SPIE Handbook of Microlithography, Micromachining and Microfabrication, Volume 1: Microlithography
Section 2.5 Systems: 2.5.4 Gaussian vector scan systems
|JC Nabity Lithography Systems||Raith GmbH||Leica Lithography Systems Ltd.|
|Alignment||Automated or manual||Automated or manual||Automated|
|Stitching||Automated, accuracy limited by stage||Automated, 0.1 um accuracy with laser stage||Automated, with laser stage|
|Energy||0-40 kV for typical SEM, but depends on target instrument||0-40 kVfor typical SEM, but dependson target instrument||10 to 100 kV|
|DAC speed||Low, > 10 us per exposure point (100 kHz)||Mid-range, >0.4 us per exposure point (2.6 MHz) but may be limited by SEM deflectors||Mid-range, > 1us per exposure point (1 MHz)|
|Throughput limited by||Settling time of scan coils, transmission rate of ISA bus||Settling time of scan coils||Settling time of scan coils|
|Stage||Support for any automated stage||optional laser controlled||optional laser controlled|
|Control computer||PC compatible ISA bus DOS/Windows||PC compatible DOS/Windows||PC compatible|
|Cost||Low, < $50k, <$30k to universities for pattern generator only. SEM purchased separately||Mid-range, > $100k for pattern generator only. SEM purchased separately||Mid- to high range, >$1000k for a complete lithography system|
|Contact||406-587-0848 406-586-9514 firstname.lastname@example.org||Germany: 49-0231-97-50000 USA: 516-293-0870, 0187 fax||USA: 708-405-0213 708-405-0147 fax, UK: 44-1223-411-123, -211 fax|
JEOL's popular JBX-5DII Gaussian vector scan system uses a LaB6 emitter running at either 25 or 50 kV. Figure 2.17 shows the 5DII with two condenser lenses and two objective lenses. Only one of the objectives is used at a time; the operator has the choice of using the long working distance lens for a field size of 800 um, or the short working distance lens, for an 80 um field at 50 kV. (The fields are twice as large at 25 kV.) The pattern generator runs at 6 MHz (> 0.167 us per exposure point) and the stage has a precision of /1024 = 0.6nm. As with all commercial systems, alignment, field stitching, and sample handling are fully automated. In fact, one drawback for research purposes is that there is no manual mode of operation. The system is capable of aligning to within 40 nm (2) and writing 30 nm wide features over an entire 5 in. wafer or mask plate. JEOL systems are known for their simple, high quality sample holders. The 5DII is one of the highest resolution (though not one of the fastest) e-beam tools in the LaB6 class.
|FIGURE 2.17 Schematic of the JEOL JBX-5DII system with LaB6 emitter. The system features two objective lenses for two different working distances (courtesy of JEOL Ltd.).|
JEOL's JBX-6000 implements a number of improvements on the 5DII. The LaB6 emitter is replaced with a thermal field emitter, eliminating the need for one of the condenser lenses. The pattern generator speed is increased to 12 Mhz, and the PDP-11 controller is replaced with a VAX. The system uses the same set of two objective lenses, and for a given objective lens the magnification is fixed (that is, the DAC's deflection is not scaled with the field size). As can be seen in the graph of figure 2.18, the ultimate spot size is somewhat improved over that of the LaB6 machine, but more importantly, the current density at smaller spot sizes is greatly improved. The JBX-6000 runs at 25 kV or 50 kV.
With higher current density comes the property that the probe size is sometimes smaller than a pixel. For example, consider a pixel grid of spacing 0.0025 um. If the rastering beam skips every n grid points, then the pixel area is (n 0.0025 um)2. With a current of 10 nA and a dose of 200 uC/cm2, we must have (n 0.0025 um)2 200 uC/cm2 = 10 nA (exposure time for one pixel), and since the minimum exposure time is 1/(12 MHz) = 0.08 us, the smallest value of n is 9. In this case the pixel spacing is 22.5 nm and the spot size, according to Fig. 2.18 is 12 nm. In this example the pixel spacing is larger than the spot size, and the exposed features may develop as a lumpy set of connected dots. The problem will be even more pronounced when using high speed resists, large field sizes, and larger currents. One solution would be to implement a faster pattern generator; however, JEOL's approach is to retain the superior noise immunity of the 12 MHz deflector and instead to use less current when necessary, or to increase the spot size by using a larger aperture. Alternatively, one can purposely defocus the beam. The NPGS system (see Sect. 18.104.22.168) attacks the problem by allowing different pixel spacings in X and Y (or in r and ).
It is interesting to note that future high resolution systems under development at Hitachi  are likely to resemble the JEOL Gaussian-spot tools, with field sizes >= 500 um and a single stage electrostatic deflector. Small fields avoid the complexities of dynamic focus and astigmatism corrections, and allow the short working distance needed to reduce the spot size. Single stage deflectors limit the bandwidth (speed) of the system, but improve intrafield stitching between deflections of coarse and fine DACs. The design tradeoff is clearly between high speed and high accuracy.
Electron beam systems from Leica Lithography Systems Ltd. (LLS) are a combination of products previously manufactured by Cambridge Instruments, the electron beam lithography division of Philips, and most recently products from the former Jenoptik Microlit Division. Leica sells eight different models of Gaussian spot vector scan machines (the EBL Nanowriter has been described above). Systems in the mid-range of resolution include the EBML-300, a LaB6 tool directly evolved from the Cambridge line, and the EBPG-5, a LaB6 machine evolved from the Philips line. The EBPG-5 is comparable to the JEOL JBX-5DII in resolution but has accelerating voltage up to 100 kV. The EBMLand EBPG are both known for their versatile control software. On Leica's high end is the VectorBeam, with optics evolved from the Philips EBPG line and control electronics and software evolved from the Cambridge EBML line. The VectorBeam (Fig. 2.19) has a thermal field emission electron source running at 100 kV and a 6 in. stage motion with up to /1024 = 0.6 nm precision. The 25 MHz pattern generator has the useful feature that it is able to hold a small pattern in a buffer, so that repeated patterns do not have to be retransmitted to the pattern generator. This can significantly decrease the transmission overhead time when writing a large array of simple figures.
|FIGURE 2.18 Probe beam diameter versus current for (a) a LaB6 cathode with a 120 um objective aperture, (b) a thermal field-emission (TFE) cathode with a 40 um objective aperture, and (c) a thermal field-emission cathode with a 100 um objective aperture. Data is from JEOL Gaussian-spot e-beam systems using 50 kV acceleration and a short working distance objective ("5th lens") (courtesy of JEOL Ltd.).|
Leica e-beam tools are also distinguished from those of JEOL by their use of a single objective lens (one working distance), and scaleable writing fields with 215=32768 or 216 =65536 pixels across the field. In the case of the EBML-300, field sizes up to 3.2 mm may be used, although the benefit of using such a large field is debatable.
|FIGURE 2.19 Schematic of the Leica VectorBeam 100 kV column with a thermally assisted field emission electron source (courtesy of Leica Lithography Systems Ltd.)|
The largest systems from Leica are also equipped with 100 kV TFE emitters, and have stages with up to 8 in. travel. Additional features include a glancing-angle laser height sensor for dynamic field size corrections, and dynamic focus/astigmatism corrections -- features more commonly found on high speed maskmaking tools. Systems using large writing fields, with deflection angles exceeding 5 to 10 milliradians, make use of a number of higher order corrections including deflection linearization maps, field rotation maps, dynamic focus and stigmation tables, and even shift corrections for the dynamic focus coil.
One of the most unique Gaussian vector scan systems is the LION-LV1 from Leica Lithographie Systeme Jena GmbH, a company better known for its large mask making machines (previously sold only in Eastern Bloc countries). The LION-LV1 combines a column designed by ICT GmbH (Heimstetten, Germany) with the pattern generator from Raith GmbH. This pattern generator has the unusual feature that it allows "continuous path control" of curves. In this mode the beam is held close to the center of the field while stage motion defines the shape of a Bezier curve. The ICT column is very similar to that used in the Leo 982 SEM,  except for the use of a beam blanker and higher bandwidth deflection coils (see Fig. 2.20). In this system, proximity effects are avoided by using beam energies as low as 1 to 2 keV. Although the voltage may be set as high as 20 kV, the system's selling point is low voltage -- avoiding both damage to the substrate and complications due to the proximity effect.
The column provides a spot size as small as 5 nm at 1 kV, through the use of an unusual compound objective lens. An electrostatic lens produces a diverging field, while the surrounding magnetic lens converges the beam. The complementary lenses reduce chromatic aberration, just as in a compound optical lens. A high resolution automated stage, substrate cassette loader, and substrate height measuring system complete the LION-LV1 as a full-featured system.
Low voltage operation avoids substrate damage and proximity effects, and offers the capability of three dimensional patterning by tailoring the electron penetration depth. However, the disadvantage is in greatly complicated resist processing. If the beam does not penetrate the resist, there will be significant effects from resist charging,  and placement errors due to charging may be dependent on the writing order and on the shape of the pattern itself. Charging may be avoided by using a resist trilayer with a conducting center (e.g., PMMA on Ti on polyimide), or by using a conducting overlayer (see sect. 2.7.1). Increased processing is required also for removing the resist layer over alignment marks. In a production environment this complexity adds significantly to the cost of ownership.
|FIGURE 2.20 Low-voltage column developed by ICT GmbH, used in the LION e-beam system from Leica. The beam blanker is directly above the anode. The objective lens combines electrostatic and magnetic elements to reduce the net chromatic aberration. Beam diameter at 1kV is approximately 5nm. (Courtesy of LLS Jena GmbH.)|
|JEOL Inc.||Leica Lithography Systems Ltd.||Leica Lithographie Systeme Jena GmbH|
|Resolution (minimum spot size)||5 nm||8 nm||5 nm|
|Field size||maximum 80 or 800 um at 50 kV||scaleable, 16 bits in up to 800 um at 50 kV or 400 um field at 100 kV||scaleable, 16 bits|
|Energy||25, 50, 100 kV||10 to 100 kV||1 to 20 kV|
|Speed of pattern generation||high, > 0.08 us per exposure point (12 MHz)||highest of class, >0.04 us per exposure point (25 MHz)||mid-range, > 0.4 us per exposure point (2.6 MHz)|
|Stage||laser controlled, 0.6 nm, 6 inch travel||laser controlled, 0.6 nm, 6 inch travel||laser controlled, 2.5 nm, 162 mm travel|
|Control computer||VAX (VMS)||VAX (VMS)||PC compatible|
|Cost||Expensive, > $3M||Expensive, >$3M||Expensive, >$1M|
|Contact||USA: 518-535-5900, Japan: 0425-42-2187, 1-2 Musashino 3-chome, Akishima Tokyo 196||USA: 708-405-0213, -0147 fax. UK: 44-1223-411-123, -211 fax||USA: 708-405-0213, -0147 fax. UK: 44-1223-411-123, -211 fax|
Next Sub-Section: 2.5.5 Gaussian Spot Mask Writers
This material is based upon work supported by the National Science Foundation under Grant No. ECCS-1542081. Any opinions, findings, conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
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