RU2353017C1 - Low-energy ion beams source for nano electronics technologies - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: invention relates to ion-beam engineering and may be used as a key element of the existing and brand-new industrial technologies of nano electronics. Low energy ion beam source includes supersonic de Laval nozzle with tube on its axis. The rod-like (or wire-like) target is supplied to the supersonic expanding part where laser beam focused to the target end evaporates and ionises the target material. The specific feature of the claimed source is an electro magnetic ion cone, installed on nozzle axis and behind outlet shearing. The electro magnetic ion cone is intended for generating and focusing ion beam and for cleaning from the blanketing gas leaking from the nozzle by pumping.

EFFECT: positive effect is achieved due to source operation and provides for increasing uninterrupted source operation period, decreasing target material losses, consumption of blanketing gas and source dimensions and decreasing cost.

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Description

The invention relates to devices for producing and managing ion beams and can be used as a key element of both existing and new industrial nanoelectronics technologies such as, for example, resistive nanolithography, implantation, dimensional doping, ion-chemical dry etching, ion microscopy, elimination of defects in lithographic masks, obtaining thin films with unique new properties (ion-beam epitaxy technique), micro- and nanomechanical processing of thin films sub ikronnoy thickness (so-called "milling" ion beams), direct "drawing" chips on a substrate with control of the computer.

A technique is known in which ions are obtained by irradiating a target substance with a focused laser beam (the so-called laser ablation). Figure 1 presents a diagram of an ion source device using laser ablation, proposed by the 1990 Nobel laureate in chemistry R.Smalley [1]. This ion beam source consists of a supersonic nozzle 1; target 3, made in the form of a rod (or wire) supplied to the source perpendicular to the plane of figure 1; a pulse valve 9 for supplying a buffer carrier gas 4; a laser beam 5 passing into an ion source through an input window 10; skimmer 11; two stages of vacuum pumping 7 and 8.

This ion source works as follows. Buffer carrier gas 4 (argon or helium) of high purity through the pulse valve 9, the opening of which is synchronized with the laser pulse, enters the subsonic part of the nozzle 1, where it blows the target 3. The focused laser beam 5 causes the evaporation and ionization of the target material 3. Received Thus, ions with energies in the region of several hundred eV (the energy spectrum of ions during laser ablation is determined by the density of the laser radiation flux per pulse) slows down in the carrier gas to thermal velocities due to okratnyh ion-atom collisions. Since a buffer gas of high purity is used, the neutralization of ions in this gas does not occur, because the ionization potentials of atoms of noble gases (argon, helium) are higher than the ionization potentials of atoms of almost all possible target substances. Then the slow ions are picked up by the gas stream and carried out from the nozzle 1 by a supersonic gas stream. Note that the supersonic flow of the buffer gas from the nozzle 1 is ensured by vacuum pumping 7. Skimmer 11, which is a conical diaphragm with a sharp edge of the inlet, cuts out its axial part from the supersonic jet along with the ions thermalized in it. Thus, the ions passing through the skimmer 11, in the form of a beam, are removed from the source under high vacuum conditions, which is ensured by pumping 8.

The disadvantage of the described ion source [1] is the low efficiency of the use of the target substance, since, firstly, a significant part of the ions formed by laser ablation is lost on the nozzle walls when they are slowed down in the buffer carrier gas, and, secondly, even for ideally When a skimmer is manufactured and optimally installed, no more than 1% of the ions removed by the supersonic jet from the nozzle can pass through the skimmer in the form of an ion beam (this directly follows from the fact that the skimmer cuts out only a small axial part of it from the jet).

Another disadvantage of the ion beam source shown schematically in FIG. 1 is its limited life. The fact is that during the operation of this source, part of the target substance 3 vaporized by the laser beam 5 inevitably precipitates on the surface of the input window 10, which over time leads to a loss of its transparency for the laser beam 5. A significant part of the ions and neutral atoms vaporized by the laser beam from the target 3 and then taken out by a supersonic jet from the nozzle 1, is deposited on the outer surface of the skimmer 11, which gradually leads to the blunting of the sharp edge of its inlet and even its “overgrowing”. As a result of these negative processes of deposition of the target substance, the installation of the ion source exits the normal and full functioning mode, and to continue operation it is necessary to stop it in order to replace and / or clean (when possible) the input window 10 and the skimmer 11. In addition, the disadvantage of this ion source [1] is also the need to use a sufficiently powerful vacuum pumping 7 to achieve the necessary parameters of a supersonic buffer gas flow in front of the inlet of the skimme ra 11, which leads to an increase in the size and cost of the installation.

The closest technical solution (prototype) to the proposed invention is the source of ion beams [2], a structural diagram of which is presented in figure 2.

This ion source consists of an axially symmetric Laval nozzle 1 having an axis 2 tube passing through a subsonic tapering part of the nozzle 1 into the region of supersonic expansion of the buffer carrier gas (expanding part of the nozzle) and target 3 in the form of a rod (or wire) supplied into the nozzle through the tube 2 (see figure 2). The buffer carrier gas 4 is supplied to the nozzle 1 through an opening in the wall of the subsonic part of the nozzle, and the laser beam 5 is focused on the end face of the target rod 3 protruding from its supporting tube 2.

This ion source works as follows. When a buffer carrier gas (for example, hydrogen, helium, neon, nitrogen, argon, krypton or xenon) flows out through a nozzle 1, a supersonic gas stream is formed through the nozzle 1, which flows around the tube 2 and closes behind the end face of the target 3. Laser interaction beam 5, focused on the end face of the target 3, causes the evaporation and ionization of the target substance, similar to how it happens in the above ion source [1]. The ions formed during laser ablation slow down in collisions with atoms of a buffer carrier gas, they are thermalized with a carrier gas, and as a result, an ion beam is formed that has a velocity and temperature (characterizing the velocity dispersion of ions) equal to the velocity and static temperature of a supersonic jet, respectively. Since the description of the invention of the ion source [2] does not contain any indication of the device by which the ion beam is removed from the gas jet under high vacuum conditions, we are entitled to assume that the authors intended to use a skimmer for this purpose, because the use of skimmers for The formation of molecular and ion beams from supersonic gas flows was at that time, and this was 1987, a common practice. Note that one of the authors of the described ion beam source [2] is also the author of the invention.

The ion source [2] has common drawbacks with the ion source [1] described above: it is the low efficiency of using the target material, the limited life of the source (due to the overgrowth of the skimmer inlet by the deposited target material) and the need to use powerful vacuum pumping for supersonic jets of buffer carrier gas. In addition, the disadvantage of the ion source [2] is the need for a relatively large consumption of expensive buffer gas of high purity. Recall that in the source [1] the decrease in the average flow rate of the buffer gas is achieved by using the pulse mode of operation of the valve 11 (see figure 1).

The aim of the present invention is to reduce the loss of target material, the consumption of a buffer carrier gas, the dimensions of the source installation while reducing its cost, as well as increasing the service life.

This goal is achieved by the fact that in a source of ion beams containing a Laval supersonic nozzle with a tube mounted on its axis, through which a target in the form of a rod (or wire) is fed into the supersonic part of the nozzle, and having a laser beam focused on the target end, which performs evaporation and ionization of the target substance, it is additionally equipped with an electromagnetic ion funnel mounted on the axis of the nozzle behind its outlet slice to clean the ion beam from the bulk of the carrier gas flowing out of the nozzle, e The focus and output in a high-vacuum conditions.

Figure 3 presents a structural diagram explaining the operation of the proposed source of low-energy ion beams for nanoelectronics technology. As well as the ion source described above [2] (see FIG. 2), the proposed source contains an axially symmetric Laval nozzle 1 having an axis 2 tube passing through the subsonic tapering part of the nozzle 1 into the region of supersonic expansion of the carrier gas ( the expanding part of the nozzle) and the target 3 in the form of a rod (or wire) fed into the jet of carrier gas through the tube 2 in the nozzle 1. The buffer carrier gas 4 also enters the nozzle 1 through an opening in the wall of the subsonic part of the nozzle, and the laser beam 5 focuses on the end of the target rod 3, you stepping from the tube supporting it 2. Additional in comparison with the prototype (see figure 2), the structural elements of the proposed source are an electromagnetic ion funnel 6 mounted on the axis of the nozzle behind its outlet slice, and an additional stage of vacuum pumping 8. The electromagnetic ion funnel 6 consists their stacks of thin ring metal electrodes with decreasing diameters of the central holes in the direction of the jet. A radio frequency electrical voltage is applied to the ring electrodes of the funnel in such a way that the adjacent electrodes are in antiphase. To wire the laser beam 5 to the target 3 through the body of the electromagnetic funnel 6 in some ring electrodes there are corresponding holes, as shown in Fig.3. The last few ring electrodes of the funnel 6 are located in the high-vacuum pumping region 8 (see Fig. 3), and additional constant voltage voltages can be applied to these electrodes, creating a small ion-accelerating electric field inside this part of the funnel 6.

The proposed source of low-energy ions works as follows. The ions produced as a result of the ionization of the target material 3 by the laser beam 5 are slowed down and cooled in collisions with the neutral atoms of the carrier gas to the velocity and static temperature of the gas jet, respectively. Upon exit from the nozzle 1, the ions are transported by the gas stream inside the electromagnetic ionic funnel 6. The radio-frequency voltage applied to the electrodes of the funnel 6 creates an effective electric field inside the funnel 6 that repels the ions to the axis and holds them inside the funnel 6. At the same time, the neutral buffer The carrier gas freely flows from the funnel 6 through the gaps between the ring electrodes due to vacuum pumping 7. As a result, the ion beam purified from the bulk of the carrier gas and focused by an electromagnetic funnel 6 It falls to under high vacuum 8, which is used to maintain a small vacuum pump performance sufficiently. When passing from the pumping area 7 to the high-vacuum area 8, the pressure of the buffer gas inside the funnel 6 decreases towards its outlet. Thus, in the region of the last ring electrodes of the funnel 6 located in the pumping region 8, the mean free path of the ions in the residual buffer gas becomes larger than the diameters of the central holes in the electrodes of the funnel 6, as well as the distances between adjacent electrodes. Therefore, in order to improve the conditions for the transport of ions through this end part of the funnel 6 to the electrodes located in the high-vacuum part of the pumping unit 8, additional constant voltages can be applied, creating along the axis of this part of the funnel 6 a small constant ion-accelerating electric field (for example, as shown below the results of computer experiments, an electric field of 0.75 V / cm is sufficient for efficient extraction of the ion beam from the source using different buffer gases lei and for ions of different masses).

It is such an previously unknown original combination of a gas-dynamic ion source [2] (prototype) with an electromagnetic ion funnel 6 (see FIG. 3) that makes it possible to achieve the stated goal. The fact that the effective electric field created by the radio frequency voltage applied to the ring electrodes of the funnel 6 repels the ions to the axis of the funnel 6, it allows practically without loss to remove from the source all the ions obtained by irradiating the target material with the laser beam 5 3. Ring electrodes of the electromagnetic funnel 6 interfere with the deposition of target material 3 sputtered by the laser beam 5 on the surface of the optics focusing the laser beam (optics not shown in Fig. 3), which significantly increases the continuous operation time th source of ions. Moreover, in the design of the proposed source there is no skimmer, due to which the normal operation of the ion source could also be disrupted due to the deposition of target material 3 on its surface.

Since, unlike the operating conditions of the ion source [2] (prototype), here, when transporting ions through an electromagnetic funnel 6, it is not necessary to maintain the supersonic flow regime of the buffer carrier gas, the optimal operating conditions of the proposed source of low-energy ions are provided at significantly lower buffer costs carrier gas through nozzle 1. In particular, this means that vacuum pumps of a much smaller manufacturer can be used to operate the installation of the proposed ion source spine and, as a consequence, the overall size and price of the equipment for the installation of the proposed source of low-energy ions are much smaller.

In order to further reduce the necessary pumping speed of vacuum pumps, it is possible to divide the electromagnetic ion funnel 6 into two separately pumped parts. A structural diagram of such an embodiment of the source is shown in FIG. 4. As can be seen from figure 4, the electromagnetic ion funnel here consists of two parts 6a and 6b, and the vacuum system now consists not of two (as in figure 3), but of three stages of differential pumping 7a, 7b and 8.

A feature of the operation of this variant of the ion source is that the pressure of the buffer gas outside the first part of the funnel 6a (see Fig. 4), supported by pumping 7a, is several times higher than the gas pressure outside the second part of the funnel 6b, supported by vacuum pumping 7b ( figure 4). As a result of this, the effective operation of the proposed source of low-energy ions is ensured in this embodiment by two small forevacuum pumps pumping both parts of the electromagnetic funnel (see Fig. 4). Moreover, the total pumping speed of these pumps is much lower than the performance of the vacuum equipment necessary for pumping the electromagnetic funnel 6 in the case of an ion source, the circuit of which is shown in Fig.3.

In order to verify the operation of the proposed ion source and study its characteristics, we performed a detailed numerical simulation of gas-dynamic cooling and the formation of low-energy ion beams in the source. Using our original program, based on solving a complete system of time-dependent Navier-Stokes equations, both subsonic and supersonic flows of a buffer carrier gas were simulated. The results of these gas-dynamic calculations were used as input by another program that, based on the Monte Carlo method, performed trajectory calculations of ions in electromagnetic funnels (see Fig. 3 and Fig. 4) under the combined action of gas-dynamic and electric fields. In fact, these were computer experiments that allowed us to study in detail the operation of the proposed ion source. A description of these computer programs, the results of their rigorous testing, and also the results of other computer experiments in the field of molecular and ion beams can be found, for example, in our works [3] and [4].

As an example of the implementation of the proposed source of low-energy ions, consider the source device (figure 3) with the following basic geometric parameters:

1. Laval nozzle 1

- tapering and expanding conical parts have a solution angle of 90 °;

- diameter of the critical section (nozzle neck) - 1.0 mm;

- the length of the supersonic part is 3.6 mm;

- the length of the subsonic part is 2.0 mm;

- The diameter of the output cut is 8.0 mm.

2. The tube 2 in the nozzle and the target rod 3

- the length of the supersonic part of the target rod is 3.6 mm;

- diameter of the target rod - 0.4 mm;

- the length of the supersonic part of the tube in the nozzle is 3.0 mm;

- the diameter of the tube in the nozzle is 0.6 mm.

3. Electromagnetic ion funnel 6

- the thickness of the ring electrodes is 0.1 mm;

- the gap between adjacent ring electrodes is 0.3 mm;

- inner input diameter - 8.0 mm;

- inner output diameter - 1.0 mm;

- the number of electrodes in the pumping area 7-111 pcs., including

39 pcs. in the input cylindrical part and 72 pcs. in the conical part;

- the number of electrodes in the field of high vacuum pumping 8-10 pieces.

For the indicated geometry of the ion source and the nozzle room temperature, gas-dynamic computer calculations were performed at the following helium and argon pressures, which were used as buffer carrier gases:

- braking pressure (in the subsonic part of the nozzle) - 120 mbar;

- pressure in the pumping area 7-1.0 mbar;

- pressure in the pumping area 8-2 · 10 ^-4 mbar.

So, as an illustration, figure 5 shows the results of gasdynamic calculations for the helium velocity field. The black lines with arrows show the directions of gas flows, the values of the gas velocity are indicated by black numbers on a white background. The color in figure 5 is used to display the values of the velocities in the gas stream: red - maximum speed, blue - minimum. In Fig. 6, the results of the same calculations as in Fig. 5 are presented in more detail for the region of the supersonic part of the nozzle and the inlet part of the electromagnetic ion funnel 6. Here, the vortex structure of the gas flow is visually visible when the tube 2 in the nozzle and the end of the target rod are viscous 3.

It turned out that for this example implementation of the proposed ion source, the helium flow rate through nozzle 1 is 27.8 mbar · l / s, which for gas pressure 1.0 mbar in region 7 (see FIG. 3) corresponds to a evacuation pump evacuation rate of 27.8 l / s. In this case, the leakage of helium into the high-vacuum pumping region 8 is only 0.06 mbar · l / s, and, therefore, a relatively small turbomolecular pump with a pumping speed of 300 l / s will be able to maintain a vacuum of 2 · 10 ^-4 mbar in this region (note that at in this vacuum, the mean free path in helium is about 1 m).

Similar gasdynamic calculations performed for a buffer argon carrier gas showed that in order to maintain a specified argon pressure in region 7 of 1.0 mbar, a fore-vacuum pump with a pumping speed of 9 l / s will be sufficient in this case.

The results of the indicated gas-dynamic calculations (temperature, density fields and velocity components of the buffer carrier gas) were used further in the trajectory calculations of ion beams using the Monte Carlo method. Trajectory calculations were performed for ions evaporated by a laser beam from copper and gold targets and formed into ion beams inside an electromagnetic ion funnel 6 (Fig. 3) in helium and argon flows. It turned out that the voltage U = 10 V applied to the ring electrodes of the funnel 6 at a frequency of F = 2 MHz is sufficient for 100% extraction of ion beams into the high-vacuum pumping region 7 (Fig. 3).

Numerical estimates of the maximum possible ion current, performed taking into account the negative effect of the space charge of the ion beam, showed that a beam with a current of 0.5 μA can be passed through without loss of electromagnetic funnel 6, which is considered in this example.

In Fig.7 and Fig.8 presents the results of Monte Carlo calculations for the distributions of the longitudinal and radial velocity components of the ion beams of copper (Cu ⁺ ) and gold (Au ⁺ ) extracted from the ion source in the high-vacuum pumping region 8. A in Fig.9 The calculation results for the cumulative radius distributions of these ion beams are presented.

The main characteristics of ion beams obtained as a result of the computer experiments described above are presented in tables 1 and 2.

Table 1 The main characteristics of ion beams obtained using helium as a buffer carrier gas Ion beam characteristic ⁶⁴ Cu ^{+ 197} Au ⁺ Longitudinal velocity V = (m / s) 465 270 energy E = (eV) 0.072 0.074 The spread of longitudinal velocities ΔV = (m / s) 225 145 temperature T = (K) 196 251 Radial velocity V⊥ (m / s) 250 130 energy E⊥ (eV) 0.021 0.017 Radial velocity spread ΔV⊥ (m / s) 170 107 temperature T⊥ (K) 112 137 Transverse emittance ε _{x, y} (π · mm · mrad) 98 89

(π·мм·мрад·[эВ]^1/2)Normalized Transverse Emittance

(π · mm · mrad · [eV] ^1/2 ) 26.3 24.4

As can be seen from tables 1 and 2, low-energy ion beams obtained using the proposed ion source have a very small normalized transverse emittance ε ^N _{x, y} and thermal ionic velocity dispersion. Therefore, their subsequent acceleration and focusing by a system of traditional electrostatic lenses makes it possible to obtain sharply focused ion beams with diameters of several nanometers for their use in various industrial nanoelectronic technologies.

table 2 The main characteristics of ion beams obtained using argon as a buffer carrier gas Ion beam characteristic ⁶⁴ Cu ^{+ 197} Au ⁺ Longitudinal velocity V = (m / s) 690 410 energy E = (eV) 0.16 0.17 The spread of longitudinal velocities ΔV = (m / s) 300 182 temperature T = (K) 348 398 Radial velocity V⊥ (m / s) 290 150 energy E⊥ (eV) 0.028 0.023 Radial velocity spread ΔV⊥ (m / s) 280 120 temperature T⊥ (K) 304 172 Transverse emittance ε _{x, y} (π · mm · mrad) 65.3 52

(π·мм·мрад·[эВ]^1/2)Normalized Transverse Emittance

(π · mm · mrad · [eV] ^1/2 ) 26.1 21.4

For example, with further acceleration of ions to energies of 50-100 keV (these are the usual energies of ion beams used in modern ion-beam industrial technologies) and for the angular divergence of the beam (beam _half- angle) θ _1/2 = 20 ° = 364 mrad ion beams with diameters of 500-700 nanometers without loss of intensity (i.e., at ion currents up to 0.5 μA). Due to the additional aperture of the ion beam, which, of course, is accompanied by corresponding losses in its intensity, beams with currents up to 1 nA will have diameters of 20-30 nanometers, and at currents up to 10 pA, beams with diameters of 2-3 nanometers.

The given sizes of focused ion beams and the corresponding currents correspond to and often even exceed the requirements of modern industry in the field of creating devices and the element base of micro- and nanoelectronics.

In order to make sure that when the ion source is operated with an electromagnetic funnel consisting of two separately pumped parts 7a and 7b (see Fig. 4), a positive effect is achieved consisting in an additional decrease in the necessary pumping speed of vacuum pumps, we performed detailed computer experiments similar to the numerical studies described above for the main version of the proposed ion source (see figure 3). In the calculations, the following pressure values of the buffer helium carrier gas were set:

- braking pressure (in the subsonic part of the nozzle) - 120 mbar;

- pressure in the pumping area 7a - 5.0 mbar;

- pressure in the pumping area 7b - 1.0 mbar;

- pressure in the high vacuum pumping area 8 - 2 · 10 ^-4 mbar.

As a result of calculations for this source operating mode, the following gas flows were obtained at various stages of the differential pumping of the ion source installation:

- helium flow rate through nozzle 1 (or total flow rate) - 27.8 mbar · l / s;

- leakage of helium from region 7a to region 7b - 0.7 mbar · l / s;

- leakage of helium from region 7b to the high vacuum region 8 - 0.06 mbar · l / s.

It should be noted that the ion-trajectory calculations by the Monte Carlo method for the reduced mode of operation of this variant of the source (see Fig. 4) showed that the ions are effectively removed from the source under high vacuum conditions, and the characteristics of the resulting ion beams are close to those given in tables 1 and 2 values.

Since the gas flow rate (expressed in mbar · l / s) is simply the product of the pressure and the pumping rate, from the above pressures and flows we directly obtain the desired values for the pumping speeds of the pumps that provide the specified source operation mode:

- pumping speed of the region 7a - 5.4 l / s;

- pumping speed of the area 7b - 0.7 l / s;

- pumping speed of the high vacuum region 8 - 300 l / s.

From the above values it is seen that the total pumping speed of regions 7a and 7b in this case is 6.1 l / s, which is 4.5 times less than the required pumping speed of region 7 in a source operating according to the scheme of FIG. 3.

Information sources

1. D. E. Powers, S. G. Hansen, M. E. Geusic, D. L. Michalopoulus and R. E. Smalley, J. Chem. Phys., Vol. 78, 1983, p. 2866.

2. V.L. Varentsov, A.A. Matyshev, Source of ion beams, Rospatent No. 1463051, priority dated 05/18/1987, registered on 03/25/1993 (prototype).

3. V.L. Varentsov, A.A. Ignatiev, Nucl. Instrum. and Meth., A 413 (1998) 447.

4. Victor Varentsov, Michiharu Wada, Nucl. Instrum. and Meth., A 532 (2004) 210.

Claims (2)

1. A source of low-energy ion beams for nanoelectronic technology, containing a supersonic Laval nozzle with a tube on the axis through which the target in the form of a rod (or wire) is fed into the expanding supersonic part of the nozzle, and having a laser beam focused on the target end, which evaporates and ionizes the substance target, characterized in that, in order to reduce the loss of target material, the consumption of buffer carrier gas, the dimensions of the source installation while reducing its cost, as well as increasing the term and the service, the source is additionally equipped with an electromagnetic ion funnel mounted on the axis of the nozzle behind its outlet slice. 2. The source according to claim 1, characterized in that, in order to further reduce the required pumping speed of vacuum pumps, an electromagnetic ion funnel consists of two separately pumped parts.

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