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Implant Angle Control on Optima MD R. D. Rathmell, B. Vanderberg, A. M. Ray, D. E. Kamenitsa, M. Harris, and K. Wu Axcelis Technologies Inc, 108 Cherry Hill Drive, Beverly, MA 01915 Abstract. Implant angle control is increasingly important with each new device node. Some devices have demonstrated a sensitivity of threshold voltage of about 100 mV/deg for implant..

    Implant Angle Control on Optima MD

    R. D. Rathmell, B. Vanderberg, A. M. Ray, D. E. Kamenitsa, M. Harris, and K.

    Wu

    Axcelis Technologies Inc, 108 Cherry Hill Drive, Beverly, MA 01915

    Abstract. Implant angle control is increasingly important with each new device node. Some devices have demonstrated a sensitivity of threshold voltage of about 100 mV/deg for implant angle and require implant angles to be held within +/-0.2; for process control. There are many sources of angle variation in single wafer implanters. Mechanical orientation can usually be controlled to high precision, but an accurate control of the implant angle requires knowledge of the actual beam angle relative to the surface or crystal planes of the wafer. In-situ methods to measure beam angles in both the horizontal and vertical planes are required and it is necessary that these methods be calibrated to the surface or crystal planes of the wafer to achieve the required angle control. Optima MD has incorporated methods to automatically measure beam angles prior to implant in both planes, and correct for any deviation from the desired implant angle. The symmetric parallelizing lens that corrects angles without bending the beam enables a method of calibrating the horizontal angle mask to crystal planes with one or two wafers. This paper discusses the methods of measurement, calibration, and accuracy of the Optima MD angle control system.

    Keywords: ion implant, angle control

    PACS: 61.72.Tt, 85.40.Ry, 06.30.Bp

    the implanter designers to minimize their contribution INTRODUCTION to the total variation.

    Each succeeding device node has placed more

    stringent requirements on angle control, and implant SOURCES OF VARIABILITY

    angle control has become a necessary component of

    implanter process control. Early medium current All ribbon beam or ribbon-like beam implanters

    implanters used raster scanned beams to span wafers today use a magnetic or electrostatic lens to image ions

    up to 150 mm diameter with scan angles of up to +/- to infinity after they diverge in one plane from a waist

    near the focal point of the lens [1-3,5]. The focal 2; across the wafer. Channeling effects on profile

    lengths of these lenses are in the range of 600 to 1000 depth were tolerated or managed by using non-

    mm. Magnetic lenses achieve parallelism by bending channeling crystal orientations. While this was

    the beam in a wedge shaped dipole magnet through an adequate angle control for devices at that time and

    these tools are still in use today, it became clear that angle ranging from 40;-70; for the central ray.

    200 mm wafers would need better angle control. Electrostatic lenses allow the central ray to continue in

    Parallel beam implanters were introduced to avoid the a straight line, while the scanned rays have the scan

    scan angle variation [1-3]. These offered implant angle cancelled by the vector sum of velocities in the

    acceleration or deceleration gap between the shaped angle control of <1;, which was satisfactory for

    electrodes which are symmetric about the central ray. devices until recently. Device structures, which can

     For some simple effects, the focal properties of lead to asymmetry in source/drain extensions of

    these lenses can be approximated by the thin lens transistors due to shadowing of gates or narrowly

    equation. If the object point of the lens or position of spaced photoresist patterns are placing even tighter

    the scan vertex moves to one side of the focal point of limits on angle control. Devices in which Vt varies by

    the lens, the image is still focused to infinity (parallel up to 100 mV/degree have been reported requiring

    rays), but at an angle Ө, where Δx is the lateral angle control of 0.2; [4]. Such tight control requires

    displacement and F is the focal length, in-situ measurement of the implant angle in horizontal

     and vertical planes and is smaller than the present

     Ө = arctan(Δx/F), (1) tolerance on crystal cut error. However, it motivates

     adjusted independently of the beam energy, it is just a Also if the implanter focusing elements make the lens. Average angle errors are corrected by steering apparent vertex move backward or forward along the the pencil beam before the scanner and P-lens beam by Δz, the beam will be converging or diverging horizontally by two electric elements of the middle at the edge of a 150 mm radius wafer by quadrupole. For a given angle error, the required

     change in the P-lens voltage and steerer is computed in 2 φ= arctan(150*Δz/(F+F*Δz)). (2) a model based closed-loop control algorithm including

     the effects of acceleration or deceleration, and angles For small errors of 5 mm in position and F = 800 mm, are usually corrected with one iteration.

     Angle measurement devices can be calibrated to the these amount to 0.36; and 0.07; angles, respectively.

    crystal planes of the wafer to provide a proper Electrostatic lenses are designed for a fixed ratio of

    reference for channeled implants and to confirm that a lens voltage to extraction voltage and for the Optima

    zero degree implant angle is actually normal to the MD lens, a 1% error in the ratio results in a 0.05; error surface of the wafer. The axially symmetric at the edge of the 300 mm wafer. A magnetic lens is

    parallelizing P-lens of the Optima MD enables a designed for a fixed bend radius or ratio of 2?unique method for horizontal angle calibration. (ME/Q)/B. For a 50; bend angle, a 1% error in the During normal operation the P-lens is set to a voltage field B due to hysteresis, for example, can result in an that cancels the scan angle to produce parallel rays. error of just under 0.5; in the average angle. The field The horizontal beam angles of the scanned beam are must be set to achieve parallelism across the wafer, measured using a moving profiler behind a mask with however, and any residual error in the average angle vertical slots at seven points across the wafer [10]. If must be corrected by tilting the wafer about a vertical the P-lens voltage is turned off, and with zero voltage axis [6]. Charge exchange of ions that have partially on the accel column, the scanned beam travels at the completed the bend on the corrector magnet may strike extraction energy of 45 keV in straight lines from the the wafer at the wrong angle, which can also result in scanner to the wafer, as shown in Figure 1. In that dose non-uniformity [7]. case, the implant angle ranges linearly approximately Deceleration/acceleration of a beam with an angle +/- 4; across the wafer, and the beam angle is easily error magnifies/demagnifies the angle by a factor M, predicted from the geometry. When a wafer is ?implanted in this mode, at some point on the wafer the M = (E/E), (3) inoutions will have the angle that is parallel to the crystal planes and a measurement of Therma Wave or sheet where E is the energy before decel/accel and E is inoutresistance will reach a minimum. the energy after. Other effects such as beam blowup As shown in Figure 2 for a portion of the wafer, and portions of divergent beams interaction with beam angles measured across the wafer form a straight apertures also affect the final effective angle at the line plot of angle vs. position while the TW values wafer. form an approximately parabolic shape with a Vertical angle errors may occur due to minimum at the position of maximum channeling. source/extraction alignment or by vertical deflectors or focusing aberrations [8].

    HORIZONTAL ANGLE

    MEASUREMENT AND CALIBRATION

     This paper will review the methods of angle

    measurement and calibration of those methods for the

    Optima MD. The beamline layout can be seen in a

    separate paper presenting an overview of the Optima

    MD [9].

     If a recipe has a limit on maximum implant angle,

    Axcelis beam tuning software, Autotune?, automatically measures and corrects angles to satisfy the recipe limit. If angles are diverging or converging FIGURE 1. Illustration of ion rays from the scanner to the as one moves from the center toward the left/right wafer with the P-lens on or off. edges of the wafer, this is corrected by adjusting the

    voltage of the Parallelizng lens, P-lens, to make the

    lens stronger or weaker. The P-lens voltage can be

     2488 1.5 1482Mask angle at -0.8 mm = 0.05;0.5 0476

    -0.5

     -1470Therma Wave valueMask angle, degreesMinimum TW value -1.5at -0.8 mm -2464 -60-40-200204060Figure 4. An illustration of the vertical beam angle faraday Position on wafer (mm) beam and that allows accurate determination the Figure 2. A horizontal diameter scan of Therma Wave center of mass vertical angle of the beam. values compared to horizontal mask angles for an implant of The fixed offset between the VBA and the wafer 45 keV, 100 uA, B+ at a dose of 1E12 and tilt/twist of surface allows one to set the chuck tilt relative to the 0;/0;.with the P-lens off. actual beam angle to the value specified in the recipe. The vertical beam angle is measured at the start of a The corresponding position on the angle plot is +0.05; batch by tilting the chuck so that the VBA is in line for this wafer. If the wafer had no crystal-cut error, with the ion beam, as shown in Figure 5a. The chuck one would calibrate the mask to read 0; at that and VBA are tilted +/- 5; about this point to collect the position. However, to compensate for crystal-cut error data. At these tilt angles, the wafer is safely above the one typically uses two wafers implanted with 180; ion beam. Then the chuck moves to the implant difference in twist. position, as shown in Figure 5b, and scans through the An example of a horizontal angle measurement is shown in Figure 3.

     1.0

    0.5

    0.0

    Angle, deg.-0.5

    -1.0 5a -150-100-50050100150

     Position, mm Figure 3. Horizontal angles for 2 keV, 750 uA B+.

VERTICAL ANGLE MEASUREMENT

     The vertical beam angle monitor, VBA, is rigidly mounted on the arm that supports the electrostatic chuck such that there is a fixed offset between the angle of the surface of the VBA and the surface of the wafer of about 30;. The VBA is a faraday with a thick slotted mask as shown in Figure 4 that limits the 5b current transmitted to the collector as the slots in the mask are tilted relative to the beam angle. This will Figure 5. a) Scan arm at tilt angle to measure vertical beam produce a current as a function of the tilt angle of the angle. b) Scan arm at an implant position with VBA out of VBA that peaks when the slots are aligned with the the beam.

     1.E+18beam at the desired tilt angle relative to the measured tilt = 0.2 deg

    beam angle, while the VBA remains fixed to the tilt 1.E+17Centertilt = 0.5 degmechanism and is safely out of the beam. 140-3 1.E+16140-6VERTICAL BEAM ANGLE 140-9CALIBRATION 140-121.E+15Conc., at/cm3 +0.2deg

    +0.5deg As with horizontal angles, it is preferred to calibrate 1.E+14the vertical beam angle measurement to the crystal 0.00.51.01.52.02.53.0planes of a wafer as the best way to verify that a zero Depth, micronsdegree tilt is indeed perpendicular to the wafer. In this Figure 7. SIMS profiles of channeled implants of 450 keV case implants are done at a range of tilt angles with P++, 5E13 at 0;/0; in {100} wafers at 5 points compared to wafers from a single boule. The VBA is used to implants at 0.2; and 0.5; tilts. measure the beam angle, then implants are performed

     at tilts ranging +/- 1.5; and the average TW data are fit

    with a second order trendline to determine the SUMMARY

    minimum accurately. The data in Figure 6 show an offset of 0.54; between the average angle of the Optima MD has complete angle control in horizontal uncalibrated VBA and the crystal planes. Results are and vertical planes of the scanned beam. not shown, but these implants are also run with 180; Measurements are made at the start of a batch and are twist to compensate for crystal-cut error. The angle automatically corrected in the horizontal plane by offset is then zeroed so that subsequent angle adjusting the voltage of the parallelizing lens or measurements are referenced to the ideal crystal plane horizontal steerer and in the vertical plane by tilting orientation, normal to the surface of the wafer. the wafer to the specified recipe tilt as referenced to Implants are then performed with constant focal length the actual vertical beam angle. This method allows scanning ensuring the wafer has the same beam focus angles to be controlled within 0.2 degrees at most and angle content at all positions regardless of tilt. conditions. The angle measurement devices can be Using these angle control schemes, an implant of calibrated to crystal planes to ensure beam orientation 450 keV P++ was made at a tilt/twist of 0;/0; and the is actually referenced to the wafer. This calibration is implanted profile was measured on center and at a needed only once as long as the wafer chuck or radius of 140 mm at four points, 3, 6, 9, and 12 measurement devices are not moved from the o’clock. As shown in Figure 7, the profiles match well calibrated positions by a disassembly of components. with each other indicating all points of the wafer were

    implanted with ions at the same angle. Profiles from REFERENCES wafers implanted at tilts of 0.2; and 0.5; show the sensitivity of this implant to small angle errors. 1. D. W. Berrian, et al, Nucl. Instr. and Meth. B37/38 500- 503 (1989) 20001.22. A. M. Ray, J. P. Dykstra, and R. B. Simonton, Nucl. Instr.

    and Meth. B74 401-404 (1993) 190013. T. Kawai, et al, Nucl. Instr. and Meth. B96 470-473

    (1995) 18000.84. A. Agarwal, private communication (2006)

    5. N. R. White, M. Sieradzki, and A. Renau, IEEE Proc. of 17000.6th11 Intl. Conf. on Ion Implantation Tech., Austin, TX 396-

    399 (1996) 16000.4Average TW value6. J. C. Olson and A. Renau, IEEE Proc. of Intl. Conf. on Normalized VBA current15000.2Ion Implantation Tech., Alpbach, Austria 670-673 (2000)

    7. M. R. LaFontaine, et al, IEEE Proc. of Intl. Conf. on Ion 0.54;14000Implantation Tech., Alpbach, Austria 246-250 (2000) -4-3-2-1012348. D. C. Sing and M. J. Rendon, Nucl. Instr. and Meth. B237 Tilt angle, degrees318-323 (2005) 9. K. W. Wenzel, et al, “Optima MD: Single-Wafer, Mid- Dose, Hybrid-Scan Ion Implanter”, These proceedings, Figure 6. Calibration of VBA (or beam tilt angle) Marseille (2006) measurement to crystal planes using 370 keV P++, 5E13 at 10. R. D. Rathmell, et al, IEEE Proc. of Intl. Conf. on Ion near 0;/0; orientation. Implantation Tech., Kyoto, Japan 392-395 (1998)

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