High Precision Measurement System Based on Coplanar XY-stage

High Precision Measurement System Based on Coplanar XY-stage Kuang-Chao Fan* a,b, Jin-Wei Miao a, Wei Gong a, You-Liang Zhang a, Fang Cheng a a School of Instrument Science and Opto-electronics Engineering, Hefei University of Technology, Hefei, China, 230009; b Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan, 106 ABSTRACT A coplanar XY-stage, together with a high precise measurement system, is presented in this paper. The proposed coplanar XY-stage fully conforms to the Abbe principle. The symmetric structural design is considered to eliminate the structure deformation due to force and temperature changes. For consisting of a high precise measurement system, a linear diffraction grating interferometer(ldgi) is employed as the position feedback sensor with the resolution to 1 nm after the waveform interpolation, an ultrasonic motor HR4 is used to generate both the long stroke motion and the nano positioning on the same stage. Three modes of HR4 are used for positioning control: the AC mode in continuous motion control for the long stroke; the gate mode to drive the motor in low velocity for the short stroke; and the DC mode in which the motor works as a piezo actuator, enabling accurate positioning of a few nanometers. The stage calibration is carried out by comparing the readings of LDGI with a Renishaw laser interferometer and repeated 5 times. Experimental results show the XY-stage has achieved positioning accuracy in less than 20nm after the compensation of systematic errors, and standard deviation is within 20 nm for travels up to 20 mm. Keywords: coplanar XY-stage; linear diffraction grating interferometer (LDGI); positioning accuracy 1. INTRODUCTION With the rapid development of nanotechnology in IC, Microsystems and bioscience processes, high precise measurement system has become a key technology and challenging task 1. Precision positioning system generally consists of four precision parts: motion actuator, mechanical stage, position feedback sensor and the control circuit to ensure locating in the ideal position 2. To achieve sub-micron positioning or even to nano level, the conventional drive and transmission mode is no longer appropriate. Traditional working stage uses servo motor and precision screw drive scheme, which has a mechanical clearance, friction and the creeping phenomenon, the accuracy of positioning only can reach microns 3. Adopting flotation guideway can eliminate effect of friction, but the high cost and bulky size limit its applications. Driven by piezoelectric elements on a flexure hinge type mechanism although can access to nanopositioning accuracy, but the limited deformation of the piezoelectric element restricts the general range of motion only to the tens of microns. The newly developed XY-stage for new nano-cmm developed by Nano Measurement Lab of Hefei University of Technology employs improved coplanar structure, which can completely reduce the Abbe error both in X direction and Y directions. Each axis is driven by an ultrasonic motor HR4, from Nanomotion Co. of Israel, to generate both the long stroke motion and the nano positioning raged by a precision position feedback sensor LDGI with 1nm resolution 4. 2. HIGH PRECISION MEASUREMENT SYSTEM 2.1 The new coplanar XY-stage Conventional XY-stage is stacked up by two linear stages composing of many components, such as ball screw, bearing, linear slide, etc. The Abbe error of the lower stage is high because of the vertical distance from the table, and the components are made in micrometer accuracy ranges 5. More considerations should be taken into account when the XY stage is used for the micro/nano accuracy motion control, for example: driver error, orientation error, Abbe error, environmental error, etc2. *miaojw_2005@163.com; phone +86 13865953420; fax +86 551 2903823 Seventh International Symposium on Precision Engineering Measurements and Instrumentation, edited by Kuang-Chao Fan, Rong-Sheng Lu, Man Song, Proc. of SPIE Vol. 8321 832119 2011 SPIE CCC code: /11/$18 doi: 10.1117/12.904072 Proc. of SPIE Vol. 8321 832119-1

A new coplanar XY-stage in symmetrical geometry and push-pull motion type has been developed, as shown the CAD drawing in Fig. 1, and photo of prototype in Fig. 2. In this structure, two measurement lines of two feedback sensors (LDGI) always coincide in one point, which is the center of the stage, no matter how the XY-stage moves. The structure which fully conforms to the Abbe principle in theory can reduce the Abbe error significantly in both vertical and horizontal directions. During the installation of the XY-stage, the sensor LDGI and nano motor HR4 are mounted onto different sides of the central table, so the vibration generated by motor will not pass through the two linear slides to the grating and affect the signals of the LDGI. In addition, the center stage is made hollow frame inside that not only reduces friction of the stage motion, but also reduces the vertical pressure caused by self-weight of the table. The oil layer is filled between the base plate and the moving table as a hydrostatic bearing for better sliding lubrication. The base plate provides a common guiding plane for the XY-stage. Four linear slides on each side of the moving table guide the table in lateral motion of each axis. This is the essence of the developed push-pull type co-planar stage design. Fig 1 symmetrical push-pull coplanar XY-stage Fig 2 Photo of the co-planar XY-stage 2.2 Drive motor-hr4 In order to achieve nano positioning accuracy, this symmetrical push-pull coplanar XY-stage is driven by the attached ultrasonic motor, model HR4, made by Nanomotion Co. of Israel. The HR4 motor is composed of four piezoelectric Proc. of SPIE Vol. 8321 832119-2

ceramic plates and can perform the motion in three modes: AC, Gate and DC mode 6. Through the integration of 3 driving modes, nano positioning accuracy can be achieved for long stroke motion. In AC mode, motor HR4 generates a successive motion at the speed of 1mm/s with a Neural Network PID Controller. Then in GATE mode, motor HR4 drives the XY-stage in pulses with short steps of 20-50nm and the average speed is controlled to 25μm/s. Lastly, in DC mode, HR4 works like a conventional PZT actuator with lower speed and nano steps. 2.3 Sensor-LDGI A linear diffraction grating interferometer (LDGI) is employed as the position feedback sensor using grating pitch as the measurement scale. The fundamental principle of LDGI is to interfere the ±1 diffraction beams, with this principle, an improved LDGI design, with exact phase shift, has been developed, as shown in Fig. 3. The laser beam is split by the polarization beam splitter (PBS1), with equal intensity. Taking the left-arm beam for instance, the P-polarized beam passes through PBS1 and is converted into a right-circular polarized beam by the quarter waveplate Q1. With the emitted angle equal to the diffraction angle of the holographic gratings (1200 lines/mm), the +1 diffraction beam will go back along the same path. It is then converted into S-polarized beam after passing through Q1 again and transmits to Q3 at PBS1. It is to avoid the beam returning back to the laser diode. The right-arm beam has the similar feature. After passing through Q3 the left-arm P-beam and right-arm S-beam will be converted into right-circular and left-circular polarized beams, respectively. The NPBS divides both beams into two split beams of equal intensity. These four beams will be separated by 0-90-180-270 degrees by PBS2 and PBS3 (set fast axis to 45 degrees) and interfere with each other. Analyzed by the Jones vector, the intensity of each photodetector can be expressed as [ 1 cos ( 2 Δ t )] [ 1 + cos ( 2 Δ t )] [ 1 + sin ( 2 Δ t )] [ 1 sin ( 2 Δ t )] where Δx is the displacement of grating and d is the grating pitch 7.8.9 1 (3) 2 (4) 3 (5) 4 (6) Δ = Δ x 4 π d (7) Fig 3 improved LDGI Fig 4 Lissjous figure of the normalized waveforms Regarding signal processing with the operation of (PD1 PD3) and (PD2 PD4), two orthogonal sinusoidal signals with π/2 phase shift can be obtained. With a software-based processor, the waveforms can be easily normalized, as shown in Proc. of SPIE Vol. 8321 832119-3

Fig. 4. With pulse counting and phase subdivision techniques, the displacement can be calculated to nanometer resolution. 3. POSITIONING EXPERIMENT RENISHAW laser interferometer was used to calibrate the positioning accuracy of the XY-stage. The positioning errors can be obtained by comparing the reading of LDGI with that of RENISHAW at different positions: 5 mm, 10 mm, 15 mm, and 20 mm. Each experiment was repeated by 5 times at the same position. As the optical axis of RENISHAW laser may not be perfectly aligned with the motion axis of the stage, with a little alignment angle, it leads to cosine systematic errors significantly. And another systematic error of the grating pitches and distortion of the grating when mounted onto the linear slide will also cause higher ordered errors. So before positioning experiments, the systematic error of each point has been obtained by averaging tested error data in the same position, and fitted by a cubic spline line to obtain a systematic error compensation function, as shown in Fig. 5. Then in the positioning experiments, the positioning errors were corrected by the compensation function. Table 1 lists all the experimental data to prove the excellent accuracy of this newly XY-stage. Fig 5 compensation functions of x axis and y axis. Table 1: Positioning errors after compensating for the systematic errors X axis 5mm error(nm) 10mm error(nm) 15mm error(nm) 20mm error(nm) 1st 7-9 10 11 2nd -7 16 19-16 3rd -9-2 6-5 4th -2 8-16 12 5th -14 9-10 10 Average -5 4 2 2 σ 8 10 14 12 Y axis 5mm error(nm) 10mm error(nm) 15mm error(nm) 20mm error(nm) 1st 3-8 7 11 2nd -8 14 12-17 3rd -9-3 7-3 Proc. of SPIE Vol. 8321 832119-4

4th 12 9 4 12 5th -17 10-11 4 Average -4 5 5 1 σ 11 9 9 13 Note: σ stands for standard deviation 4. CONCLUSIONS In this paper, an improved coplanar XY-stage is presented. It is driven by the ultrasonic motor HR4, and detected by sensor LDGI on a symmetric mechanism. Positioning experiment results show that, XY-stage has achieved less 20nm positioning accuracy after compensation of systematic errors, and standard deviation within 20 nm for travels up to 20 mm. It will be integrated into a nano-cmm and used as the XY motion. REFERENCES [1] J GER Gerd, GRNWALD Rainer, MANSKE Eberhard, HAUSOTTE Tino, F βl Roland, A Nanopositioning and Nanomeasuring Machine: Operation-Measured Results, Nanotechnology and Precision Engineering, Vol.6, No.2, 81-84(2004) [2] Katerina Moloni, Ensuring Nano Positioning Accuracy Requires Sensor Monitoring and Careful XY stage Design, SPIE OE magazine, 40-41(2002) [3] Weili Wang, Yetai Fei, Kuangchao Fan, Investigation of Nanometer XY Positioning Stage, Nano/Micro Engineered and Molecular Systems, 2006. NEMS '06. 1st IEEE International Conference on, 320-324(2006) [4] Kuang-Chao Fan, Fang Cheng, Ye-Jin Chen, Nanopositioning Control on a Commercial Linear Stage by Software Error Correction, Nanotechnology and Precision Engineering, Vol.1, No.4, 1-9(2006) [5] Kuang-Chao Fan, Ye-Tai Fei, Xiao-Fen Yu, Wei-Li Wang, Ye-Jin Chen, Study of a noncontact type micro- CMM with arch-bridge and nanopositioning stages, Robotics and Computer-Integrated Manufacturing 23, 276 284(2007) [6] Cheng Fang, Kuang-Chao Fan, Ye-Tai Fei, Measurement of Pre-Travel Distance of a Touch Probe for Nano- CMM, Nanotechnology and Precision Engineering, Vol.8, No.2,107-112(2010) [7] XIA Hao-jie, FEI Ye-tai, FAN Guang-zhao, CHENG Fang, 2D Nano-Displacement Measurement with Diffraction Grating, Nanotechnology and Precision Engineering, Vol.12, No.4, 311-314(2007) [8] Liu Yu sheng, Fan Guang zhao, Chen Ye jin, A Research on Diffraction Grating Interferometer With High Accuracy, Industrial Measurement, Vol.16, No.2, 1-3(2006) [9] Kuang-Chao Fan, Zi-Fa Lai, Peitsung Wu, Yung-Chang Chen, Yejin Chen and Gerd J ager, A displacement spindle in a micro/nano level, Meas. Sci. Technol. 18, 1710 1717(2007) Proc. of SPIE Vol. 8321 832119-5