Method of controlling an implanter operating in plasma immersion

Abstract

A method of controlling an implanter operating in plasma immersion, the method including the steps of: an implantation stage (1) during which the plasma AP is ignited and the substrate is negatively biased S; a neutralization stage (2) during which the plasma AP is ignited and the substrate has a positive or zero bias S applied thereto; a suppression stage (3) during which the plasma AP is extinguished; and an expulsion stage (4) for expelling negatively charged particles from the substrate and during which the plasma AP is extinguished. The method is remarkable in that the duration of the expulsion stage is longer than 5 s. The invention also provides a power supply for biasing an implanter.

Claims

1. A power supply for biasing an implanter, the power supply comprising: a first electricity generator having its positive pole connected to ground; a first switch having its first pole connected to the negative pole of said first generator, and having its second pole connected to the output terminal of the power supply; and a second switch having its first pole connected to a compensation terminal, and having its second pole connected to said output terminal; the power supply being characterized in that it includes a third switch having its first pole connected to the negative pole of an auxiliary generator, and its second pole connected to the output terminal, the positive pole of said auxiliary generator being connected to a link terminal.

2. A power supply according to claim 1, characterized in that said link terminal is connected to ground.

3. A power supply according to claim 1, characterized in that the implanter includes a recovery electrode, and said link terminal is connected to the recovery electrode.

4. A power supply according to claim 1, characterized in that said compensation terminal is connected to ground.

5. A power supply according to claim 1, characterized in that said compensation terminal is connected to the positive pole of a second generator, the negative pole of the second generator being connected to ground.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention appears below in greater detail from the following description of embodiments and implementations given by way of illustration and with reference to the accompanying figures, in which:

(2) FIG. 1 shows a high voltage power supply of the prior art;

(3) FIG. 2 shows an ion implanter provided with a control module;

(4) FIG. 3 shows a first embodiment of a high voltage electrical power supply of the invention;

(5) FIG. 4 is a timing diagram showing the method of the invention for this first embodiment;

(6) FIG. 5 shows a second embodiment of a high voltage electrical power supply of the invention; and

(7) FIG. 6 is a timing diagram showing the method of the invention for this second embodiment.

(8) Elements present in more than one of the figures are given the same references in each of them.

DETAILED DESCRIPTION OF THE INVENTION

(9) With reference to FIG. 2, an ion implanter comprises a plurality of elements arranged inside and outside a vacuum enclosure ENV. For microelectronic applications, it is recommended to use an enclosure made of aluminum alloy if it is desired to limit contamination by metallic elements such as iron, chromium, nickel, or cobalt. It is also possible to use a coating of silicon or of silicon carbide.

(10) A substrate carrier turntable PPS, in the form of a disk in a horizontal plane that is movable about its vertical axis AXT receives the substrate SUB that is to be subjected to ion implantation.

(11) In the proximity of the substrate SUB, there is a recovery electrode such as a ring that is coaxial around the substrate SUB. This electrode may also be in the form of protective liner plates LIN arranged vertically at the periphery of the substrate SUB in order to protect the enclosure ENV.

(12) The top portion of the enclosure ENV receives the body of the plasma source CS that is cylindrical about a vertical axis AXP. The body is made of quartz. It is surrounded on the outside firstly by confinement coils BOCi, BOCj, and secondly by an external radiofrequency antenna ANT. The plasma-generating gas input ING lies on the vertical axis AXP of the source body CS.

(13) This vertical axis AXP encounters the surface of the substrate carrier turntable PPS on which the substrate SUB for implanting is arranged.

(14) It is possible to use any type of pulsed plasma source: discharge; inductively coupled plasma (ICP); helicon; microwaves, arc. These sources can operate at pressure levels that are low enough for the electric field created between the high voltage turntable PPS and the grounded enclosure ENV not to ignite a discharge plasma which would disturb the pulsed operation of the source.

(15) The control module of the ion implanter essentially comprises three elements: a substrate power supply PS for supplying high voltage to the substrate SUB; a plasma power supply AP for powering the radiofrequency antenna ANT and the confinement coils BOCi, BOCj (the term power supply is used broadly herein since naturally there is one power supply for the antenna and at least one distinct power supply for the coils); and a control circuit CC for controlling these two power supplies.

(16) With reference to FIG. 3, the substrate power supply PS comprises: a first high voltage generator HT having its positive pole connected to ground; a first switch SW1 having its first pole connected to the negative pole of the first generator HT and having its second pole connected to the output terminal S of the power supply; a second switch SW2 having its first pole connected to the output terminal S, and having its second pole connected to the positive pole of a second generator GP; the second generator GP having its negative pole connected to ground; and preferably, a regulator capacitor Cr connected in parallel with the first generator HT.

(17) The output terminal S is connected to the substrate carrier turntable PPS of the implanter.

(18) The positive voltage from the second generator GP generally lies in the range 0 to 100 volts (V), and it is advantageously selected to be substantially equal to the plasma potential, which often lies in the range +10 V to +20 V.

(19) With reference to FIG. 4, the control circuit CC controls the plasma power supply AP and the two switches SW1, SW2 of the substrate power supply PS as follows.

(20) At the beginning of the cycle, the implantation stage [1] takes place: the plasma power supply AP is on; the first switch SW1 is closed; and the second switch SW2 is open.

(21) This implantation stage typically has a duration lying in the range 5 s to 100 s.

(22) Following the implantation stage, there is a utilization stage [2]: the plasma power supply AP is still on; the first switch SW1 is open; and the second switch SW2 is closed.

(23) The electrons of the plasma are attracted by the positive surface potential so neutralization takes place. It should be observed at this point that it is possible to eliminate the second generator GP, in which case the second switch SW2 is connected to ground. This neutralization stage has a typical duration lying in the range 50 s to 200 s.

(24) There follows a suppression stage [3]: the plasma power supply AP is now inhibited; the first switch SW1 remains open; and the second switch SW2 remains closed.

(25) This suppression stage has a typical duration in the range 50 s to 200 s.

(26) After this suppression stage, there follows an expulsion stage [4]: the plasma power supply AP is still inhibited; the second switch SW2 opens; and negatively charged particles are expelled by means of one or the other or both of the following two operations: closing the first switch SW1; and/or connecting the recovery electrode LIN to a positive voltage (connection not shown in the figure).

(27) The negative particles that have accumulated on the surface of the substrate SUB are expelled by the electrostatic effect. This expulsion stage has a typical duration lying in the range 5 s to 100 s.

(28) If the recovery electrode LIN is positively biased, it accumulates these particles and it can dump them once it is no longer biased, at the risk of contaminating the substrate once again. It is therefore prudent to provide a period for cleaning the electrode. By way of example, the positive bias is stopped and it is even possible that a negative bias is applied in the presence of a strong flow of gas so that the particles are evacuated via the pumping system. This cleaning operation must naturally take place outside any implantation treatment, e.g. between treating two substrates or two batches or indeed when performing maintenance on the implanter.

(29) Thereafter, there operationally follows a fifth stage referred to as the preparation stage [5]: the plasma power supply AP is still inhibited; the first switch SW1 is open; and the second switch SW2 is closed.

(30) This preparation stage has a typical duration lying in the range 50 s to 200 s.

(31) By way of example, the above method is used to implant boron by means of a BF.sub.3 plasma on a silicon substrate having a diameter of 300 millimeters (mm). The negative bias voltage HT is equal to 2 kV. The implanted dose is 10.sup.16/cm.sup.2.

(32) Under such conditions, the number of particles of diameter greater than 65 nanometers (nm) on the substrate lies in the range 1 to 8. Without using this method, the number of particles of diameter greater than 65 nm present on the substrate would lie in the range 30 to 1000.

(33) It is nevertheless possible that the plasma might be ignited by discharge during the expulsion stage [4]. This situation can be improved by modifying the substrate power supply PS somewhat.

(34) With reference to FIG. 5, the substrate power supply PS comprises: a first high voltage generator HT having its positive pole connected to ground; a first switch SW1 having its first pole connected to the negative pole of the first generator HT, and having its second pole connected to the output terminal S of this power supply; a second switch SW2 having its first pole connected to the output terminal S, and having its second pole connected to a compensation terminal P; this compensation terminal P is connected: either to ground (configuration not shown in the figure); or else to the positive pole of a second generator GP (as shown in the figure); the second generator GP has its negative pole connected to ground; a third switch SW3 has its first pole connected to the output terminal S, and has its second pole connected to the negative pole of an auxiliary generator GA; the auxiliary generator GA has its positive pole connected to a link terminal L that is connected to ground, or possibly to the recovery electrode LIN; and preferably, a regulator capacitor Cr connected in parallel with the first generator HT.

(35) The output terminal S is connected to the substrate carrier turntable PPS of the implanter.

(36) The voltage of the auxiliary generator GA is less than the voltage of the first generator HT so as to avoid ignition in the enclosure.

(37) With reference to FIG. 6, the method is then modified as follows.

(38) At the beginning of a cycle, the implantation stage [1] takes place: the plasma power supply AP is on; the first switch SW1 is closed; the second switch SW2 is open; and the third switch SW3 is open.

(39) This implantation stage has a typical duration lying in the range 5 s to 100 s.

(40) After this implantation stage, there follows a neutralization stage [2]: the plasma power supply AP is still on; the first switch SW1 is open; the second switch SW2 is closed; and the third switch SW3 is open.

(41) This neutralization stage has a typical duration lying in the range 50 s to 200 s.

(42) There follows a suppression stage [3]: the plasma power supply AP is now inhibited; the first switch SW1 remains open; the second switch SW2 remains closed; and the third switch SW3 remains open.

(43) This suppression stage has a typical duration lying in the range 50 s to 200 s.

(44) After this suppression stage, there follows an expulsion stage [4]: the plasma power supply AP is still inhibited; the first switch SW1 remains open; the second switch SW2 opens; and the third switch SW3 closes, and optionally the recovery electrode LIN is connected to a positive voltage (connection not shown in the figure), e.g. to the positive pole of the auxiliary generator GA.

(45) The substrate is once more negatively biased, but without plasma. This expulsion stage has a typical duration lying in the range 5 s to 100 s.

(46) Thereafter, there necessarily follows a fifth stage referred to as the preparation stage [5]: the plasma power supply AP is still inhibited; the first switch SW1 remains open; the second switch SW2 closes; and the third switch SW3 opens.

(47) This preparation stage has a typical duration lying in the range 50 s to 200 s.

(48) The implementations of the invention described above have been selected because they are concrete in nature. Nevertheless, it is not possible to list exhaustively all possible implementations covered by the invention. In particular, any step or any means described may be replaced by an equivalent step or by equivalent means without going beyond the ambit of the present invention.