DESIGN AND FINE MOTION TEST OF A PRECISION X-THETA STAGE IN VACUUM

Gyungho Khim1, Jongyoup Shim1, and Chun-Hong Park1 1Department of Ultra Precision Machines and Systems

Korea Institute of Machinery and Materials Daejeon, South Korea

INSTRUCTIONS

Recently, precision stages in vacuum are required in many applications such as mastering processes for high density optical disc [1-2], and lithography equipments for wafer or mask. These processes rely on electron beams, which are operable in only high vacuum. In this paper, a precision X-Theta stage aimed at being used for vacuum applications, representatively electron beam mastering equipments, is developed. A brief design method and experiment result focused on fine motion using commercialized controller are introduced. DESIGN OF A X-THETA STAGE

In the mastering processes of optical disc, two degree of freedom motions, linear(X) and rotary(Theta) motions, are mainly required. The linear stage is designed using linear motion ball bearings(THK, HR2555M) while rotary stage used air bearings because rotational accuracy is more important. The HR model used for linear stage has characteristics of very low height and compact size, and easy adjustment of clearance. Coreless linear motor(Trilogy, 210-4N) and linear scale(Heidenhain, LIP481R) were also used in the linear stage. These are chosen to be vacuum-compatible. Maximum stroke is 250 mm and a scale resolution is set to 5 nm. FIGURE 1 shows a structure of the linear stage.

FIGURE 1. Structure of a linear stage

The rotary stage used a vacuum-compatible air bearings(carbon porous) for extremely high

accuracy, and a slotless direct drive motor (Aerotech, S130-39-B) to suppress unwanted motion errors caused by cogging force, and a thin type of rotary encoder(Sony, BH20) to shorten the overall height of the stage. The Turbo PMAC2 controller (Delta Tau) and linear amplifier (Varedan, LA400) are used for both stages. The commutation for motor driving is carried out using not a hall sensor but encoder signal for a better smoothing motion. The rotary encoder has a resolution of 0.162 µrad (0.03 arcsec) by means of dividing the analog sine signal using interpolator board (Delta Tau, ACC-51P) The air leakage problem from the air bearings has been solved using differential exhaust method [2-3]. Sufficient vacuum level for electron beams can be obtained using three steps of exhaust system. The first exhaust line is open to the atmosphere, and the second, third exhausts are connected outside vacuum pumps. The first exhaust line is also used as feedthroughs for a motor, encoder cables, and compressed air line as well as exhaust of air. This region is connected to outside atmosphere using bellows, and always maintains atmospheric condition. Therefore non-vacuum-compatible components can be used. This is one of the big merits. FIGURE 2 shows a structure of rotary stage.

FIGURE 2. Structure of a rotary stage

Linear motor

Linear scale

Linear motion bearing

Journal bearingThrust bearing

Direct drive motor

Rotary encoder

2nd exhaust

3rd exhaust

Compressedair in

Workpiece

1st exhaust: Feedthrough for motor & encoder cable, compressed air line

(Atmospheric condition)

Shaftd3

d2

ls3

ls2

ls1

d1

Air leak

The gap between the inside of housing and the outside of shaft in differential exhaust system is very small (5~10 µm). This small gap prevents air from leaking into the vacuum chamber. The design parameters such as diameter of exhaust lines (di), seal lengths(lsi), and pumping speeds of vacuum pumps(Si) are optimally decided using optimized method with genetic algorithm [3]. Table 1 shows design results. We can also know which type of pumps should be used for the second and third exhaust lines by means of the inlet pressures(Ppi) calculated in Table 1. VACUUM LEVEL TEST

FIGURE 3 shows the X-Theta stage installed inside the vacuum chamber. The stages were fabricated with anodized aluminum 7075. It was used because of high hardness characteristic in spite of a disadvantage in outgassing. FIGURE 4 represents a vacuum chamber and various feedthroughs for signals and flows. The vacuum chamber was installed on the anti-vibration table. FIGURE 5 demonstrates a pressure variation of a vacuum chamber obtained while the air bearing is working. Turbo molecular pump(TMP) of a 550 l/s was used for exhausting the vacuum chamber. The chamber pressure was measured with a full-range vacuum gauge (PFEIFFER Vacuum, TPG 261). The chamber pressure reached 5.1×10

-4 Pa within 29 hours of

operation without activating the air bearing. Then, the second and third exhaust pumps were operated for preparation of activating the air bearing. After 1 hour, the chamber pressure was reduced to 4.7×10

-4 Pa. Compressed air of 0.4

MPa was supplied to the air bearing. The chamber pressure suddenly increased to 5.2×10

-4 Pa, but then began to decrease slowly

to 4.3×10-4 Pa after 5 hours later. This level of

vacuum pressure is sufficient for using electron beams. Moreover, it is expected that high vacuum level less than 10

-4 Pa could be

achieved with increasing time, as shown in FIGURE 5.

FIGURE 3. X-Theta stage installed inside the vacuum chamber

FIGURE 4. Vacuum chamber

FIGURE 6 shows a pressure variation of a vacuum chamber obtained while the air bearing(rotary stage) is moving. The stage is rotated with velocities of 100, 200, and 300 rpm, and the pressures are continuously measured. The pressure rise is evaluated by means of “100 log (P/P0),” where P0 is a pressure just before moving, and P is a pressure measured during moving. The pressure increased with velocity, however, it showed only increase of 0.5 % for a velocity of 300 rpm. This amount of increase is negligible.

Rotary

stage

Linear stage

E-beam

Vacuum gauge

Feedthroughs

Exhaust line

TABLE 1. Optimal design of exhaust system

Chamber pressure (Pa)

Seal lengths of i-th exhaust lines,

lsi (mm)

Diameters of i-th exhaust lines, di (mm)

Pumping speeds of i-th vacuum pump,

Si (liter/s)

Inlet pressures of i-thvacuum pump, Ppi (Pa)

ls1 ls2 ls3 d2 d3 S1 S2 S3 Pp1 Pp2 Pp3

Optimal design

2.3�10-5 4.8 10.2 11.6 16.2 24.2 Atm. 24.5 61.9 Atm 40.7 3.8×10-3

FIGURE 5. Pressure variation while air bearing is working

FIGURE 6. Pressure variation during air bearing movement FINE MOTION TEST

Vacuum-compatible grease was firstly used as a lubricant in the linear motion bearing, however, its friction was considerably high due to a high viscosity characteristic. Instead of grease lubrication, solid lubrication was adopted. DLC (Diamond Like Carbon) coating on the linear motion bearing was carried out by sputtering deposition because the DLC coating exhibits super-low friction and high hardness, and high atomic structural stability in vacuum [4]. Capacitance sensor(ADE, 2803V, 0.5 nm resolution) and acquisition board(Dewetron, Dewe-43, 24bit resolution) were used to evaluate a fine motion and the repeatability. The displacements were measured after low pass filtering for eliminating environmental noise and electrical noise. FIGURE 7 shows a fine motion step response of 5 nm, which is the same as

scale resolution, in the linear stage. The result showed the stage exactly moved 5 nm, and better fine motion would be obtained if the higher scale resolution were used. FIGURE 8 shows the bi-directional repeatability at a point. The displacements were measured for stopping at a zero position while the stage reciprocally moved 1000 counts within the measurement range of the capacitance sensor. The result showed 2σ (σ:standard deviation) of 0.005 µm.

FIGURE 7. Fine motion step response in the linear stage

FIGURE 8. Bi-directional repeatability in the linear stage

FIGURE 9. Setup for fine motion step response in the rotary stage The fine motion step response in the rotary stage was measured using a bar installed upper table of the rotary stage, and converted to angular position, as shown in FIGURE 9. The result showed the stage exactly moved a resolution of 0.162 µrad, which is the same as encoder resolution, as shown in FIGURE 10. The bi-directional repeatability was also

0 5 10 15 20 25 30 3510

-4

10-2

100

102

104

106

27 28 29 30 31 32 33 34 35

4.2

4.5

4.8

5.1

5.4

5.7

4.3x10-4 Pa

RP On

Air Bearing On

TMP On

Pressure (Pa)

Time (hr)

3rd TMP On2nd, 3rd RP On

Air Bearing On

Pressure (10-4 Pa)

Time (hr)

0 20 40 60 80 1007.56

7.60

7.64

7.68

0 20 40 60 80 100-0.2

0.0

0.2

0.4

0.6

P

P0

Stop

Start

Rotation speed

100 rpm

200 rpm

300 rpm

Pressure (10-4 Pa)

Rotation speed

100 rpm

200 rpm

300 rpm

Time (sec)

100 log(P/P

0)

0 10 20 30 40 50 60

0

5

10

15

20

25

301 cnt/step

5 nm/cnt

Displacement (nm)

Time (s)

0 20 40 60 80 100 120-0.2

-0.1

0.0

0.1

0.22σ=0.005 µm

Time (s)

Displacement (µm)

evaluated like the method done in the linear stage. The result showed 2σ of 0.054 µrad, as shown in FIGURE 11.

FIGURE 10. Fine motion step response in the rotary stage

FIGURE 11. Bi-directional repeatability in the rotary stage SIMULTANEOUS MOTION TEST OF X-THETA STAGE

Simultaneous motion between a linear and rotary stage was evaluated. When machining optical disc, the constant linear velocity control is required. So the velocity of the rotary stage must decrease with increasing radius (that is, position of the linear stage). The simultaneous motion performance was carried out using only PMAC controller without a DSP board and advanced control scheme. The linear stage must move 320 nm while the rotary stage is moving one revolution for a 25 GB Blu-ray disk. FIGURE 12 shows the experimental result that following error is lower than ±10 nm. FIGURE 13

demonstrates the comparison between command and actual velocity. Actual velocity was calculated from position of the linear stage and velocity of the rotary stage at the moment. The result showed that actual velocity is well coincided with command velocity. CONCLUSIONS

This paper describes a design and fine motion performance of a precision X-Theta stage for vacuum applications. The linear stage(X) which is designed with solid-lubricated linear motion ball bearings showed the same step response as scale resolution(5 nm). The rotary stage

(Theta) which is equipped with vacuum-compatible porous air bearings showed a vacuum level of 10

-4 Pa, and the same step

response as encoder resolution(0.162 µrad). The following error caused by simultaneous motion of X-Theta stage showed ±10 nm.

FIGURE 12. Simultaneous motion test of X-Theta stage

FIGURE 13. Comparison between command and actual velocity REFERENCES

[1] Kojima Y, et al., High Density Mastering Using Electron Beam. Japanese Journal of Applied Physics. 1998; 37: 2137-2143.

[2] Furuki M, et al., Electron Beam Recording with a Novel Differential Pumping Head Realizing more than 50 GB/Layer Capacity Disc. Japanese Journal of Applied Physics. 2003; 43: 759-763

[3] Khim G, et al., A Vacuum-Compatible Air Bearing: Design Analysis and Optimization. Key Engineering Material. 2007; 339: 37-44.

[4] Andersson J, et al., Friction of Diamond-Like Carbon Films in Different Atmospheres. Wear. 2003; 254: 1070-1075.

0 10 20 30 40 50 60

0.0

0.3

0.6

0.9 1 cnt/step

0.162 µrad/cnt

Displacement (µrad)

Time (s)

0 10 20 30 40 50 60 70 80 90-2

-1

0

1

22σ=0.054 µrad

Time (s)

Displacement (µrad)

0 500 1000 1500 2000

0.00

0.16

0.32

0.48

0.64

0 500 1000 1500 2000-20

-10

0

10

20

Linear position (mm)

Rotary position (rev)

Following Error (nm)

0 100 200 300 400 500 600 700 800499.96

499.98

500.00

500.02

500.04

500.06

Actual velocity

Command velocity

Time (s)

Linear velocity (mm/s)