification image of defect on wire grid. Image has been electronically rotated to bring wire to a nearly vertical position. The cross wire is not at a right angle to the vertical wire, possibly as a result of a deformation of the sample when it was fit into the test assembly. The defect is approximately 0.5 # m wide.

140

A . D . F E I N E R M A N A N D D. A. C R E W E

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FIGURE 39. Line scan data from a high-magnification image of one period of the 1000-mesh grid. The scan signals were electrically rotated so that the beam was deflected perpendicular to the wires. The 10-90% rise time of 2.1 # m corresponds to a Gaussian probe sigma of 0.75 #m. In its present configuration the MSEM is spherical-aberration limited, so the Gaussian probe is a good approximation to the actual beam.

I X . SUMMARY AND FUTURE PROSPECTS

Microfabrication techniques have advanced to the point where conductors, semi-conductors, and insulators can be positioned in complex threedimensional arrangements with very high precision. This is equivalent to a conventional machinist's operating miniature milling machines and lathes with micron-sized bits. This flexible machining capability allows electric and magnetic fields to be created that can accelerate, focus, steer, and/or align charged particles, because the fields occupy a volume of space rather than simply existing next to a surface. Specific fabrication techniques developed at UIC include stacking silicon chips with Pyrex fibers, selective anodic bonding (slicing), and using a LIGA lathe. These techniques are being used to integrate chargedparticle sources, electrodes, and detectors into various miniature instruments including a subcentimeter SEM, a 10-cm time-of-flight mass spectrometer, a 10-cm nuclear magnetic resonance instrument, and a 5-m linear accelerator/undulator capable of producing hard X-rays. Analytical instruments of this size will allow the analytical laboratory to be brought to the sample, which will be essential when the sample must be observed in situ (e.g., at a toxic waste site or in outer space).

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