ARC-BEAM POSITION MONITORING AND POSITION CONTROL IN PICVD COATING SYSTEMS

Abstract

A method to stabilize position and shape of a plasma beam established between a cathode and an anode, where an electrical field is established between the cathode and the anode and where the shortest electrical field line between the cathode and the anode defines a reference line, wherein at least one oriented electromagnetic coil is provided and the at least one oriented electromagnetic coil has its coil axis oriented in a non-colinear manner to the reference line in such a way that at least one of the straight lines which are intersecting both of the coil openings and which are parallel to the coil axis intersects with the reference line and where a current is sent through the at least one oriented electromagnetic coil in order to establish a magnetic field which is used to deflect or attract the plasma beam.

Claims

1. Method to stabilize position and shape of a plasma beam established between a cathode and an anode, where an electrical field is established between the cathode and the anode and where the shortest electrical field line between the cathode and the anode defines a reference line, characterized in that at least one oriented electromagnetic coil is provided and the at least one oriented electromagnetic coil is with its coil axis oriented in a non-colinear manner to the reference line in such a way that at least one of the straight lines which are intersecting both of the coil openings and which are parallel to the coil axis intersects with the reference line and where a current is send through the at least one oriented electromagnetic coil in order to establish a magnetic field which is used to deflect or attract the plasma beam.

2. Method according to claim 1, characterized in that the non-colinear orientation is a perpendicular orientation.

3. Method according to claim 1, characterized in that at least one mirrored electromagnetic coil is provided oriented and positioned with respect to the at least one oriented electromagnetic coil in a mirrored manner, the reference line being used as mirror axis, the oriented and the mirrored electromagnetic coils forming a first pair of electromagnetic coils.

4. Method according to claim 1, characterized in that a second pair of electromagnetic coils is provided and arranged in such a manner that their coil axis is oriented in a non-collinear manner with the reference line in such a way that at least one of the straight lines which are intersecting the coil openings and which are parallel to the coil axis intersects with the reference line and the second pair is as well oriented in a non-colinear manner with the axis of the first pair of electromagnetic coils.

5. Method according to claim 1, characterized in that orientation of the second pair of electromagnetic coils is perpendicular to the reference line.

6. Method according to claim 1, characterized in that a magnetic field is generated by a coil or by two or more coils arranged in such a way that field lines are essentially parallel to the axis from cathode to anode and the field strength is approximately homogeneous.

7. Method according to claim 1, characterized in that a set of coils producing magnetic fields Bx, By is provided, wherein the set of coils is arranged in such a way with respect to the reference line that the magnetic fields Bx, By are oriented perpendicular to an axial field Bz stabilizing the arc beam.

8. Method according to claim 1, characterized in that a static position of the arc beam is extracted from a modulated voltage signal by using a demodulation technique.

9. Method according to claim 1, characterized in that an offset position of the arc beam can be expressed in coordinates x and y by a transformation from polar to cartesian coordinates.

10. Method according to claim 1, characterized in that a curvature of arc impedance paraboloid is used for specific process conditions to determine the positioning of the arc beam.

11. Method according to claim 1, characterized in that static offset currents Isx and Isy are used for correction of the actual arc beam position.

12. Method according to claim 1, characterized in that a correction process for arc beam centering is iteratively repeated.

13. Method according to claim 10, characterized in that the curvature of arc impedance paraboloid is determined with manually testing the arc voltage for specific settings of the static offset currents Isx and Isy.

14. Method according to claim 13, characterized in that intentionally non-equal coil modulation currents Imx and Imy are chosen for determination of the curvature required for determination of the beam position.

15. An arc-beam PICVD coating system for coating parts, characterized in that the arc-beam PICVD coating system is configured to carry out the method according to claim 1.

16. Method according to claim 14, wherein uniaxial symmetry of the impedance paraboloid is assumed, the values of the curvature of arc impedance paraboloid are determined ahead of the process for each step, and the curvature required for determination of the beam position is simultaneously obtained for several steps of the determination procedure.

17. Method according to claim 5, wherein orientation of the second pair of electromagnetic coils is perpendicular to the coil axis of the first pair of electromagnetic coils.

18. Method according to claim 7, wherein the magnetic fields Bx, By are used to determine the arc beam position, wherein a determination of the arc impedance is used for determination of the arc beam position.

19. Method according to claim 1, characterized in that a static position of the arc beam is extracted from a modulated voltage signal by using a phase sensitive quadrature demodulation applied to the modulated voltage signal, wherein both an amplitude and a phase delay to an imposed field modulation is detected.

20. Method according to claim 9, wherein an amplitude provides a static offset position from a center alignment and a phase delay gives an angle direction of a misalignment of the arc beam.

21. Method according to claim 1, characterized in that static offset currents Isx and Isy are used for correction of the actual arc beam position by superimposing the static offset currents Isx and Isy to a coil modulation signal, wherein the arc beam is centered by using the estimation that Isx=?dIx and Isy=?dIy, wherein a check of the centering of the beam is indicated by a disappearance of the coil modulation signal.

22. Method according to claim 12, wherein the iteration procedure is automated as an on-line arc beam centering method which maintains the arc beam position in a centered position over time.

Description

DETAILED DESCRIPTION

[0033] A PICVD System setup according to the invention is shown in FIG. 1c.

[0034] The two key elements of remotely determining the actual beam position in the system, and adjusting the beam position within the system to compensate deviations are addressed by a set of coils which produce magnetic fields Bx, By perpendicular to the axial field Bz for stabilizing the arc beam (FIG. 1c). By using 2 pairs of coils 10a,10b arranged with their axis preferably perpendicular in their directions, a magnetic field vector (Bx,By) in any direction non-linear to the and preferably perpendicular to the arc beam can be generated. The plasma electrons of the arc beam follow the magnetic field lines in passing from cathode to anode. By applying a (small compared to Bz) horizontal field in the system, the magnetic field lines from the axial coils are deflected and consequently the beam is deflected on their path between cathode and anode in the presence of such a horizontal field. With specific settings of the coil's currents generating a horizontal field component, beam defections in any horizontal direction can be obtained.

[0035] With setting and adjusting amount and direction of preferably static currents in the x,y-coils, any undesired beam deflection can be compensated and the beam can properly be brought to a centered position. Additionally, any off center beam position (within limits) can also be achieved by intentionally using appropriate coil currents to generate a static deflection field, for example in the case of parts to be coated that are not concentrically arranged with the system axis.

[0036] The horizontal magnetic fields (Bx,By) generated by the coils can also be utilized for determining the beam position, which is the second topic to be addressed, namely an arc beam position monitoring. When the arc beam is deflected with horizontal fields, the arc impedance (=arc voltage divided by arc current) increases, since the electrons migrating along the magnetic field lines must deviate from this path when approaching the vicinity of the anode to reach the anode surface.

[0037] With a small circular aperture cathode and a conical shaped anode, the system has an axial symmetry, and the impedance of the arc beam increases by the same amount independent of the horizontal direction of its deflection. Since the impedance has a minimum when the arc beam is fully aligned with the axial magnetic field, the impedance rises in first order approximation quadratically with the beam defection.

[0038] Thus the impedance map versus beam deflection is a paraboloid with its minimum on the vertical axis defined by the conical anode tip (see FIG. 2a).

[0039] Alternative to the conical shaped anode, a cylindric hollow anode can also be used, which has a parabolic impedance dependency by its symmetry as well.

[0040] The variation of impedance with the arc beam (lateral) position can be used for determining the position of the arc beam. For the rest of this text, the arc beam impedance R.sub.arc is represented by the arc-beam voltage signal V.sub.arc=R.sub.arc.Math.I.sub.arc, assuming the arc is driven by a constant current source.

[0041] By applying phase shifted current modulations in the x- and y-coils, a horizontal magnetic field component is generated that is dynamically rotating in the horizontal plane with a specific modulation frequency fm (modulation period Tp=1/fm). In the simplest case, the coils are perpendicularly arranged in the horizontal plane and denoted by x- and y-direction as shown in FIG. 1b. The coil current modulation to the first pair of coils (x-coils) can be chosen as a sine-function, with the modulation to the is second pair (y-coils) which can be chosen as a sine-function phase shifted by 90?.

[0042] The arc beam following this magnetic field component is moving on a correspondingly circular path about its static direction. The arc voltage (or impedance) is then affected by the motion of the arc beam. In the case of any misalignment of the arc beam w.r.t to the anode axis, the arc voltage gets modulated synchronously with the moving arc beam. On the impedance paraboloid, the corresponding path is a ellipse. FIGS. 3a, 3b and 3c summarizes the situation described above.

[0043] With proper deflection of the beam with superposed preferably static currents to the coils, the arc beam can be aligned with the anode axis, resulting in a disappearance of the modulation of the arc voltage. In this case, the beam moves on a circle at a fixed impedance value on the impedance paraboloid. This level depends on the modulation amplitudes alone. This absence of modulation signal in the arc voltage is the indicator that the arc beam is well centered.

[0044] During the course of one modulation period Tp, the coating rate on parts that are closer to the arc beam in the first half of the modulation period is momentarily larger than with the undeflected beam, in the second half of the period, the coating rate is correspondingly smaller. For a total coating process lasting much longer than this modulation period, the variation of the coating rate averages to the static thickness without beam modulation. As a typical example in PICVD processes for micrometer-thickness diamond coatings lasting up to several hours, typical modulation periods may be in the range of seconds to 10-seconds.

[0045] The static position of the arc beam can be extracted from the modulated voltage signal as shown in FIG. 3c by using a demodulation technique. Specifically, with phase sensitive quadrature demodulation (also commonly called Lock-In technique) applied to the modulated voltage signal, both the amplitude and the phase delay to the imposed field modulation can be detected. FIG. 5 shows a schematic implementation of the demodulation. The amplitude provides the static offset position from the center alignment, the phase delay gives the angle direction of the misalignment of the beam. Equivalently, the offset position of the beam can be expressed in coordinates x and y by the usual transformation from polar to cartesian coordinates.

[0046] The modulation of the horizontal magnetic fields (Bx,By) by the coils is generated with the modulation of the coils currents according to

Ix=Imx.Math.cos(2.Math.?.Math.f.Math.t)

Iy=Imy.Math.sin(2.Math.?.Math.f.Math.t)

[0047] where f is the modulation frequency and Imx and Imy the corresponding amplitudes of the x-coils and y-coils, and t is the time. With the synchronous harmonic (or 1f-) demodulation signals denoted by V cos and V sin, the static correction current to the x-coil and y-coils can be calculated by:

dIx=2.Math.A.Math.(V cos.Math.cos ?+V sin.Math.sin ?)/(c2.Math.I arc.Math.Imx)

dIy=2.Math.A.Math.(V sin.Math.cos ??V cos.Math.sin ?)/(c2.Math.I arc.Math.Imy)

[0048] Where ? is a modulation hardware and frequency dependent signal system phase shift, A is a correspondingly dependent amplitude factor from signal damping, c2 is the curvature of arc impedance paraboloid for specific process conditions. For simplicity, uniaxial symmetry of the impedance paraboloid is assumed. Using a previously determined sensitivity of the beam deflection on the coil current, which can be assumed to be linear with a slope parameter p.sub.scal that is both dependent of the coil arrangement and geometry as well as on the axial field strength Bz, the geometric beam position is then

xb=p.sub.scal.Math.dIx

yb=p.sub.scal.Math.dIy

[0049] In order to correct the actual beam position, a static offset currents Isx and Isy must be superposed to the coil modulation signal

Ix=Isx+Imx.Math.cos(2.Math.?f.Math.t)

Iy=Isy+Imy.Math.sin(2.Math.?.Math.f.Math.t)

[0050] With Isx=?dIx and Isy=?dIy, the beam can be centered, which is characterized by the disappearing modulation signal of the arc voltage, as depicted in FIG. 4c. The arc beam rotates about the center position shown in FIG. 4a, the arc voltage is constant at the level given by the impedance times the arc current.

[0051] In reality, any other imposed or intrinsic fluctuations of voltage signal are suppressed in demodulation but may still result in fluctuations of the correction values dIx and dIy. The correction process for arc beam centering may be iteratively repeated according to

Isx.sub.n+1=Isx.sub.n?dIx.sub.n

Isy.sub.n+1=Isy.sub.n?dIy.sub.n

[0052] where Isx.sub.n and Isy.sub.n are previous settings of static currents applied to the x- and y-coils, and dIx.sub.n and dIy.sub.n are the correction currents obtained in the n.sup.th iteration step. With the position sensitivities of the beam to the coil currents p.sub.scal, the arc beam position is iteratively corrected according to

xc.sub.n+1=xc.sub.n?dIx.sub.n/p.sub.scal

yc.sub.n+1=yc.sub.n?dIy.sub.n/p.sub.scal

[0053] The sequences of (xc.sub.n,yc.sub.n) represent the arc beam position after each update of the beam position with corrections dIx.sub.n and dIy.sub.n.

[0054] This iteration procedure may be automated as an on-line arc beam centering method which maintains the arc beam position in a centered position over time. Those can originate form changes in process conditions during a more complex process recipe sequence of process steps which affect the arc beam position.

[0055] So far, it was assumed that the arc beam should be centered about the system axis to achieve equal coating uniformity about this axis. However, this can be considered as a special case of maintaining the arc beam at the predefined position. Adding specified offset currents Ipx and lpy to the x- and y-coils the arc beam is deflected by a fixed amount ?x=Ipx/p.sub.scal and ?y=Ipy/p.sub.scal from the position without these currents.

[0056] For an ideally centered arc beam, the arc-beam is then at position (xp,yp)=(?x,?y). The demodulation method then detects this shift in arc-beam position as new correction currents dIx and dIy, which contain the amounts Ipx and Ipy. The iteration formulas for successively correcting the arc beam position is then

Isx.sub.n+1=Isx.sub.n?(dIx.sub.n?Ipx)

Isy.sub.n+1=Isy.sub.n?(dIy.sub.n?Ipy)

[0057] For the arc-beam position, this translates into

xp.sub.n+1=xp.sub.n?(dIx.sub.n?Ipx)/p.sub.scal

yp.sub.n+1=yp.sub.n?(dIy.sub.n?Ipy)/p.sub.scal

[0058] The arc-beam then iteratively fluctuates about the desired position (xp,yp).

[0059] The curvature c2 of arc impedance paraboloid may be determined with manually testing the arc voltage for specific settings of the Isx and Isy. In more complex processes comprising of several steps with different process parameters, the shape of the paraboloid might change, and the c2 values must be determined ahead of the process for each step.

[0060] Avoiding this determination of c2 would be desired by a method that does this automatically for given process conditions.

[0061] Using the already applied modulation of the coils, the arc voltage signal is also fed into a 2f-demodulation scheme. Denoting the 2f-demodulation quadrature signals as V cos 2 and V sin 2, and still assuming the rotationally symmetric impedance paraboloid, c2 can then be obtained as

c2=8.Math.((V cos 2).sup.2+(V sin 2).sup.2).sup.0.5/(Imx.sup.2?Imy.sup.2)

[0062] Thus, for intentionally non-equally chosen coil modulation currents Imx and Imy, the curvature required for the determination of the beam position can simultaneously obtained for each step in the above described method for automatic correction of the beam position. In this case of non-equal chosen coil modulation currents Imx and Imy, the arc beam circulates on a elliptic path about the center position, as depicted in FIG. 6a. The arc voltage modulation consists of both of a 1f and a 2f component (FIG. 6b). FIG. 6c schematically shows the additional 2f-demodulation signals fed into the offset controller providing the actual value of c2.

[0063] Alternatively, the curvature of the arc impedance paraboloid can also be obtained from the arc voltage V0 without any perpendicular magnetic fields deflecting the arc beam, and the time-averaged arc voltage V.sub.avg over many periods of the modulated horizontal magnetic fields (Bx,By), e.g. the same number of periods as used for generating the demodulation V cos and V sin. With the paraboloid formula for the arc voltage dependent on the currents Ix and Iy to the horizontal coils V=V0+0.5.Math.c2.Math.(Ix.sup.2+Iy.sup.2), the time averaged voltage can be calculated for an averaging over a fixed number of modulation periods with fixed currents Isx and Isy, and the sinusoidal modulation amplitudes Imx and Imy as

c2=2.Math.(V.sub.avg?V0)/(Isx.sup.2+Isy.sup.2+0.5.Math.(Imx.sup.2+Imy.sup.2))

[0064] In this way, the necessary parameter c2 for obtaining the demodulation signals and the arc beam positions can be simultaneously obtained, which is an advantage if this parameter might change over the duration of the coating process.

[0065] The invention will now be described on the basis of a specific example.

[0066] In a typical arrangement of a PICVD system for diamond coatings like described in [1]-[5], beam deflection sensitivity w.r.t. coil currents are about 2 mm/A, as both observed from visual defection experiments with larger currents, and from field line calculations for the considered geometry. In our example the axial magnetic field Bz was 10 mT, and the horizontal field generated by the coils at the location of the arc beam was about 0.1 mT per ampere coil current.

[0067] The dependence of the arc voltage to the coil current has been observed with static deflections to be approximately c2=0.3 V/A.sup.2. With intrinsic voltage fluctuations on level of a few 10 mV from various other sources for a demodulation of voltage signal traces of 10 modulation periods, the position accuracy with a deflection sensitivity of 2 mm/A is in the range of less than 1 mm, below the level typically demanded to achieve a coating thickness deviation of less than 10%. Using longer lasting traces for the demodulation, better position accuracy could be achieved, however it takes also longer to obtain the position values.

[0068] The above described situation is a special case of arrangement and modulation-demodulation scheme. In general, the x- and y-coils do not need to be perpendicular to each other for generating a rotating horizontal magnetic field component that deflects the beam. In this case, the modulation amplitudes and the phase must be adapted and this must correctly be considered in the demodulation scheme, making the formulas for obtaining the beam center positions more complex. Additionally, the modulations signal of y-coil does not have to be in sin-function form, it can also have the more general form cos(2*pi*f*t+?), with ???. In this case, there is a 2f component of the arc voltage also with equal modulation amplitudes Imx and Imy applied on the x- and y-coils. As with the previous generalization, the formulas for the obtaining the beam positions become more complex.

[0069] For some special cases, it might be desirable not to have cos- or sin-type modulation functions. The modulation-demodulation scheme can equally applied to this case, but again, the correct formulas for detecting and correcting arc-beam center or position offsets become more complicated.

[0070] In the example as described two pairs of coils (x-coils and y-coils) were used in order to deflect the plasma beam. It should be noted however that the number of coil pairs can be increased. One possibility would be to have 3 or 4 or N pairs of coils kind of surrounding the plasma arc beam.

[0071] The generation of the magnetic field perpendicular to the arc beams can also be done with more than 2 pairs of coils. This can make sense if, for example, the arrangement of the 2 pairs has some geometric or other unwanted limitations. Then the wave functions of the currents to the coils must be adapted accordingly beyond simple trigonometric sin and cos functions.

[0072] In the system considered so far as depicted in FIG. 1a and FIG. 1b, the cathode and anode are arranged on opposite sides of the system. The invention can be extended to system configurations as shown in FIGS. 7a and 7b. The cathode is located on a side of the system rotated in its emission direction with respect to the anode located at the bottom of the system and the arc beam forms a bent column from cathode to anode. With proper choice of field strength both from coil 9a at the cathode and coil 9 at the anode defined be the corresponding coil currents, the arc beam path can be tailored and stabilized such that it extends from the cathode on a horizontal path, then bends towards the anode in a rather short path and finally goes in an almost straight vertical path to the anode.

[0073] By proper modulation of both the currents to coil 9a and coil 9, the arc beam can be deflected in x-direction forth and back. With an optional coil 9b an additional magnetic field can be generated to extend and simplify the motion of the arc beam in x-direction. The deflection of the arc beam in y-direction can be achieved with a pair of coils 10b as in the system described in previous FIGS. 1a and 1b, By synchronized modulations both of the currents to coils 9a, 9, and optional coil 9b with currents to coils 10b, a closed path of arc beam motion can be generated, ideally in an elliptic or even circular path, and the same demodulation techniques can be applied to determine the arc beam position as described above.

[0074] The system described in FIGS. 7a and 7b can additionally be extended by an additional cathode source that is located on the opposite side of the system on the same level, as shown in FIG. 8.

[0075] This might become necessary if the arc beam current should be increased beyond the limit of one cathode source. This cathode source again is equipped with a coil 9b generating the magnetic field for this source. Now, the field from coils 9a, 9b and 9 generate the magnetic field for stabilizing the arc beam, and the modulation scheme is for deflecting the must include all the currents to the corresponding coils. In such a system with more than one arc beam, mutual interactions between the arc beams must be considered because the arc beam's own magnetic fields causes an interaction with the other arc beam. Controlling multiple arc beams obviously might result in more complicated current controls to the individual coils for guiding and stabilizing the combined arc beams to the common anode.

[0076] In the same manner, the system could further be extended with additional cathode sources located on perpendicular sides of the system if even more arc beam current is required. In such a system, coils 10b for deflecting the beam in y-direction might not be used anymore, the coils of these additional sources may be used for this purpose.