Method For The Production Of An Ion Beam Having A Large Cross-sectional Area

Abstract

A method and a device for the production of a substantially parallel ion beam having a large cross-sectional area, wherein a first double deflection of a parallel incident-ion pencil is carried out in a same plane X and a second double deflection is carried out in a perpendicular plane Y by means of at least two alternating fields having different periods, said fields being parallel to said planes X and Y and perpendicular to the direction of the incident-ion beam. This invention relates to a method and a device for the production of a substantially parallel ion beam having a large cross-sectional area. In the techniques which make use of ion beams, it is frequently useful to have available a substantial beam cross-section while retaining parallel ion paths. This is particularly true in the technique which involves doping of semiconductors by ion implantation and which consists in bombarding a crystal with a beam of heavy ions (P.sup.+, B.sup.+, Al.sup.+, Ga.sup.+, Te.sup.+, etc . . . ). The ion beam must conform to particular characteristics. On the one hand, the beam must be homogeneous and cover a large area. On the other hand, the paths of the incident ions must coincide with the crystal axis of the target with a very high degree of accuracy : by way of example, this latter is 10 to 15 minutes of arc in respect of an energy of incident ions of 180 keV. It is accordingly the object of this invention to obtain an ion beam having these different properties. To this end, the invention proposes a method of production of a substantially parallel ion beam having a large cross-sectional area, wherein a first double deflection of a parallel incident-ion pencil is carried out in a same plane X and a second double deflection is carried out in a perpendicular plane Y by means of at least two alternating fields having different periods which are parallel to said planes X and Y and perpendicular to the direction of the incident-ion pencil. In a first advantageous form, at least one of the said two double deflections contained in perpendicular planes is carried out by means of two alternating electric fields having the same period which are separated from each other by a non-deviating zone having instantaneously the same intensity and opposite directions, said period being very great with respect to the transit time during which an ion undergoes said double deflection. In a second advantageous form, at least one of the said two double deflections contained in the perpendicular planes is carried out by means of an alternating electric field having a period substantially equal to the transit time during which an ion undergoes said double deflection, the maximum radial distance r.sub.max of an ion which has been subjected to a double deflection with respect to the center of said incident-ion pencil being given by the relation , wherein V.sub.max is the difference in maximum potential established between two parallel flat plates in oppositely-facing relation which are spaced at a distance d, wherein L is the length of said plates in the direction of said incident-ion pencil and wherein V.sub.a is the voltage of acceleration of the ions in said direction. The present invention is also directed to a device for carrying out said method. Said device comprises an ion source which is capable of delivering a parallel ion pencil, two flat electrodes parallel to a plane X which are located in oppositely-facing relation and occupy symmetrical positions with respect to said incident-ion pencil, one electrode being brought to zero potential, two further flat electrodes perpendicular to said plane X which are located opposite to each other and occupy symmetrical positions with respect to said incident-ion pencil, one electrode being brought to zero potential, means for establishing a potential difference between two parallel electrodes in oppositely-facing relation. A better understanding of the invention will be gained from the following description of modes of execution of the invention which are given by way of example without any limitation being implied.

Description

Reference is made in the description to the accompanying drawings in which

FIGS. 1 and 2 represent the general diagram of two methods which make it possible to obtain a double deflection contained in a plane, and

FIG. 3 represents one advantageous mode of execution of the invention in which the two methods for obtaining a double deflection are employed in combination.

In order to obtain a uniform radial ion-density of the beam, it has been found necessary to sweep the parallel pencil of incident ions derived from an ion source along two axes located at right angles on the one hand to each other and on the other hand to the direction of the incident ions. In the particular case of the technique of ion implantation employed in the construction of integrated circuits, it is necessary to take into account the ion channeling conditions which are sought (parallel relation of the ion path with the crystal axis of the sample) and the implantation time which is desired for the construction of an integrated circuit. The solution proposed for obtaining parallel entrance and exit beams consists in producing a double deflection which is contained in a single plane. By subjecting the incident ions to this operation either in two consecutive steps or simultaneously in two perpendicular planes X and Y, sweeping of the incident-ion pencil which is thus achieved is similar to the sweep to which an electron beam is subjected in a cathode-ray tube.

In order to select the values of pencil-sweeping frequencies, the ion implantation time which is desired for the construction of an integrated circuit must first be taken into account ; it is then necessary to prevent the formation of Lissajous figures, that is to say to give a high value to the ratio f.sub.x /f.sub.y, wherein f.sub.x and f.sub.y designate the sweep frequencies along two perpendicular axes X and Y. By way of example, the following pair of values can be chosen:

f.sub.x = 10 kc/s

f.sub.y = 1 mc/s

In FIG. 1, the double deflection is obtained from two identical pairs of electrically conducting parallel flat plates located on each side of the incident-ion pencil, said pairs being spaced at a distance b from each other over which the ions do not undergo a deviation (rectilineal path - non-deviating zone). Two diagonally opposite and therefore not oppositely-facing plates are connected to the zero potential and the two other plates are brought to an alternating-current potential. The plate-polarization voltage V is represented in FIG. 1. The voltage oscillating time or period which is equal to 100 .mu.secs in this particular case is very great with respect to the time of transit .tau. of the ions within said plates. The ionized particle therefore "sees" virtually a direct-current voltage. In consequence, the angles of deviation to which an ion is subjected within the two identical pairs of plates are therefore equal but in opposite directions, thus producing parallel paths of the ion at the entrance and at the exit. The radial distance r of an ion with respect to the center of the incident-ion pencil will be greater as the plate-polarization voltage V is higher. Its maximum value r.sub.max is given by the relation :

wherein V.sub.a is the ion acceleration voltage, l is the length of the plates along the axis of the ion pencil at the entrance of the plates, d is the distance between two parallel oppositely-facing plates. For example, if l = 10 cm, b = 22 cm, V.sub.a = 30 kV, d = 15 cm, r.sub.max = 3 cm, we will have V.sub.max = 12 kV. Moreover, in order to obtain an exit beam which is symmetrical with respect to the axis of propagation, the applied voltage V must be symmetrical or, in other words, the signal rise time is equal to the fall time.

If the transit time .tau. is not negligible compared with the voltage oscillating time, it is evidently possible to introduce a time-delay in the signal which is applied to the second pair of plates encountered by the ions relative to the first pair in order that the deflecting voltages should remain identical at the moment when the ions pass at the level of each pair of plates.

In FIG. 2, the double deflection contained in a same plane is obtained by means of a single pair of electrically conducting parallel plates located opposite to each other and on each side of the incident-ion pencil. One of the plates is connected to the zero potential and the other is connected to the polarizing voltage V. This alternating-current voltage has an oscillating time or period which is equal to the time of transit .tau. of the ions within the interior of the plates. Since an ion is always deflected for a period of time which corresponds to the period of the plate-polarization signal, it follows on the one hand that the paths of the ion at the entrance and at the exit are parallel and on the other hand that the radial distance r at the exit depends on the value of the polarization voltage V at the instant at which the ion penetrates between the plates. As in the case of FIG. 1, the exit beam is radially homogeneous when the applied polarizing signal is symmetrical, that is to say when the rise time is equal to the fall time (for example a signal having the shape of a sine-wave or of an isosceles triangle).

The inventor has shown that, in a single system of plates, if a periodic polarizing signal is applied and the period T of said signal is equal to a whole number which is k times the transit time .tau. of the particle within the system of plates, the value r.sub.max of the distance from one ion to the center of the incident-ion pencil is given by the relation :

r.sub.max = (1/16) (V.sub.max)/(V.sub.a d) L.sup.2 /k

wherein V.sub.a is the ion acceleration voltage, L is the length of the plates along the axis of propagation of the ions, d is the distance between the plates and V.sub.max is the maximum value of the polarizing voltage. It will be noted that, in order to have a minimum polarizing voltage V.sub.max, the period T of the polarizing signal must be chosen equal to the transit time .tau. (k = 1).

It is clear that the two methods can be combined. Thus, a first double deflection in a plane X is carried out in accordance with the method described with reference to FIG. 1 and a second double deflection which can be carried out at the same time as the first is effected in a plane Y which is perpendicular to X in accordance with the method described with reference to FIG. 2.

In FIG. 3 which shows an advantageous mode of execution of the invention, an ion source 1 emits a parallel ion pencil 2 along an axis Z. Two identical pairs of electrically conducting parallel flat plates 3-4 and 5-6 are disposed symmetrically on each side of the beam and said pairs are spaced so as to form a non-deviating zone in a plane parallel to said plates. One pair of parallel flat plates 7 and 8 which are located in oppositely-facing relation is disposed at right angles to the plates 3, 4, 5 and 6 and extends over the same portion of the axis Z as said plates 3, 4, 5 and 6. The plates 4, 5 and 7 are connected to the zero potential and means (not shown) serve to polarize the plates 3, 6 and 8, the two first plates (namely the plates 3 and 6) being polarized in the same manner. The voltage applied to the plate 8 is sinusoidal and has a period or oscillating time which is equal to 1 .mu.sec and the maximum value V.sub.max of the voltage is 12 kV. The voltage applied to the plates 3 and 6 is periodic and has an oscillating time equal to 100 .mu.secs, the maximum voltage V.sub.max is 12 kV and the polarizing signal over one-half period has the shape of an isosceles triangle. The lengths have the following values in millimeters : l = 100, b = 230, L = 430, d = 150.

It is readily apparent that the present invention is not limited solely to the embodiment which has been illustrated and described by way of example and that the scope of this patent also extends to alternative forms of either all or part of the arrangements herein described which remain within the definition of equivalent means as well as to any applications of such arrangements.

In particular, the deflections of the beam can be carried out at X and at Y successively in two separate zones and not simultaneously in the same zone and the electrostatic deflections can be replaced by electromagnetic deflections.

Claims

What we claim is:

1. The method of increasing the cross-sectional area of a parallel ion beam, the steps of causing a first double deflection of a parallel incident ion beam in a plane X by first deflecting the ions away from the axis of the incident ion beam and then deflecting the ions back toward said axis such that the resulting ion beam is parallel to the incident ion beam, and causing a second double deflection of the ion beam in a plane Y which is perpendicular to the plane X by first deflecting the ions away from the axis of the incident ion beam and then deflecting the ions back towards said axis such that the resulting ion beam is parallel to the incident ion beam, said two double deflections being caused, respectively, by at least two alternating electric fields having different periods, said fields being parallel to said planes X and Y, respectively, and perpendicular to the direction of the incident ion beam, said double deflection in both X and Y planes producing a parallel ion beam of increased cross-sectional area from that of said incident ion beam, at least one of said two double deflections being caused by the application of alternating current potential to one electrode of two spaced electrodes between which the ion beam passes while maintaining the other of the two spaced electrodes at ground potential to thereby produce an alternating electric field having a period substantially equal to k times the transit time during which an ion undergoes said double deflection, wherein k is a whole number, and the maximum radial distance r.sub.max of an ion subjected to a double deflection with respect to the center of said incident ion beam being:

r.sub.max = ( 1/16) (V.sub.max)/(V.sub.a.d) L.sup.2 /k

wherein v.sub.max is the maximum potential difference established between said two electrodes spaced at a distance d from each other, L is the length of said electrode in the direction of said incident ion beam and V.sub.a is the ion acceleration voltage in said direction.

2. A method according to claim 1, said alternating electric fields being produced by an electric signal having a sinusoidal shape.

3. A method according to claim 1, said alternating electric fields being produced by an electric signal having the shape of an isosceles triangle in each half-period.