Electron source

Abstract

The invention concerns a source supplying an adjustable energy electron beam, comprising a plasma chamber (P) consisting of an enclosure (1) having an inner surface of a first value (S1) and an extraction gate (2) having a surface of a second value (S2), the gate potential being different from that of the enclosure and adjustable. The invention is characterized in that the plasma is excited and confined in multipolar or multidipolar magnetic structures, the ratio of the second value (S2) over the first value (S1) being close to: D=1/β {square root}2πcme/mi exp (−½), wherein: β is the proportion of electrons of the plasma P, me the electron mass, and mi is the mass of positively charged ions.

Description

The present invention relates to the field of electron sources. More specifically, the present invention relates to the forming of extended electron beams intended to be injected into a plasma, under vacuum, or in any reactive gaseous atmosphere at reduced pressure.

On manufacturing of circuits in semiconductor devices, certain deposition or etch steps performed in ionized gases use electron sources. Indeed, it may for example appear to be necessary to negatively bias the surface of a substrate by bombarding it with an electron beam. An electron source may further be necessary to generate a plasma, to increase the ionization rate of a plasma, or yet to accelerate the ions of a plasma in which a substrate is placed. Further, different electron beam powers are desired to be used according to the considered application. For example, it may be necessary to have an electron beam with a power on the order of 100 eV to enable erosion (etch) of a substrate.

Electron sources of point type such as a heated emissive cathode are already available. However, such sources have the disadvantage of providing beams with a very small cross-section. The maximum possible processing surface area is thus very limited. Further, such sources can seldom be used in the considered applications, since the involved gases (plasma) risk reacting with the electrode.

Part 5.5.2 “Electron beam characteristics of miniaturized ECR plasma cathodes” of work “Electron Cyclotron Resonance Ion Sources and ECR Plasmas” by R. Geller, published by the Institute of Physics Publishing Bristol and Philadelphia (pages 352-353) describes electron sources in which electrons are extracted from a plasma. However, such sources cannot be sources of large surface area. Indeed, to be able to increase the electron extraction surface area of a plasma, the plasma volume has to be increased. Then, obtaining the electromagnetic field necessary to excite the plasma at the cyclotron resonance imposes generation of a continuous axial magnetic field of excessively high intensity. The generation of such a magnetic field, intended to confine the trajectory of the extracted electrons around the magnetic field lines imposes using particularly complex, bulky, and expensive installations. Further, the presence of a continuous axial magnetic field may be a problem in certain applications. This limits the extraction surface area to a few square millimeters. Further, electron sources of this type generate electron beams of an intensity limited to the order of one hundred milliamperes. Further, the current density exhibits an exponential-type variation along with the beam extraction power. It is thus impossible to extract on an extended surface area an electron beam of a current density which remains relatively steady when the beam extraction potential is modified. Further, said potential is at most on the order of some hundred volts.

The present invention aims at providing an electron source which exhibits at least some of the following features:

• • usable for different application fields,
  • capable of providing a beam, with as high as possible an electron current density,
  • relatively steady when the beam power is varied,
  • beam power controllable within a wide range,
  • surface area as extended as possible, for example, with a diameter from a few centimeters to a few tens of centimeters, or more,
  • free of the problems of known sources, and especially, exhibiting no axial magnetic field at the level or downstream of the extraction.

To achieve these objects, the present invention provides a source providing an electron beam of settable power, comprising a plasma chamber formed of an enclosure having an internal surface area of a first value and of an extraction grid having a surface area of a second value, the grid potential being different from that of the enclosure and being settable, characterized in that the plasma is excited and confined in multipolar or multidipolar magnetic structures, the ratio of the second value to the first value being close to the following upper limit: $D=\frac{1}{\beta }\sqrt{\frac{2\pi m_e}{m_i}}\exp \left(-1/2\right),$ where:

• • β is the electron ratio of plasma P,
  • m_e is the mass of the electron, and
  • m_i is the mass of the positively-charged ions, whereby the electron current density of the extracted beam is substantially steady when the grid-enclosure potential difference varies.

According to an embodiment of the present invention, the plasma is excited and confined by microwaves at the distributed electronic cyclotron resonance.

According to an embodiment of the present invention, the ratio between the second value and the first value is selected to be between D/10 and D.

According to an embodiment of the present invention, the ratio between the second value and the first value is selected to be between D/2 and D.

According to an embodiment of the present invention, the grid is divided into a plurality of elementary grid portions.

According to an embodiment of the present invention, various grid portions are gathered and set to distinct potentials.

According to an embodiment of the present invention, the source is followed by a post-acceleration enclosure.

According to an embodiment of the present invention, the gate is formed of parallel tubes run through by a cooling fluid.

The foregoing objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which:

FIG. 1 schematically illustrates a charged particle extraction plasma chamber;

FIG. 2 illustrates the electronic and ionic currents at the level of a surface of a chamber according to FIG. 1; and

FIG. 3 schematically illustrates an alternative electron source according to the present invention.

According to the present invention, a plasma chamber using excitation and confinement structures using multipolar or multidipolar magnetic structures at the distributed electron cyclotron resonance such as described, for example, in French patent applications No 85/08836, 93/02414, 94/13499, and 99/10291, is used as an electron source.

Such low-pressure plasma excitation devices advantageously enable increasing the surface area of extraction of an electron beam without imposing large magnetic field volumes, and while avoiding the presence of a magnetic field at the level and downstream of the extraction.

FIG. 1 schematically and partially illustrates a specific embodiment of the present invention. A confinement chamber of a plasma P is formed of an enclosure 1 with an internal surface area S₁ and of an extraction grid 2 of surface area S₂. Grid 2, isolated from enclosure 1, is biased by a voltage VB to potential V₂ with respect to this enclosure. Voltage VB is settable by a used. Electrons are desired to be extracted from plasma P. Potential V₂ of grid 2 must be greater than potential V₁ of enclosure 1, that is, voltage VB is positive (V_B=(V₂−V₁)>0). The multipolar or multidipolar magnetic excitation structures are not shown in FIG. 1. These will be, for example, structures similar to those described in the above-mentioned French patent applications.

The electron beam thus generated penetrates through grid 2 into an enclosure 3 of confinement of a processing atmosphere (gas, ionized gas, or plasma) of an element to be processed 4, such as a semiconductor substrate. The beam coming from grid 2 penetrates into enclosure 3 and enables processing and/or negatively biasing a surface S₄ of substrate 4.

FIG. 2 illustrates the theoretical variation of the currents corresponding to the particles extracted from plasma P according to the value of extraction voltage V_B. An electron current I_e is considered as being positive and a positive ion current I_i is considered as being negative. More specifically, it is known, for example from French patent application No 95/04729 issued to the CNRS, that the electron current I_e and the positive ion current I_i that can be extracted from plasma P depend on difference U=V_B−V_P, where V_P is the potential of plasma P. When U is negative and ranges between V_f and zero, electron current I_e increases according to an exponential law to a maximum saturation value I_esat. When U is positive, that is, when bias potential V_B exceeds the potential of plasma V_P, the extracted electron current remains constant at maximum saturation voltage I_esat. However, the extracted positive ion current I_i remains substantially constant as long as U is negative. When U is positive, ion current I_i becomes zero. FIG. 2 also illustrates in dotted lines real current I_r, that is, the algebraic sum of electron and positive ion currents I_e and I_i. The value of U for which electron current I_e is equal to positive ion current I_i, that is, for which real current I_r is zero, is called the floating potential V_f.

A priori, to obtain a source such as defined hereabove, it should be enough to apply a voltage V_B sufficiently high to be in the saturation field, to the right of the curve of FIG. 2. In practice, account must be taken of an adjustment with respect to one another of potential V_P of plasma P and of potentials V₁ of enclosure 1 and V₂ of grid 2. Indeed, such a self-adjustment enables guaranteeing at any time the neutrality of plasma P. To keep the neutrality principle, the positive ion current on all electrode surfaces must at any time exactly compensate for the electron current on these surfaces.

To reach saturation I_esat on grid 2, the characteristic of FIG. 2 shows that the adjustment must be performed so that potential V₂ of grid 2 is greater than potential V_P. Similarly, to respect the neutrality principle, that is, so that surface area S₁ of enclosure 1 can absorb the ion current compensating for this saturation current I_esat, potential V₁ of enclosure 1 must be smaller than potential V_f. Surface area S₁ then receives both an ionic saturation current and an electronic current, as illustrated in FIG. 2.

In steady state, the saturation electronic current towards grid 2 is given by the following relation: $I_2=I_{\mathrm{esat}}=-e\beta {\mathrm{nS}}_2\sqrt{\frac{k_B{T}_e}{2\pi m_e}},$ where:

• • −e is the electronic charge,
  • β is the electron ratio of plasma P,
  • n the ion density of plasma P,
  • S₂ the surface area of grid 2,
  • k_B is Boltzmann's constant,
  • T_e is the electron temperature in the beam, and
  • m_e is the mass of the electron.

The current collected by enclosure 1 is the sum of a positive ionic saturation current and of an electronic current and can be expressed as follows: $I_1={\mathrm{enS}}_1\sqrt{\frac{k_B{T}_e}{m_i}}\exp \left(-1/2\right)-e\beta {\mathrm{nS}}_1\sqrt{\frac{k_B{T}_e}{2\pi m_e}}\exp \left[\frac{e\left(V_1-V_P\right)}{k_B{T}_e}\right],$ where m_i is the mass of the positively-charged ions.

Then, to respect to plasma neutrality condition, there must be I₁+I₂=0, which results in the following relation: $V_P-V_1=-\frac{k_B{T}_e}{e}\ln \left[\frac{1}{\beta }\sqrt{\frac{2\pi m_e}{m_i}}\exp \left(-1/2\right)-\frac{S_2}{S_1}\right].$

Further, the automatic adjustment of the potentials must be performed so that this difference is positive. The surface area ratio must thus be such that: $\frac{S_2}{S_1}<\frac{1}{\beta }\sqrt{\frac{2\pi m_e}{m_i}}\exp \left(-1/2\right).$

To simplify the rest of the discussion, the optimal limit thus defined will be called D, that is: $D=\frac{1}{\beta }\sqrt{\frac{2\pi m_e}{m_i}}\exp \left(-1/2\right)\cong \frac{1.5}{\beta }\sqrt{\frac{m_e}{m_i}}$

This condition being fulfilled, surface area S₁ takes a potential V₁ close to floating potential V_f, potential V₂ of surface area S₂ takes a positive value with respect to plasma potential V_p and receives an electronic current density equal to the electronic saturation current density. The power of the electrons in the beam output by grid 2 then is on the order of extraction power eV_B, assuming that the thermal power of the electrons k_BT_e is negligible as compared to V_B. The beam power thus only depends on the potential difference V_B applied by the user between grid 2 and enclosure 1.

If the upper limit thus defined is exceeded, the obtained source exhibits an extended surface area, but the density of the electronic current of the extracted beam is no longer substantially steady with respect to the extraction power and varies exponentially with the extraction power.

According to the present invention, it is however possible to obtain a beam with an extended cross-section of substantially constant intensity (I_esat) and of controlled variable power (eV_B). More specifically, it is possible to vary at the output of the extraction chamber the electron power within a range from a few eV to a few hundreds of eV, instead of some ten eV with prior devices. This result is obtained without modifying the level of the extracted electronic current. Said current depends on the plasma density and may reach current densities of from a few tens to a few hundreds of mA/cm².

According to the present invention, the current is substantially steady, that is, its variation according to extraction voltage U does not have an exponential character (of type e^U), but is of type U^α, with α<½, for example according to a {square root}{square root over (U)} law.

According to the present invention, it is also possible to optimize extraction surface area S₂. Surface area S₂ is chosen so that ratio S₂/S₁ of the extraction surface area to the internal surface area of enclosure 1 of the chamber is smaller than limit D, but as close as possible thereto.

To increase the surface area S₄ that can be processed, the grid may be split as illustrated in FIG. 3, the sum of surface areas S₂₁, S₂₂, . . . , S_2n of the grid portions corresponding to above-mentioned surface area S₂. Indeed, by a beam dispersion effect, the processed surface area is greater than the extraction surface area. The openings are formed close to one another so that the beams partially cover one another at the level of element 4 as they scatter, so that surface area S₄ is continuously bombarded. All the grid portions can be biased to a same voltage V_B. It may also be chosen to gather the grid portions into assemblies biased to distinct voltages.

Another advantage of this splitting of extraction surface area S₂ is to guarantee a better cooling down thereof. Indeed, if an electron current with a relatively high density I_esat is desired to be extracted, a relatively high thermal power generation can be observed at the grid level. Now, it is easier to cool down a plurality of elementary grids than a continuous extraction grid, especially because the surface separating two elementary grids can be used as a radiator or cooled down by the flowing of a fluid. It is thus possible according to the present invention to ensure an efficient cooling down even for relatively high extraction powers while guaranteeing an extended extraction surface area. For the extraction from plasmas of very high densities, a direct cooling down of the grid will have to be provided, by forming it by means of a bundle of parallel tubes of a diameter on the order of one mm spaced apart by a distance on the order of one mm.

As a non-limiting example, if the plasma is an argon plasma having atomic number 40, β=1 and ratio S₂/S₁ must be smaller than D=1.5/(1836.40)^1/2, that is, 1/180. In a practical example, the good operation of the system in the specific case where the extraction grid had a 4-cm diameter and where the plasma chamber was a cylinder with a 20-cm height and a 25-cm diameter has been confirmed. In this case, S₂=12.5 cm² and S₁=2550 cm², ratio S₂/S₁ thus is 1/204, which fulfills the required condition. The electron current that could be extracted from the plasma was practically 0.5 ampere under an extraction voltage V₂-V₁ of 60 V.

Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the plasma may be formed from gases other than argon, for example, lighter gases such as hydrogen or helium.

Further, it will be within the abilities of those skilled in the art to complete the electron source illustrated in FIG. 1 with the appropriate devices necessary to its operation, described for example in French patent application No 99/10291. Similarly, processing enclosure 3 may be completed in any appropriate fashion. A post-acceleration grid or electrode may for example be provided between extraction grid 2 of the electron source and processing enclosure 3.

Claims

1. A source providing an electron beam of settable power, comprising a plasma chamber (P) formed of an enclosure (1) having an internal surface area of a first value (S1) and of an extraction grid (2) having a surface area of a second value (S2), the grid potential being different from that of the enclosure and being settable, characterized in that the plasma is excited and confined in multipolar or multidipolar magnetic structures, the ratio of the second value (S2) to the first value (S1) being close to the following upper limit:

2. The source of claim 1, characterized in that the plasma is excited and confined by microwaves at the distributed electron cyclotron resonance.

3. The source of claim 1, characterized in that the ratio between the second value (S2) and the first value (S1) is selected to be between D/10 and D.

4. The source of claim 1, characterized in that the ratio between the second value (S2) and the first value (S1) is selected to be between D/2 and D.

5. The source of claim 1, characterized in that the grid is divided into a plurality of elementary grid portions (S21, S22, . . . S2n).

6. The source of claim 5, characterized in that various grid portions are gathered and set to distinct potentials.

7. The source of claim 1, characterized in that it is followed by a post-acceleration enclosure.

8. The source of claim 1 or 5, characterized in that the gate is formed of parallel tubes run through by a cooling fluid.