JPS6212624B2 - - Google Patents

Description

[Detailed Description of the Invention] (1) Field of Application of the Invention The present invention is applicable to uniform ion beam application fields such as ion implantation, ion beam deposition, ion beam processing, and ion plating. The present invention relates to a method and apparatus for performing ion beam irradiation.

(2) Prior art In the ion implantation method, the area of the sample to be irradiated is usually large compared to the cross-sectional area of the ion beam.
In order to perform implantation with good uniformity, a method is used in which the ion beam or the irradiated sample is scanned. Beam scanning methods include electromagnetic methods and mechanical methods.

However, whether or not uniform implantation can be achieved by scanning is not always guaranteed due to various factors such as scanning distortion and beam current fluctuations. For this reason, it is desirable to control scanning so that uniform irradiation can be performed even during implantation.
Attempts to control scanning have so far been made only in mechanical scanning, and the following methods are known. In other words, in a method that uses a stationary beam to scan the irradiated sample with a combination of rotational movement and parallel movement, the rotational movement is kept constant and the speed or distance of the parallel movement is determined by the beam current flowing into the irradiated sample or A method of controlling based on the integral value, and
In the method of scanning the irradiated sample by a combination of parallel movements in two directions, the speed of parallel movement or the distance of movement is controlled in the same manner as described above. In these mechanical scanning methods, since a stationary beam is used, it is easy to monitor the relative position of the beam center with respect to the irradiated sample, so the scanning control has the advantage of being relatively easy. However, since the scanning speed is slow and the number of scans cannot be increased, mechanical scanning has the disadvantage that it is not suitable for low-density implantation. Mechanical scanning also has the disadvantage of a complicated mechanism. However, in mechanical scanning, it is generally difficult to control the rotational movement where the scanning speed is the highest, and since reciprocating scanning cannot be performed, a tendency toward non-uniformity in the rotational direction may occur.

On the other hand, in the electromagnetic scanning method which has a high scanning speed and is suitable for low concentration implantation, scan control has not been conventionally performed in the field of ion implantation. The closest conventional technology to electromagnetic scanning control is probably ion beam lithography, which is similar to electron beam lithography, but ion beam lithography is currently under development and is particularly applicable to large area irradiation, such as ion implantation. However, it is still far from practical use due to the problem of scanning distortion and the high cost of the equipment.

(3) Object of the invention An object of the present invention is to provide means for controlling electromagnetic scanning in order to perform ion irradiation with good uniformity in an ion implantation method.

(4) General explanation of the invention The control principle of uniform irradiation of the present invention will be explained in a one-dimensional case using FIG. 1. Now, assume that the beam intensity distribution shape 1 is expressed by a function F(s) normalized by ∫ ^∞ _−∞ F(s) ds = 1, and the beam current intensity at time t is expressed as I(t) ,
Let ξ(t) be the coordinate of the beam center position 2 on the X axis. Here, the beam center position refers to the center of gravity of the beam intensity distribution where the left and right areas of F(s) are equal. By one-dimensional scanning in the X direction, the beam represented by the product of the above functions F and I moves along the X axis
This means that it is being scanned as represented by (t). At this time, if the coordinate of the point of interest 3 on the X-axis is x, the amount of ions irradiated to the point 3 during time T is D
(x, T) is given by D(x, T)=1/ε∫ ^T ₀ I(t)F(x−ξ(t)dt, where ε is the elementary charge.Here, if the center position of the beam The integral value of the moving distance and beam current is proportional to k, where k is the proportionality constant ξ
(t) - ξ (0) = k∫ ^t ₀ I(s) ds, and point 3 is within the central region 4 excluding the beam spread from the edge of the scanning region. , if the beam crosses region 4 only once during time T, then from the above equation ξ(t)−ξ(o)=K∫ ^t ₀ I(s) ds, I
The following relationship is obtained: (t)=1/kdξ(t)/dt. Using this relationship, D
(x, T)=1/kε∫ ^x− 〓 ^(o) _x− 〓 _(t) F(
s) ds, and in effect, x−ξ(t) becomes −∞, x−ξ(o)
can be made to +∞, respectively, so D
(x, T)=1/kε, which is constant regardless of x.
That is, uniform irradiation is achieved within region 4. It is clear that even when the beam crosses the region 4 repeatedly, uniform irradiation can be achieved by performing control to satisfy the above relationship each time it crosses the region 4.
That is, in order to scan such that the beam crosses region 4 for each charge amount Q, the scanning width is 2L and k = 2L/
Choose Q, and of course 2L includes region 4.
What should be noted here is that the beam center position ξ(0) at the irradiation start time t=0 can be arbitrarily selected. A convenient choice for control is that if scanning is not performed, that is, the beam is not electromagnetically deflected, the point on the X axis where the beam center is located is the origin of the coordinates, and ξ (0) = -L. That is. On the other hand, the beam current integral ∫I(t)dt is reset every time the value becomes Q. In this way, by controlled scanning, the beam center position starts from -L, crosses region 4 at a controlled speed, reaches L, instantaneously returns to -L, and repeats the same movement again. However, in such a scan, from L to -
There remains a concern that the return line returning to L may cause non-uniform irradiation. Therefore, the integral ∫I(t)dt of the beam current is Q
When , the integral sign is reversed and I(t) is obtained from Q.
It is arranged so that it gradually returns to 0 according to dt, and when it reaches 0, the integral sign is reversed to return to the initial state, and this is repeated. In this way, the beam center position starts from -L, reaches L, and then returns to -L again at a controlled speed, repeating a reciprocating motion. In order to achieve uniform irradiation, it goes without saying that the number of scans should be selected so that the area 4 is traversed just an integral number of times or a sufficiently large number of times. As mentioned above, by controlling according to the above-mentioned principle,
Uniform irradiation is possible regardless of scanning distortion such that the electrostatic deflection voltage and deflection distance are not accurately proportional, temporal fluctuations in the beam current, and even without detailed information regarding the beam intensity distribution shape F.

Now, according to the above principle, when scanning the irradiated sample and the ion beam having a small cross section relative to each other, in order to control the scanning to achieve uniform irradiation, it is necessary to The position of the beam relative to the sample and the amount of irradiated ions must be detected and controlled simultaneously.

In the case of a scanning method, the present invention has a function almost equivalent to that of a one-dimensional uniform resistor as shown in FIG. , a Faraday cup having a large number of divided collectors interconnected by resistors is used as a monitor, and according to the above-mentioned principle, the integral value of the amount of beam current flowing into the Faraday cup is proportional to the distance traveled by the beam center. Uniform implantation is achieved by controlling the electrostatic deflection voltage as shown in .

It will be obvious that what has been described above can easily be applied to magnetic scanning as well. In addition, to control two-dimensional scanning, the above-mentioned one-dimensional direction control may be combined in two directions, or scanning in one direction may be controlled by mechanical translation controlled by the above logic. It may also be done by scanning. In this way, even if mechanical scanning is used in one-dimensional direction, it is possible to apply it to low-density implantation as long as at least one-dimensional scanning is performed at high speed by an electromagnetic method.

(5) Examples Hereinafter, the present invention will be explained in detail with reference to examples. The structure of a Faraday cup used as a monitor is schematically shown in FIG. 2b. The opening area of the Faraday cup is 15 x 0.5 cm ² with 15 cm in the control direction y and 0.5 cm in the direction x perpendicular thereto. collector 21
is a flat metal plate 22 placed parallel to the x direction and tilted approximately 15 degrees to the beam incidence direction Z, and
They are arranged at intervals of mm and connected through a resistor 23 of approximately 1 kΩ. Ideally, the collector should be a uniform resistor, but due to deterioration due to beam irradiation or
In order to avoid deterioration of positional resolution due to secondary electron emission, the structure shown in the figure is adopted. Of course, this Faraday cup also has a secondary electron suppression electrode 24.

FIG. 3 illustrates a specific example of the structure of the monitor in more detail. Figure a is an elevational view, b is a side view, and c is a top view. A metal flat plate 32, a metal film resistor 33, and a secondary electronic control electrode 34 constituting the collector are connected to a ground potential container 35 through ceramics.
is fixed. An opening 3 is provided on the top surface of the container 35.
6 is provided.

Note that it would also be possible to use something like a channel plate or an image sensor as a monitor.

Next, an example of scanning control for uniform irradiation will be explained using FIG. 4. A beam 51 extracted from the ion source and subjected to mass analysis is adjusted by a quadrupole lens 52 to have a substantially circular cross section of about 1 cm ² . This beam is transmitted to the monitor 53 having the above structure.
Approximately 2 cm in the x direction by the scanning electrode 55 at the position of
10kHz to the width of y
It is scanned at about 100Hz in a width of about 8cm in the direction.
The current output from the monitor 53 is controlled by the circuit 56.
It is converted into a voltage corresponding to the beam center position in the direction and the integral value of the current.

A detailed embodiment of the circuit 56 will now be described using the block diagram shown in FIG.

First, _the current outputs i ₂ , i ₂ from the monitor 53 are converted into voltages corresponding to the beam center position.
Since i ₂ is a pulsating current caused by scanning in the x direction by the electrode 55 in FIG. 4, the input signal of the circuit 56 is
After converting into voltage by voltage converters 101 and 101', the waveform is shaped into a waveform having a plateau of an appropriate width by pulse stretchers 102 and 102'.
03, the divider 104 calculates i ₂ /(i ₁ + i ₂ ), and regardless of the time difference between the i ₁ and i ₂ inputs, only the stable calculated value is converted to a peak holder using the gated output generator 105. 106 to produce a voltage signal corresponding to the beam center position in a manner that minimizes the influence of scanning in the x direction. This position signal voltage is further subtracted by a constant voltage using a subtracter 107 so that the voltage signal corresponding to the beam center position when no voltage is applied to the scanning electrode 55' in FIG. is the voltage signal output by circuit 56 corresponding to the beam center position. At this time, a timing circuit is used to calculate only inputs that are above the noise level, send only stable calculated values to the peak holder 106, and further remove the influence of subsequent inputs until one calculation is completed. 108 is required. Such a timing circuit 108 is similar to one often used for signal processing of position-sensitive detectors in nuclear measurements, and is used to open and close the gates of the pulse stretcher 102 and output generator 105 shown in FIG. In addition, the pulse stretcher 102 is reset. The operation will be explained from the start of the operation. First, if I=(i ₁ +i ₂ ) is equal to or higher than the noise level and a plateau equal to or higher than a certain value is not detected, the gate of the pulse stretcher 102 is opened and division is started. At this time, the gate of output generator 105 is closed. Then,
If a plateau is detected, the gate of pulse stretcher 102 is closed. At this time, if i ₂ is above the noise level, the division value is considered stable, so after a certain period of time, the gate of the output generator is opened and the division value becomes the peak holder. While being held, pulse stretcher 102 is reset. The gate of the pulse stretcher 102 does not open until this series of operations is completed. When the series of operations described above is completed, the division circuit 104 returns to the initial state while holding the division value in the peak holder, and repeats the same operation. Note that in the timing circuit 108, 110, 110'
is the noise disk, 111 is the plateau detector,
112 is a delay output monostable FF, 113 is a reset monostable FF, and 114 is a gate monostable FF.
F, and these functions are clear from the above.

On the other hand, when converting to an integral value, the sign of the integral is inverted every time the value Q set according to the beam current is reached, and the integral value is converted into an approximately triangular wave whose frequency is approximately 100kHz. The setting value Q is selected so that 115 is an adder, 116 is an inverter, 117 is an analog integrator, 118,1
18' is a comparator, and 119 is a bistable F.F.

Next, the half of the wave height of this approximately triangular wave is
The DC voltage corresponding to Q/2 is subtracted by the subtracter 120 so that the voltage is 0V, and the voltage output by the circuit 56 corresponds to the integrated value of the current. This approximately triangular wave is amplified to a high voltage according to the energy of the ions so that the scanning width is 8 cm at the position of the monitor 3 in FIG. 2, and is applied to the scanning electrode 5.
If there were no scanning distortion, the beam center position converted into voltage form and the beam current integral value converted into voltage form should be proportional to each other, but in reality, there is a difference due to the presence of scanning distortion. arise. By controlling the voltage applied to the scanning electrode 55' by the circuit 57 so as to correct this difference, uniform beam scanning in the y direction can be achieved. A block diagram of circuit 57 is shown in FIG. Both 121 and 122 are subtractors. It is well known that the voltage applied to ideal parallel plate deflection electrodes is proportional to the scanning distance. Note that 64 is an amplifier and 65 is a triangular wave generator.

Of the rectangular beam uniformly scanned in the y direction as described above, the portion of the sample to be irradiated that is not blocked by the 1 cm wide slit 58 placed at the position of the monitor 53 in the arrangement shown in FIG. be guided effectively. The scanning in the x direction by the scanning electrode 55 is for ensuring that a portion of the beam is received by the monitor 53. The beam that has passed through the slit 58 is deflected by about 7 degrees to remove neutral beams by a parallel scanning electrode 59 bent in the x direction, and the beam is deflected by about 1 Hz to a width of about 12 cm in the x direction at the sample position.
is scanned. The scanning in the y direction by the scanning electrode 55' has a width of approximately 15 cm at the sample position. A monitor 60 having the same structure as the monitor 3 is placed at the sample position in order to control the voltage applied to the scanning electrode 59. circuits 61 and 62
has the same function as the circuits 56 and 57 and is for uniform irradiation in the x direction. thus,
A beam controlled in two dimensions, x and y, is irradiated onto an irradiated sample 63 with a diameter of approximately 10 cm, and uniform implantation is achieved with scanning distortion and beam current fluctuations corrected.

In addition, if the scanning electrode is well designed and there is almost no scanning distortion, the irradiated sample holder 81 can be used as the collector of one Faraday cup 82, as shown in FIG. By detecting only the integrated value of the current, it is possible to perform very simple uniform scanning control for correcting beam current fluctuations. That is, circuit 80
As mentioned above, the beam current integral value is x
It is only necessary to convert the signal into approximately triangular waves of two appropriate frequencies for scanning, amplify the signal, and apply the signal to the x-direction scanning electrode 55 and the y-direction scanning electrode 55'. 83 and 84 are amplifiers. A block diagram of circuit 80 is shown in FIG. The operation of this circuit can be easily understood from the explanation of FIG. 91 is a current/voltage converter;
92, 92' are inverters, 93, 93' are analog integrators, 94, 94' are subtracters, 95, 95',
96 and 96' are comparators, and 97 and 97' are bistable F.F.

Although analog control is used in the above embodiment, digital control using a small computer may be possible in some cases.

(6) Summary As explained above, according to the present invention, in the ion implantation method, beam scanning control for eliminating the influence of scanning distortion and beam current fluctuation and performing uniform irradiation can be performed in a manner that has not been previously possible. It is now possible to perform electromagnetic scanning, which has not been done before, and it is expected to improve the controllability of ion implantation as a semiconductor process technology for ion implantation in low concentration regions.

[Brief explanation of the drawing]

FIG. 1 is a diagram for explaining the principle of the present invention, FIGS. 2 and 3 are diagrams showing one configuration example of a monitor used in the present invention, and FIGS. 4 and 8 are diagrams showing embodiments of the present invention. FIG. 5, FIG. 6, FIG. 7, and FIG. 9 are diagrams showing main parts of embodiments of the present invention.

Claims (1)

[Claims] 1. Electromagnetically controlling the scanning of the beam so that the relative moving distance of the beam center with respect to the object to be irradiated and the integral value of the beam current incident on the object to be irradiated or the beam monitor placed at a desired location are in a proportional relationship. An ion beam irradiation device characterized by having a means for