Nuclear Instruments and Methods in Physics Research B37/38 (1989) 525-527 North-Holland, Amsterdam

525

SCAN COMPENSATION FOR VARIAN SERIAL IMPLANTERS

Peter FISHER

Varian /Extrion Division, Gloucester, Massachusetts 01930, USA

Implantation uniformity improvements have been realized by introducing intentional distortion, or compensation, to the waveform applied to the electrostatic beam deflection system. The analog correction is accomplished by transforming the (linear) triangle wave signal from the scan controller into a quadratic waveform, which is used to intentionally vary the distribution of dopant on the target wafer. The controls are calibrated in percent, corresponding to the percent change in dose gradient introduced in the appropriate axis. Dual end station models have independent controls for each end station, with provisions for reversing the polarity of the correction signal for zero degree end stations or research cube applications.

1. Introduction

The electrostatic deflection system used in the Varian serial implanters has been enhanced by the introduction of electronic compensation of the scan waveform which is applied to the deflection plates. The method em- ployed uses a quadratic equation to define the relation- ship between voltage and time; therefore, the time de- rivative of the waveform varies linearly with time. This causes a smooth change in the rate at which the ion beam sweeps across the surface of the wafer. The mag- nitude of this change can be selected by the user to optimize the scan waveform to the configuration of the implanter.

2. First order effects

This approach lends itself to the various geometries of implanter end stations produced at Varian. The dual end station implanters are available with platens for 7 o or 0 o implant angles, and the single end station models allow for any implant angle from 0 o to 20 O. The platen angle selection affects the implant angle in the horizon- tal axis for the dual end station models and the vertical axis for the single end station models. Thus, different arrangements for scan compensation are required in each case. Fig. 1 illustrates the relationship between the scan angle B and the position of the ion beam on the target plane. The Z axis corresponds to a O” implant, while the X axis corresponds to the 7 o case. To a first order approximation, the deflection X is directly pro- portional to the voltage V applied to the scan plates:

X = K&m tJ = K,V. (1)

This is because the tangent of the deflection angle 0 is directly proportional to the applied voltage; the con-

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stant of proportionality is dependent on the deflection plate geometry [ 1,2]. As the angle a varies from 0 ‘, the linear relationship between V and X is replaced by a nonlinear relationship between V and Z (eqs. (2) and (3)). This nonlinearity becomes more pronounced as the angle a increases [2]:

Z(cos a)

‘= K,(l - Z(sin (Y)/L) ’

z= K,V cos a + K,V(sin a)/L

For positive, nonzero values of the angle a in fig. 1, the nonlinear relationship between V and Z has the effect of slowing down the rate at which the beam sweeps across the wafer surface when the voltage V

increases at a constant rate (no compensation). This causes the dose to increase with deflection angle, pro- ducing a characteristic dose gradient in uniformity maps with the contour lines running perpendicular to the scan axis in question. This describes the situation in the horizontal axis for 0” implants in both single and dual

DEFLECTION PLATES

Fig. 1. Illustration of the geometric relationship between the deflection angle (8) and the beam position on the target

surface for implant angles of 0 o ( 2 axis) and 7 o (X axis).

V. NEW IMPLANTATION EQUIPMENT

526 P. Fisher / Scan compensation for Varian serial implanten

(a’ ;.;c x1; *Y:-“;> ~e:~g;l.

‘.___ : ,A ~ 0 = 0.40% 0 = 0.59% 0 = 0.45%

O” implant angle 7O implant angle 7O implant angle

no compensation no compensation no compensation

100 mm 150mm 150mm

0 = 0.29% Cl = 0.31%

O0 implant angle 7O implant angle

with compensation with compensation

100 mm 150mm

o = 0.25%

7O implant angle

vyith compensation 150mm

Fig. 2. Illustration of uniformity maps with and without scan compensation. All implants are done on 300XP implanters, 3 X 10“’ dose, 180 keV, As+. All maps are plotted on the Prometrix Omnimap System. b signifies the outside edge, farthest away from the

beamline centerline.

end station implanters, where an uncompensated wafer map will exhibit higher dose at the outside edge of the wafer farthest away from the centerline of the beamline (fig. 2a). The implant angle at the center of the wafer is defined as the difference between 90 o and the angle of incidence of the beam at the center of the wafer, and is equal to the offset angle 0, (where the beam strikes the wafer center) minus the angle a. For 0” implants in Varian implanters these angles are both 7O.

3. Second order effects

For 7O implants on dual end station implanters, the angle a is zero and the Z and X axes coincide. Pq. (1) predicts a linear relationship between V and X, suggest- ing that no compensation is required in this case. In practice, a characteristic pattern occurs with higher dose toward the inside edge of the wafer. This is caused by the second order effects such as the fringing fields and interaction with the suppression bias electrodes in the deflection system. The deviation from the ideal condi- tions in eq. (1) has been quantified in the literature as an additive term which increases with the deflection voltage V [3,4], which agrees with the results obtained experimentally and with computer simulations on the deflection system [5]. These effects are repeatable, thus the same compensation approach effectively eliminates

the dose gradient. The compensation is implemented by reversing the polarity of the quadratic scan waveform relative to the 0” case, causing the sweep rate of the beam across the wafer to decrease as the voltage in- creases. Polarity selection and control settings are inde- pendent for each end station, permitting optimum com- pensation for combinations of 0 o and 7 o configura- tions. As the platen angle selections on dual end station implanters affect only the horizontal axis, compensation for these systems affect only the horizontal scan wave- form.

4. Single end stations

Compensation for single end station implanters is applied in both horizontal and vertical axes. This is because the implant angle variation affects the vertical axis, while the horizontal axis requires compensation due to the offset angle described previously. The di- agram in fig. 1 can be applied to the vertical axis in these systems by positioning the wafer center on the L

axis (eliminating the offset angle) and using negative values for angle Q. In this case, --(I! corresponds di- rectly to the implant angle. No vertical compensation is used for 0 o implants; as the implant angle is increased, the increasing vertical compensation causes the sweep rate of the beam across the wafer surface to decrease as

P. Fisher / Scan compensation for Varian serial implanten 527

Fig. 3. Photograph of the beam controller with compensation for 300XP (dual correction controls for each end station.

the applied voltage increases. This corrects for the lower dose at the top edge of the wafer which would otherwise occur without compensation. Independent controls for horizontal and vertical compensation are provided for

these systems.

5. Experimental results

The wafer maps in fig. 2 illustrate the effects of compensation on uniformity [6,7]. These implants are done on the 300XP dual end station implanter using 180 keV As* at a dose of 3 x 1014, with the major flat rotated 45O from the top towards the centerline of the beamline. This recipe is chosen to minimize the effects of channeling. Scan compensation does not affect chan- neling, so care was taken to separate channeling effects out of the experiments. Note the higher dose on the inside edge of the 7O implants without compensation

(figs. 2c and 2e), whereas the 0 o implant shows a higher dose at the outside edge. With compensation applied at 2.5% from edge to edge in each case, the dose gradients are essentially leveled out with substantial improve-

ments in uniformity.

end station implanter) with independent

6. Conclusion

The application of intentional distortion, or com- pensation, to the linear output of the scan controller has been shown to yield significant uniformity improve- ments on Varian’s serial implanters. The compensation

approach works with any configuration of platen angles in the product line, accommodating the latest models as well as the installed customer base.

References

[II

121 [31

141 151

A. Zhigarev, Electon Optics and Electron-Beam Devices (Mir Publishing, Moscow, 1975) pp. 321-327. J.H. Keller, Radiat. Eff. 44 (1979) 72. 0. Kemperer and M.E. Bamett, Electron Optics (Cam- bridge University Press, London, 1971) pp. 442, 443. A. Recknagel, Z. Phys. 111 (1938) 61. J.E. Boers, SNOW Program for Simulation of Ion Beam Devices, Sandia National Laboratories Report SAND79- 1017 (1982).

161 Varian Extrion Model 300XP Ion Implanter.

171 Prometrix Omnimap System.

V. NEW IMPLANTATION EQUIPMENT