DEVICE OF TREATMENT Of a SAMPLE PER IMPULSE ELECTRONIC BEAM

The present invention relates to a device for processing a sample with electron beam pulse. The invention is particularly useful in the treatment of surface layers of the semiconductor materials.

Indeed, when the is implanted for example in a semiconductor substrate, by bombardment thereof, foreign particles or impurities, it is known that it is desired to perform, after implant, a step known as the word annealing for rendering electrically active the implanted impurities and for re-shaping the crystal lattice of the substrate which has been disturbed upon bombardment.

One of the techniques most used to make the annealing step is to an elevated temperature (of the order of 900 to 1200 °C) the implanted substrate for a certain time.

Another technique, latest, which consists in bringing to the surface, or at the first implanted layers, a high energy density, for a very short time, so that very high temperatures are reached locally during this time. In some cases, the temperatures reached liquefy the first layers of the substrate, for curing damage created during implantation.

Since the temperature rise is very localized and of short duration (less than millisecond), the remainder of the substrate is not affected.

The input of energy to the surface or on the first layers of the substrate can be made, either by using light energy to the firing means, for example a laser or a flash tube, or using an intense beam of particles such as electrons. The light beams or particles can be narrow or broad beams or non-pulsed, and can scan the substrate to "annealing" thereof at different points.

Known devices to process samples, with an electron beam and intense pulsed, comprise, in general a diode field emission plasma and known as the Anglo-Saxon expression of "field-plasma emitting diode". Such a device has been represented in Figure 1.

The device thus comprises, a diode cathode 2, generally graphite and provided with a plurality of grooves, and an anode 4 consisting of an anode itself 4a and 4b of a grid, these two elements 4a and 4b being carried at the same potential. The diode is generally placed in a vacuum chamber 6.

Furthermore, these devices include a high voltage generator 8, connected to an energy storage system under high voltage may be made 10 example of a coaxial line or a capacitor, and a system 12 pulse triggering, connected to a switch 14 for example of the type known as spark gap the Anglo-Saxon expression of "Spark gap switch". The devices include, further, a supply system 16 for presenting the sample to be processed in the vacuum chamber 18 6, 20 and systems for measuring the voltage and current supplied by the generator 6 over time. The anode itself 4a supports the sample 18.

Such devices have been described, for example in a U.S. Patent 3,950 187 № of Kirkpatrick listing "Method and apparatus involving pulsed-electron-beam processing of semiconductor devices" and in an article of "Journal of Applied Physics", flight. 50, no. 2 of February 1979 entitled "Pulsed-electron-beam annealing of ion implantation Stamping".

In these devices, the application of a large voltage between the gate 4b of the anode and the cathode 2 by means of for example a capacitor 10, that was previously charged by the generator 8, creates an intense electric field in the vicinity of the cathode of the diode 2. The application of the charging voltage of the capacitor 10 is provided by the spark gap 14 controlled by the system pulse triggering 12.

The electric field generated is intensified the vicinity of the cathode by the presence of microprotrusions, known as English word of "whiskers", from the grooves (not shown) which is provided on the cathode. S microprotrusions The create the electric field emission. The electrical power and the provided is such that there will be an explosion of the microprotrusions corresponding to an explosive vaporization and ionization thereof. The microboules plasma thus formed become in turn an electron source promoting the rapid rise in the current which promotes itself the explosion. In few nanoseconds, each of these plasma microboules extends sufficiently so that the cathode is covered with a continuous plasma sheath. The sheath sees, by relaxing effect, increasing the thickness thereof until it reaches the lattice of the anode, producing shortcircuit of the diode known as the Anglo-Saxon expression of "diode-closure".

The capacitor 10 charged by the generator 8, continuous to be discharged into the diode. The voltage applied between anode and cathode by the capacitor has permitted, followed by the short circuit, extract and accelerate the electrons created to form an electron beam and intense pulsed may be used, for example, annealing the sample 18.

By use in such a device an anode itself 4a for supporting the sample 18, leading thus to positively biasing said sample, allows electrons to readily penetrate therein, while the ions formed can not penetrate in the sample, which is automatically pressed. Well is obtained, and, an electron beam and intense pulsed.

Therefore, the high voltage generator 8, via the capacitor 10, serves both to the creation of the plasma sheath that will be the source of electrons and the extraction and acceleration thereof.

Such arrangements can create electron beams having energy between 10 and 50 kilo-electron Volts which are (KeV), whose intensity is 100 to a few thousand amperes per square centimeter and whose pulse duration is a few tens of nanoseconds to a few microseconds.

For more details on the operating principle of such devices, reference is made to an article of the "Journal of Applied Physics",. 45 flight, of June 1974 No.6 listing "Plasma-induced field emission and the identifying of high-current wave scattering electron flow".

The devices, simple design have a number of disadvantages. These drawbacks are, in particular:

-the need for generating a high voltage abruptly in the space between the anode and the cathode in conditions such that 1' intensity is to be very important, i.e. between 100 and 10,000 amperes;

the characteristics of the electron beam, i.e. its energy and current density cannot be selected independently of the conditions wherein the plasma is generated. For example, if it is desired to obtain an electron beam of low energy, i.e. of the order of 20 KeV, it is not obvious that the corresponding electric field sufficient to generate the plasma sheath;

-a system with very high power to generate the plasma, while that which does not require.

Just The invention relates to a device for processing a sample with electron beam pulse capable of solving these various disadvantages.

The device of the invention, using the same principles as described above to separate the two following functions:

creating the plasma sheath, and

-extracting and accelerating the electrons created.

More specifically, the invention relates to a device for processing a sample with electron beam of the type previously described, but in which the vacuum tube comprises three independent electrodes. The device is characterized in that it comprises, further:

A)-A first circuit to create impulse a plasma from the cathode of the vacuum tube, the first circuit comprising:

a)-a capacitor of capacitance charged by means of the continuous high voltage generator, said capacitor being provided with first and second armatures,

b)-a spark gap for initiating the discharge of the capacitor of capacitance therein so as to produce a pulse of high voltage between the cathode and the grid, said spark gap being provided in series with said capacitor, and

c)-means for generating pulses controlling the spark gap; and

B)-A second circuit to create between the anode and the cathode an electric field for accelerating and extract the electrons produced in such a manner that they reach bombarding the sample, the second circuit comprising a capacitor of capacitance C ^ much greater than the capacitance C ^, the capacitor capacitance Cj is charged by means of the continuous high voltage generator and can be discharged through the vacuum tube itself.

According to a preferred embodiment of the invention, the first circuit comprises, further, two resistors R ^ and R2, which is in series with the capacitor capacity C ^, so that one of the terminals of the resistance R ^ is connected to the first frame of the said capacitor and that one of the terminals of the resistance R2 is connected to the second armature of said capacitor, the spark gap being, then, connected to the first capacitor plate capacity and also to the other terminal of the resistor R2 and it also comprises the capacity capacitor C2 connected in parallel with the circuit Rj ^ C₁ R₂ "

In this embodiment, and due to the high value resistors and R2 with respect to the impedance of the rest of the first circuit, the voltage applied between cathode and gate at the time of the spark gap is triggered is the sum of the voltages stored in the capacitors and C capacity is twice the voltage of the DC voltage generator.

Other features and advantages of the invention shall become apparent better still from the description which will follow, exemplary explanatory but in no way limiting, with reference to the drawings appended in which:

-figure 1, already described, schematically shows a sample processing device of the prior art, and

-figure 2 shows schematically a sample processing device according to the invention.

The device of the invention, schematized in Figure 2, comprises among other things a discharge tube comprising, as previously, a cathode 22 provided with a plurality of grooves (not shown) and an anode 24 consisting of an anode itself 24a and 24b of a grid. The tube enables the production of a beam of electrons and capable of intense pulsed which a sample 26 over the anode proper 24a and supported. The use of the anode itself 24a as sample support is used for positively biasing thereof, resulting in the penetration of the electrons in said sample and the distance of the ions also formed. The electrodes and sample are placed in a vacuum chamber as 28. Means 30 are provided for supplying to the interior of the enclosure 28 the sample 26 to be treated.

The device includes, also, a continuous high voltage generator 32 for charging at a voltage V a capacitor of capacitance and a capacitor of capacitance C2 connected in parallel. The capabilities and C2 are such that the capacitance C2 is much greater than the capacitance C ^, i.e. that the capacitance C2 is 10 to 50 times as large as a capacity C ^.

The capacitor capacitance is, further, connected in series with two resistors R and R2 ^ identical located on either side of said capacitor. Indeed, the resistance R^a one of its terminals connected to ground and the other terminal to one of the armatures of the capacitor capacitance C ^, and the resistance R₂ has one of its terminals connected to the positive terminal of the high voltage generator 32 and the other terminal to the other reinforcement said capacitor.

Furthermore, the group consisting of the two resistors R ^ and R2 and the capacitor capacity

can be connected in series with a resistor r and the capacity capacitor C2 can be connected in series with a resistor R. The resistance R is much greater than the resistance r, it will be shown below in an exemplary embodiment.

The device comprises, further, a spark gap 34 controlled by a pulse generator 36. The spark gap 34 is connected to the positive terminal of the high voltage generator 32 through the resistor R and on the other hand, to the frame of the capacitor capacitance C connected to the resistor R ^ ^.

In the device of the invention, the cathode 22 of the diode is connected to ground, the gate 24b of the anode is connected to the terminal of the resistor R2 which is connected to one of the armatures of the capacitor capacitance C ^, and the anode itself 24a to the other terminal of the resistor R2 through the resistor r.

The circuit mainly comprising the capacity capacitor C ^, the capacity capacitor C2 and the spark gap 34, controlled by the pulse generator 36, is a low-power circuit (C ^ being less than C2) for generating a plasma by creating a vaporization and ionization the vicinity of the cathode 22. The plasma is generated by applying to the terminals of the grid-to-cathode space of the sum of the voltages stored in the capacitors of capacities and under the control of the spark gap 34.

The circuit mainly comprising the capacity capacitor C £is an high power for the extraction and acceleration electrons generated by creating a strong electric field between the anode per se and 24a the cathode 22. The circuit, unlike those of the prior art devices, does not include switch or spark gap, thereby avoiding problems connected with the snap closing of the high power circuit.

The device of the invention has another advantage of its independence of the two functions, i.e. the creation of the plasma and on the other hand, the extraction and acceleration electrons. This to select the extraction voltage acceleration and without affecting the creation of the plasma, the power required is a function of the voltage supplied by the high voltage generator 32 and the value of a capacitance C2 of the corresponding capacitor.

As is the has already said, the operation of the device of the invention is equal to that of the prior art, if not in that the circuit does not include high power switch. Furthermore, the use of the two identical resistor R and R^₂ of very high impedance with respect to those of the spark gap 34 and capabilities and C₂ provides, when the pulse generator 36 triggers the spark gap 34 causing the discharge of the capacitor of capacitance C ^, a pulse between the gate 24b of the anode and the cathode of the diode 22, whose voltage is twice that provided by the high voltage generator 32. This is linked to the assembly of different components constituting the low-power circuit.

One will maintaining an example and embodiments examples sample processing.

The generator high-voltage charge the capacitors and capabilities C₂ 1 nanofarads and 40 respectively at a voltage of 30 kilovolts. The resistors R and r are respectively two resistors 30 Mégohms (Ma) and 2a and the resistors R and R^₂ resistors 300 Ma.

Tests sample processing have been made on silicon wafers on a surface area of about 2.5 cm of diameter by means of a diode, the cathode, of circular shape, had a diameter of 2.5 cm, whose distance between the cathode and the grid was 4 to 6 mm and of which surface between the gate and the anode actual 0 to 3.5 cm.

The energy delivered for annealing the wafers can be controlled, by means of the acceleration voltage, itself controlled, and by means of the distance between the anode and the gate. This distance also allows control of the duration of the pulse. Furthermore, the supplied energy depends on the nature of the cathode. Indeed, the nature of the cathode to control the nature of the plasma, thus the rate of expansion, thus the duration of the pulses and thus of the input energy.

The test has been carried out by means of graphite cathodes, aluminum, antimony and silicon, respectively corresponding to a expansion rate/received 3 cm, 2 cm of/received, of 1 cm and 2 cm/received/ps.

Examples sample processing described above provide a sample annealing the pre-ion implantation, but of course any other type of treatment can be contemplated.

On the other hand can be, without departing from the scope of the invention, provide different schemes of that which has been described; for example the trigger circuit adapted to create the plasma between cathode and gate, low power circuit, and the acceleration electrons to the anode, high power circuit, can be independent, high-voltage generators include distinct and/or separate switches.