WO2004053945A2

WO2004053945A2 - Re-crystallization of semiconductor surface film and doping of semiconductor by energetic cluster irradiation

Info

Publication number: WO2004053945A2
Application number: PCT/US2003/039754
Authority: WO
Inventors: Allen R. Kirkpatrick; John J. Hautala; Martin D. Tabat; Thomas G. Tetreault; Sean Kirkpatrick
Original assignee: Epion Corporation
Priority date: 2002-12-12
Filing date: 2003-12-12
Publication date: 2004-06-24
Also published as: EP1584104A2; AU2003299614A8; WO2004053945A3; AU2003299614A1; JP2006510196A; EP1584104A4

Abstract

Method of gas-cluster ion beam 128 processing of damaged semiconductor films 308 for re-crystallization and/or for activating a dopant in a semiconductor film 308 with reduced dopant diffusion, and semiconductor devices formed using the method. The method is useful, for example, for restoring crystallinity and/or for electrically activating a dopant species after shallow dopant ion implantation for forming shallow junctions. In one embodiment of the method, dopant atoms incorporated into the gas-clusters produces shallow doping of a semiconductor 302 with simultaneous electrical activation of the dopant atoms and re-crystallization of the semiconductor.

Description

Re-Crystallization Of Semiconductor Surface Film And Doping Of Semiconductor By Energetic Cluster

Irradiation

Field Of The Invention

This invention relates generally to the doping and/or re- crystallization of semiconductor surfaces and, more particularly to the re-crystallization of a semiconductor surface by energetic gas-cluster ion irradiation and to the impurity doping of semiconductor surface with self-annealing or self-activation by energetic gas-cluster ion irradiation.

Background Of The Invention

The useful characteristics of semiconductor materials such as silicon, germanium and gallium arsenide and other semiconductors are contingent upon the purity and crystal structure of the semiconductor material. Dopant atoms incorporated into semiconductor materials for the purpose of altering electrical properties, forming electronic junctions, etc., are often introduced into a semiconductor surface by conventional ion implantation. During the conventional process of ion implantation, ionized dopant atoms are physically deposited into a crystalline semiconductor material, but it is well known that, in doing so, the crystal lattice of the semiconductor becomes damaged by the implantation process. In order for the implanted dopant atoms to become electrically active within the semiconductor and to restore the desirable crystallinity of the semiconductor, the semiconductor crystal lattice structure must be restored and the implanted dopant atoms must occupy lattice sites within the restored crystal lattice by substitution. Processes typically employed to produce crystal lattice restoration and electrical activation of implanted dopant atoms include elevated temperature thermal annealing, pulsed laser beam annealing and pulsed electron beam annealing.

For some semiconductor products, an important requirement for the introduction of dopants into the semiconductor surface is that the maximum depth to which the dopant has penetrated after completion of the lattice re-crystallization and dopant activation processes must be kept very shallow, often only a few hundred Angstroms or less. By using very low energy conventional ion implantation such shallow introduction of dopant is feasible by using very low implantation energies on the order of less than 1000 eN or in some cases even less than 200 eN. However, when such low energy conventional implants are re-crystallized or activated according to previously known techniques, diffusion of the shallow dopant atoms results in deeper redistribution of the dopant atoms and thus in formation of deeper junctions than desirable.

The use of a gas-cluster ion beam (GCIB) for etching, cleaning, and smoothing surfaces is known in the art (see for example, US patent 5,814,194, Deguchi, et al.) For purposes of this discussion, gas- clusters are nano-sized aggregates of materials that are gaseous under conditions of standard temperature and pressure. Such clusters may consist of loosely bound or more tightly bound aggregate s of molecules numbering from a few to several thousand molecules. The clusters can be ionized by electron bombardment or other means, permitting them to be formed into directed beams of controllable energy. Such ions each typically carry positive charges of q-e (where e is the magnitude of the electronic charge and q is an integer of from one to several representing the charge state of the cluster ion). The larger sized clusters are often the most useful because of their ability to carry substantial energy per cluster ion, while yet having only modest energy per molecule. The clusters disintegrate on impact, with each individual molecule carrying only a small fraction of the total cluster energy. Consequently the impact effects of large clusters are substantial, but are limited to a very shallow surface region. This makes ion clusters effective for a variety of surface modification processes, without the tendency to produce deeper subsurface damage characteristic of conventional ion beam processing.

Means for creation of and acceleration of such GCIBs are described in the reference (USP 5,814,194) previously cited. Presently available ion cluster sources produce clusters ions having a wide distribution of sizes, N (where N = the number of molecules in each cluster - in the case of monatomic gases like argon, an atom of the monatomic gas will be referred to as either an atom or a molecule and an ionized atom of such a monatomic gas will be referred to as either an ionized atom, or a molecular ion, or simply a monomer ion - throughout this discussion). It is therefore an object of this invention to provide for re- crystallization of a semiconductor surface by energetic gas cluster ion irradiation.

It is another object of this invention to provide for the activation of shallowly implanted dopant atoms in a semiconductor material with reduced redistribution of the dopant atoms by diffusion induced by the activation by utilizing energetic gas-cluster ions for the activation.

It is a further object of this invention to provide for the introduction of dopant atoms in the ultra-shallow sub-surface regions of a semiconductor material by irradiation of energetic gas-cluster ions comprising dopant atoms or comprising dopant and inert atoms.

A still further object of this invention is to provide for the production of an ultra-shallow junction by the introduction of dopant atoms in the ultra-shallow sub-surface regions of a semiconductor material and for the activation of the dopant and for the re- crystallization of the semiconductor surface by irradiation of energetic gas-cluster ions comprising dopant atoms or comprising dopant and inert atoms.

Summary Of The Invention

The objects set forth above as well as further and other objects and advantages of the present invention are achieved by the embodiments of the invention described hereinbelow. Upon impact of an energetic gas-cluster on the surface of a solid target, penetration of the atoms of the cluster into the target surface is typically very shallow because the penetration depth is determined by the low energy of each individual constituent atom. Gas-clusters dissociate upon impact and the individual gas atoms then become free to recoil and escape from the surface of the target. Other than energy carried away by the escaping individual gas atoms, the total energy of the energetic cluster prior to impact becomes deposited into the impact zone on the target surface. The dimensions of a target impact zone are dependent on the energy of the cluster but are of the order of the cross- sectional dimensions of the impacting cluster and are small, for example roughly 30 Angstroms in diameter for an ionic cluster comprised of 1000 atoms. As a result of the deposition of most of the total energy carried by the cluster into the small impact zone on the target, an intense thermal transient occurs within the target material at the impact site. The thermal transient dissipates as energy is lost from the impact zone by conduction deeper into the target. Duration of the thermal transient is determined by the conductivity of the target material but will typically be less than 10^"6 second.

In the vicinity of a cluster impact site, a volume of the target surface can momentarily reach temperatures of many hundreds to several thousands of degrees Kelvin. As an example, impact of a cluster carrying 10 keN total energy is estimated to be capable of producing a momentary temperature increase of about 2000 degrees Kelvin throughout a hemispherical zone extending to almost 100 Angstroms below a silicon surface.

Following initiation of an elevated temperature transient within the target volume below an energetic cluster impact site, the affected zone cools from below the surface back to the surface. If a damaged crystal lattice condition, such as that caused by ion implantation of dopant atoms, exists within a layer near the target surface, the transient temperature conditions produced by energetic cluster impact can be employed to cause recovery of the damaged lattice. For this to occur, a sufficient thermal transient must be created in a volume extending through the damaged region to the undamaged silicon crystal below. During dissipation of the transient temperature conditions, cooling must proceed from the undamaged crystal lattice below the damaged layer back through the damage layer to the surface. Upon restoration of the crystal lattice within the damaged region, dopant atoms will become incorporated into lattice sites and electrical activation will occur.

Gases such as argon, xenon, oxygen, nitrogen, carbon dioxide, for example, not for limitation, can be employed to form the energetic gas-clusters for producing the crystal lattice restoration and dopant activation effects described above. Additionally, if a gas containing an appropriate semiconductor dopant atom such as boron is added to, or used as, the gas to form the energetic gas-clusters, the energetic gas- cluster impact can deposit dopant atoms into a semiconductor lattice and simultaneously produce recovery of any damage to the lattice. For some semiconductor products, an important requirement for the introduction of dopants into the semiconductor surface is that the maximum depth to which the dopant has penetrated after completion of the lattice re-crystallization and dopant activation processes must be kept very shallow, often only a few hundred Angstroms or less. The depth to which the effects produced by energetic gas-cluster impact can be controlled by controlling the energy of the impinging gas-cluster ion beam. Consequently the energetic gas- cluster methods which have been described for semiconductor annealing and dopant activation, or for dopant introduction and self- annealing, can facilitate very shallow resulting depth of the introduced and activated dopants.

Brief Description Of The Figures of the Drawing

For a better understanding of the present invention, together with other and further objects thereof, reference is made to the accompanying drawing and detailed description, wherein:

Figure 1 is a schematic showing the basic elements of a prior art GCIB processing apparatus that uses an electrostatically scanned beam;

Figure 2 is a schematic showing the basic elements of a prior art GCIB processing apparatus that uses a stationary beam with mechanical scanning of the workpiece; Figure 3 is a schematic of a portion of a semiconductor wafer with a damaged and/or doped surface film;

Figure 4 is a schematic of a portion of a semiconductor wafer with a damaged and doped surface film during gas-cluster ion beam irradiation according to an embodiment of the invention;

Figure 5 is a schematic enlarging a portion of the semiconductor wafer from Figure 4, showing additional detail;

Figure 6 is a schematic of a portion of a semiconductor wafer, showing re-crystallization and electrical activation of a region-impacted by a gas-cluster ion beam, according to the invention;

Figure 7 is a schematic of a portion of a semiconductor wafer, showing re-crystallization and electrical activation of surface regions impacted by many gas-cluster ions;

Figure 8 is a schematic of a portion of a semiconductor wafer, showing re-crystallization and electrical activation of surface film after completion of gas-cluster ion beam processing according to the invention;

Figure 9 is a schematic of a portion of a semiconductor wafer being impacted by a gas-cluster ion comprising a mixture of dopant gas and electrically inert gas molecules, according to an embodiment of the invention;

Figure 10 is a schematic enlarging a portion of the semiconductor wafer from Figure 9, showing additional detail; Figure 11 is a schematic of a portion of a semiconductor wafer, showing doping with re-crystallization and electrical activation of a region-impacted by a gas-cluster ion beam, according to an embodiment of the invention; and

Figure 12 is a schematic of a portion of a semiconductor wafer, showing doping with re-crystallization and electrical activation of surface film after completion of gas-cluster ion beam processing according to an embodiment of the invention.

Detailed Description Of Certain Preferred Embodiments Of The

Invention

Figure 1 shows schematically of the basic elements of a typical configuration for a GCIB processing apparatus 100 of a form known in prior art but adapted for practice of the present invention, and which may be described as follows: a vacuum vessel 102 is divided into three communicating chambers, a source chamber 104, an ionization/acceleration chamber 106, and a processing chamber 108. The three chambers are evacuated to suitable operating pressures by vacuum pumping systems 146a, 146b, and 146c, respectively. A condensable source gas 112 (for example, argon or N₂) stored in a gas storage cylinder 111 is admitted under pressure through gas metering valve 113 and gas feed tube 114 into stagnation chamber 116 and is ejected into the substantially lower pressure vacuum through a properly shaped nozzle 110. A supersonic gas jet 118 results. Cooling, which results from the expansion in the jet, causes a portion of the gas jet 118 to condense into clusters, each consisting of from several to several thousand weakly bound atoms or molecules. A gas skimmer aperture 120 partially separates the gas molecules that have not condensed into a cluster jet from the cluster jet so as to minimize pressure in the downstream regions where such higher pressures would be detrimental (e.g., ionizer 122, high voltage electrodes 126, and processing chamber 108). Suitable condensable source gases 112 include, but are not necessarily limited to argon, nitrogen, carbon dioxide, oxygen, and other gases.

After the supersonic gas jet 118 containing gas-clusters has been formed, the clusters are ionized in an ionizer 122. The ionizer 122 is typically an electron impact ionizer that produces thermoelectrons from one or more incandescent filaments 124 and accelerates and directs the electrons causing them to collide with the gas-clusters in the gas jet 118, where the jet passes through the ionizer 122. The electron impact ejects electrons from the clusters, causing a portion the clusters to become positively ionized. A set of suitably biased high voltage electrodes 126 extracts the cluster ions from the ionizer, forming a gas cluster ion beam (GCIB), then accelerates them to a desired energy (typically from 1 keN to several tens of keN) and focuses them to form a GCIB 128. Filament power supply 136 provides filament voltage N to heat the ionizer filament 124. Anode power supply 134 provides anode voltage N_A to accelerate thermoelectrons emitted from filament 124 to cause them to irradiate the cluster containing gas jet 118 to produce ions. Extraction power supply 138 provides extraction voltage N_E to bias a high voltage electrode to extract ions from the ionizing region of ionizer 122 and to form a GCIB 128. Accelerator power supply 140 provides acceleration voltage VA_CC to bias a high voltage electrode with respect to the ionizer 122 so as to result in a total GCIB acceleration energy equal to N_Acc electron volts (eN). One or more lens power supplies (142 and 144 shown for example) may be provided to bias high voltage electrodes with focusing voltages (V and N_L2 for example) to focus the GCIB 128.

A workpiece 152, which may be a semiconductor wafer or other workpiece to be processed by GCIB processing, is held on a workpiece holder 150, disposed in the path of the GCIB 128. Since most applications contemplate the processing of large workpieces with spatially uniform results, a scanning system is desirable to uniformly scan the GCIB 128 across large areas to produce spatially homogeneous results. Two pairs of orthogonally oriented electrostatic scan plates 130 and 132 can be utilized to produce a raster or other scanning pattern across the desired processing area. When beam scanning is performed, the GCIB 128 is converted into a scanned GCIB 148, which scans the entire surface of workpiece 152.

Figure 2 shows a schematic of the basic elements of a prior art mechanically scanning GCIB processing apparatus 200 adapted for use in the present invention having a stationary beam with a mechanically scanned workpiece 152, and having a conventional faraday cup for beam measurement and a conventional thermionic neutralizer. GCIB formation is similar to as shown in Figure 1, except there is additional provision for an optional second source gas 222 (typically different from the source gas 112) stored in a gas storage cylinder 221 with a gas metering valve 223 and connecting through gas feed tube 114 into stagnation chamber 116. This arrangement allows for controllably selecting between two differing source gasses 112 and 222 or for controllably forming a mixture of two source gasses for use in forming gas-clusters. Also, in the mechanically scanning GCIB processing apparatus 200 of Figure 2, the GCIB 128 is stationary (not electrostatically scanned as in the GCIB processing apparatus 100) and the workpiece 152 is mechanically scanned through the GCIB 128 to distribute the effects of the GCIB 128 over a surface of the workpiece 152.

An X-scan actuator 202 provides linear motion of the workpiece holder 150 in the direction of X-scan motion 208 (into and out of the plane of the paper). A Y-scan actuator 204 provides linear motion of the workpiece holder 150 in the direction of Y-scan motion 210, which is typically orthogonal to the X-scan motion 208. The combination of X-scanning and Y-scanning motions moves the workpiece 152, held by the workpiece holder 150 in a raster-like scanning motion through GCIB 128 to cause a uniform irradiation of a surface of the workpiece 152 by the GCIB 128 for uniform processing of the workpiece 152. The workpiece holder 150 disposes the workpiece 152 at an angle with respect to the axis of the GCIB 128 so that the GCIB 128 has an angle of beam incidence 206 with respect to the workpiece 152 surface. The angle of beam incidence 206 may be 90 degrees or some other angle, but is typically 90 degrees or very near 90 degrees. During Y-scanning, the workpiece 152 held by workpiece holder 150 moves from the position shown to the alternate position "A", indicated by the designators 152A and 150A respectively. Notice that in moving between the two positions, the workpiece 152 is scanned through the GCIB 128 and in both extreme positions, is moved completely out of the path of the GCIB 128 (over-scanned). Though not shown explicitly in Figure 2, similar scamiing and over- scan is performed in the (typically) orthogonal X-scan motion 208 direction (in and out of the plane of the paper).

A beam current sensor 218 is disposed beyond the workpiece holder 150 in the path of the GCIB 128 so as to intercept a sample of the GCIB 128 when the workpiece holder 150 is scanned out of the path of the GCIB 128. The beam current sensor 218 is typically a faraday cup or the like, closed except for a beam-entry opening, and is affixed to the wall of the vacuum vessel 102 with an electrically insulating mount 212.

A controller 220, which may be a microcomputer based controller connects to the X-scan actuator 202 and the Y-scan actuator 204 through electrical cable 216 and controls the X-scan actuator 202 and the Y-scan actuator 204 so as to place the workpiece 152 into or out of the GCIB 128 and to scan the workpiece 152 uniformly relative to the GCIB 128 to achieve uniform processing of the workpiece 152 by the GCIB 128. Controller 220 receives the sampled beam current collected by the beam current sensor 218 by way of lead 214 and thereby monitors the GCIB and controls the GCIB dose received by the workpiece 152 by removing the workpiece 152 from the GCIB 128 when a predetermined desired dose has been delivered.

Figure 3 is a schematic cross-section view 300 of a semiconductor wafer portion with a thickness 302 with a damaged and/or doped surface film 308. The figure is not drawn to scale. The bulk semiconductor 304 is typically a single crystal material with a high degree of crystallinity and has a surface 306 with the damaged and/or doped film having a thickness 310. The damaged and/or doped film 308 is common feature of semiconductor wafers during various stages of typical integrated circuit or semiconductor device fabrication. In one example, for illustration - not for limitation, when a semiconductor wafer is at a stage of fabrication immediately following shallow doping by a low energy ion implantation process, a damaged and/or doped film 308 having a thickness 310, which may be on the order of a few tens to several tens of Angstroms, exists adjacent the surface 306 of the semiconductor wafer 302. The damage in the damaged and/or doped film 308 arises from radiation damage during an ion implantation step and typically comprises crystalline lattice damage, which may be of any degree from modest to extreme, even including complete amorphization of the previously crystalline material. Additionally the film contains dopant atoms of a donor or acceptor species deposited by the ion implantation process (for example if the bulk semiconductor is silicon, boron, phosphorous, arsenic, and antimony are some typical dopant species). It is usually desired that after ion implantation of dopants, any damage to crystallinity should be repaired and the implanted dopant atoms should be placed into substitutional sites in the restored crystalline lattice in order to effect their electrical activity (activation) in the semiconductor. In the prior art, such re-crystallization and activation is sometimes referred to as annealing and has been typically been performed by means of a process such as heating in a furnace, treating with a rapid optical or thermal pulse, irradiation with a laser or electron beam, etc. Such process typically result in a period of heating of the implanted film with subsequent re-crystallization of the damaged semiconductor and with most of the dopant atoms achieving substitutional lattice sites and electrical activation. All these annealing processes have a drawback, in that they result in diffusion of many of the dopant atoms to depths considerably deeper than the as-implanted film thickness 310. Accordingly, electrical effects of the dopant atoms extend to greater than desired depths and/or semiconductor junctions may be formed at depths deeper than desired. A first embodiment of this invention provides a method of re-crystallizing and electrically activating (annealing) such a damaged and doped film with dramatically improved control of depth redistribution of the dopant atoms as compared to prior art methods.

Figure 4 is a schematic cross-section view 350 of a semiconductor wafer portion with a thickness 302 with a damaged and/or doped film 308 similar to the prior art type illustrated in Figure 3, but, shown at an early stage of processing by gas-cluster ion beam irradiation according to the first embodiment of the invention. For simplicity, only a single gas-cluster ion 352 and gas-cluster ion-impact region 360 are shown in Figure 4. During practice of the first embodiment of the invention, a gas-cluster ion beam processing system, which may be, for example, of the type shown in Figure 1 or in Figure 2 is employed to direct a gas-cluster ion beam onto the surface of a semiconductor wafer 302. Semiconductor wafer 302 is the workpiece 152 as shown in of Figure 1 or of Figure 2. For example, cluster ions (gas-cluster ion 352 shown for example) comprised of a gas or gasses that is/are not a dopant for the bulk semiconductor 304 is utilized in formation of the gas-cluster ion beam. Gas-cluster ions, such as the gas-cluster ion 352, for example, follow trajectories such as trajectory 354, for example, impacting the surface 306 of the semiconductor wafer. Such a gas-cluster ion 352 has an energy preselected and controlled by the gas-cluster ion beam processing system and its operational and control parameters. Upon impact with the surface 306 of the semiconductor wafer 302, the gas-cluster ion 352 forms a gas-cluster ion-impact region 360. According to the first embodiment of the present invention, the pre-selected and controlled energy of the gas-cluster ion 352 is such as to provide a gas-cluster ion- impact region 360 that extends through the damaged and/or doped film 308 into the bulk semiconductor 304. The gas-cluster ion-impact region 360 comprises two portions, a gas-cluster ion-impact region portion 356 within the damaged and/or doped film 308 and a gas- cluster ion-impact region portion 358 within the bulk semiconductor 304.

Figure 5 is a schematic 400 enlarging a portion ofthe semiconductor wafer 302 shown in Figure 4, showing additional detail. Gas-cluster ion 352 is comprised of multiple molecules (one molecule 408 is indicated for example) comprising a gas or gasses that is/are not a dopant for the bulk semiconductor 304. Such molecule(s) 408 may be, for example - not for limitation, molecules of argon or other inert gas. The gas-cluster ion-impact region 360 has a boundary 402. The volume ofthe gas-cluster ion-impact region 360 and hence it's depth of penetration ofthe surface 306 of the semiconductor is dependent on the preselected and controlled energy ofthe gas-cluster ion 352. Upon impact of an energetic gas-cluster ion 352 on the surface 306, the gas-cluster ion dissociates and the individual gas molecule(s) 408 then become free to recoil and escape from the surface ofthe target. Other than a small energy carried away by the escaping individual gas atoms, the total energy ofthe energetic gas-cluster ion becomes deposited into the gas-cluster ion-impact region 360. The dimensions ofthe gas-cluster ion-impact region 360 are dependent on the energy of the cluster but are of the order of the cross-sectional dimensions ofthe impacting gas-cluster ion and are small, for example roughly 30 - 100 Angstroms in diameter, depending on the preselected gas-cluster ion energy. As a result ofthe deposition of most ofthe total energy carried by the gas-cluster ion 352 into the small gas-cluster ion-impact region 360, an intense thermal transient occurs within the material in the gas-cluster ion-impact region 360. The heat deposited in the gas-cluster ion-impact region 360 dissipates by conduction in the directions 404 deeper into the surrounding semiconductor material. Duration ofthe thermal transient is determined by the conductivity of the surrounding material but will typically be less than 10^"6 second.

In the gas-cluster ion-impact region 360 material can momentarily reach temperatures of many hundreds to several thousands of degrees Kelvin. As an example, impact of a gas-cluster ion 352 when carrying 10 keN total energy is estimated to be capable of producing a momentary temperature increase of about 2000 degrees Kelvin throughout an gas-cluster ion-impact region 360 extending to almost 100 Angstroms below the surface 306. Without being bound to a particular theory, it is believed that during the thermal transient, thermal agitation is high enough to possibly melt the material in the gas-cluster ion-impact region 360. As the gas-cluster ion-impact region 360 cools by thermal conduction in the directions 404, re-growth of crystalline material proceeds in the directions 406 from the initially undamaged highly crystalline gas-cluster ion-impact region portion 358 ofthe gas-cluster ion-impact region 360 through the gas-cluster ion- impact region portion 356 of the gas-cluster ion-impact region 360. As a result, crystallinity is restored within much of the gas-cluster ion- impact region 360 following the thermal transient due to the impact of gas-cluster ion 352. Also resulting from the re-crystallization of material in the gas-cluster ion-impact region 360, many dopant atoms present in the gas-cluster ion-impact region portion 356 of the gas- cluster ion-impact region 360 move mto substitutional lattice sites and become electrically activated.

Figure 6 is a schematic 450 of a portion of a semiconductor wafer 302, showing re-crystallization and electrical activation of a region 452 impacted by a gas-cluster ion, according to the first embodiment ofthe invention. The portions 356 and 358 ofthe gas- cluster ion-impact region 360 of Figure 5 become all or at least partially re-crystallized and dopant is activated to form the region 452.

Figure 7 is a schematic 500 of a portion of a semiconductor wafer 302, being processed by gas-cluster ion beam according to the method ofthe first embodiment ofthe invention showing re- crystallization and electrical activation of surface regions impacted by many gas-cluster ions. As the process continues, larger and larger regions 502 ofthe original damaged and/or doped film 308 experience re-crystallization and electrical activation of dopant. Islands 504 ofthe original damaged and/or doped film 308 shrink and are gradually replaced by the re-crystallized and activated material. The re- crystallized and activated regions 502 extend to a depth 506 below the surface 306 and depth 506 is somewhat greater than the original thickness 310 ofthe original damaged and/or doped film 308. The depth 506 is determined by the energy ofthe gas-cluster in beam used for processing and may be predetermined and pre-selected either by conventional modeling or by experimentally processing a matrix of gas- cluster ion beam energies and selecting the gas-cluster ion beam energy that provides acceptable re-crystallization and activation without resulting in depth 506 exceeding the semiconductor integrated circuit or semiconductor device process requirements.

Figure 8 is a schematic 550 of a portion of a semiconductor wafer 302, showing re-crystallization and electrical activation of surface film after completion of gas-cluster ion beam processing according to the first embodiment ofthe invention. A uniformly re- crystallized and electrically activated film 552, having a depth 506 replaces the original damaged and/or doped film 308.

Figure 9 is a schematic 600 of a portion of a semiconductor wafer 602 being impacted by a gas-cluster ion 606 comprising a mixture of dopant gas and electrically inert gas molecules, according to a second embodiment ofthe invention. The figure is not drawn to scale. The semiconductor wafer 602 has a surface 604 and is typically a single crystal material with a high degree of crystallinity and may be at any of several stages of processing for fabricating an integrated circuit or semiconductor device. In this example, for illustration - not for limitation, a semiconductor wafer 602 is at a stage of fabrication where it is desired to dope a thin surface film with an acceptor or donor impurity and to electrically activate the dopant for formation of a shallow junction or shallow doped region. A gas-cluster ion 606 having a trajectory 608 is shown impacting surface 604 of semiconductor wafer 602, where it forms a gas-cluster ion-impact region 610. According to this second embodiment ofthe invention, gas-cluster ion 606 has been formed so that it is a gas that includes an acceptor or donor species for the semiconductor wafer 602. For example, if the semiconductor wafer 602 is silicon, the gas might include boron trifluoride, diborane, arsine, arsenic pentafluoride, phosphorous pentafluoride, phosphine, stibine, etc.

Figure 10 is a schematic 650 enlarging a portion o the semiconductor wafer 602 from Figure 9, showing additional detail. Gas-cluster ion 606 comprises multiple molecules of at least one gas that includes a dopant atom for the semiconductor wafer 602. The gas- cluster ion 606 may consist entirely of a gas that comprises a dopant for the semiconductor wafer 602, or the gas-cluster ion 606 may consist of a mixture of gases such that at least one of the two or more gasses in the mixture comprises a dopant atom for the semiconductor wafer 602. Such a gas-cluster ion 606 may be formed in a GCIB processing apparatus 100 or 200 as shown in or similar to those shown in Figure 1 or in Figure 2, for examples. When a mixture of gasses is desired in gas clusters formed in GCIB processing apparatus, a premixed gas mixture with the desired mix can be provided in a single gas storage cylinder 111 (Figure 1 or Figure 2) or alternatively separate differing source gases 112 and 222 can be provided in gas storage cylinders 111 and 221 (Figure 2) and then mixed in desired proportions as they flow to the stagnation chamber 116 (Figure 2) by suitable adjustment of gas metering valves 113 and 223 (Figure 2). Thus it is possible to generate gas-cluster ion beams consisting entirely of a single gas including a dopant atom in its molecules, or to generate gas-cluster ion-beams with a mix of two or more gasses, wherein at least one is a gas including the dopant atom in its molecules and wherein at least one gas is a gas that is not a dopant.

Returning to discussion of Figure 10, the gas-cluster ion 606, is shown (for example and not for limitation) to comprise non-dopant molecules (non-dopant molecule 652 indicated as a representative example) and dopant molecules comprising a dopant atom (dopant molecule 654 indicated as a representative example). It is recognized that a wide range of mixtures of dopant and non-dopant molecules are useful in the present invention and that the clusters used for the process ofthe invention can be formed entirely from dopant gas molecules, or at the other extreme, the ratio of dopant gas molecules to non-dopant gas molecules may be so low that some or many gas- cluster ions do not contain even a single dopant gas molecule, but wherein at least a portion ofthe gas-cluster ions in a gas-cluster ion beam comprise one or more molecules of dopant gas. In the example illustrated in Figure 10, the gas-cluster ion 606 is formed from a mix of gases with a majority of non-dopant molecules 652 and a minority of dopant molecules 654. When, for example, the semiconductor wafer 602 is silicon, the dopant gas might be for example diborane, comprising the dopant atom boron. The non-dopant gas might be for example argon, which is an inert gas. The gas-cluster ion-impact region 610 has a boundary 656. The volume ofthe gas-cluster ion-impact region 610 and hence it's depth of penetration ofthe surface ofthe semiconductor is dependent on the preselected and controlled energy ofthe gas-cluster ion 606. Upon impact of an energetic gas-cluster ion 606 on the surface 604, the gas-cluster ion 606 dissociates and the individual non-dopant molecules 652 and dopant molecules 654 then become free. Inert gas molecules typically recoil and escape from the surface 604 ofthe semiconductor wafer 602. Some molecules including some ofthe dopant molecules 654 become imbedded in the surface. Other than a small energy carried away by the escaping individual gas atoms, the total energy ofthe energetic gas-cluster ion 606 becomes deposited into the gas-cluster ion-impact region 610. The dimensions o the gas-cluster ion-impact region 610 are dependent on the energy of the cluster but are ofthe order ofthe cross-sectional dimensions ofthe impacting gas-cluster ion and are small, for example roughly 30 - 100 Angstroms in diameter, depending on the preselected gas-cluster ion energy. As a result o the deposition of most ofthe total energy carried by the gas-cluster ion 606 into the small gas-cluster ion-impact region 610, an intense thermal transient occurs within the material in the gas-cluster ion-impact region 610. The heat deposited in the gas- cluster ion-impact region 610 dissipates by conduction in the directions 658 deeper into the surrounding semiconductor material. Duration o the thermal transient is determined by the thermal conductivity of the surrounding material but will typically be less than 10^"6 second.

In the gas-cluster ion-impact region 610 material can momentarily reach temperatures of many hundreds to several thousands of degrees Kelvin. As an example, impact of a gas-cluster ion 606 when carrying 10 keN total energy is estimated to be capable of producing a momentary temperature increase of about 2000 degrees Kelvin throughout an gas-cluster ion-impact region 610 extending to almost 100 Angstroms below the surface 604. Without being bound to a particular theory, it is believed that during the thermal transient, thermal agitation is high enough to possibly melt the material in the gas-cluster ion-impact region 610. As the gas-cluster ion-impact region 610 cools by thermal conduction in the directions 658, re-growth of crystalline material proceeds in the directions 660 from the highly crystalline semiconductor wafer 602 through the gas-cluster ion-impact region 610. As a result, crystallinity is restored within the gas-cluster ion-impact region 610 following the cluster ion thermal transient and some dopant atoms originating in the gas dopant molecules 654 remain in the semiconductor and occupy substitutional lattice sites in the semiconductor material in the gas-cluster ion-impact region 610. The dopant atoms present in the of the gas-cluster ion-impact region 610 thus become electrically activated. The semiconductor wafer 602 thus becomes doped, electrically activated, and re-crystallized in the gas- cluster ion-impact region 610. The doping concentration is controlled by pre-selecting and controlling the ratio of dopant to non-dopant molecules in the gas mixture used for forming the gas-cluster ion beam.

Figure 11 is a schematic 700 of a portion of a semiconductor wafer 602, showmg doping with re-crystallization and electrical activation of a region impacted by a gas-cluster ion, according to the second embodiment ofthe present invention. After the gas-cluster ion- impact event described in Figure 10, upon dissipation ofthe thermal transient, a doped, electrically activated, and re-crystallized region 702 replaces the gas-cluster ion-impact region 610 of Figure 10. Doped, electrically activated, and re-crystallized region 702 extends to a depth 704 below the surface 604 of semiconductor wafer 602. Figure 12 is a schematic 750 of a portion of a semiconductor wafer 602, showmg a doped, electrically activated, and re-crystallized film 752 formed by completion of gas-cluster ion beam processing according to the second embodiment ofthe present invention. With continued gas-cluster ion irradiation, additional doped, electrically activated, and re-crystallized region regions similar to the doped, electrically activated, and re- crystallized region 702 (Figure 11) form, overlap, and eventually develop the doped, electrically activated, and re-crystallized film 752, extending to a depth 704 below the surface 604 ofthe semiconductor wafer 602. By gas-cluster ion beam processing of a semiconductor surface using gas cluster ions comprising molecules of dopant gas a doped, electrically activated, and re-crystallized film 752 is formed on semiconductor wafer 602 in a single processing step. By avoiding the need for conventional annealing, undesired diffusion ofthe dopant atoms is avoided and the film depth 704 is controlled by selection of the gas-cluster ion beam energy. Shallow doped semiconductor surface films of a few hundred Angstroms down to a few tens of Angstroms are formed and hence similarly shallow junctions may be formed.

Although the invention has been described with respect to various embodiments, it should be realized this invention is also capable of a wide variety of further and other embodiments within the spirit ofthe invention.

Claims

We claim:

1. A method for re-crystallizing a damaged semiconductor thin film at the surface of a substantially crystalline semiconductor substrate, comprising the steps of: providing a reduced pressure vacuum chamber containing a substrate holder for holding the substantially crystalline semiconductor substrate; holding the substantially crystalline semiconductor substrate on said substrate holder within said reduced pressure vacuum chamber; selecting an energy for gas cluster ion processing; generating a gas cluster ion beam within said reduced pressure vacuum chamber; accelerating the gas cluster ion beam to said selcted energy; and directing the accelerated gas cluster ion beam onto and within at least a portion ofthe damaged semiconductor thin film to process the film to reduce the amount of damage and improve the crystallinity thereof.

2. The method of claim 1 , wherein the workpiece is a semiconductor substrate from an ion irradiation process of a type causing radiation damage to the thin film.

The method of claim 2, wherein the ion irradiation process is ion implantation using molecular or atomic ions.

The method of claim 1, wherem the step of selecting an energy further comprises the step of determining a depth for reducing damage and improving crystallinity, said depth being less than about 200 angstroms, and wherein the beam is directed to a level within the film consistent with each determination.

The method of claim 1, wherein the directing step further comprises controllably scanning the accelerated cluster ion beam.

6. A method for electrically activating dopant atoms in a doped semiconductor thin film overlying the surface of a substantially crystalline semiconductor substrate, comprising the steps of: providing a reduced pressure vacuum chamber containing a substrate holder for holding the substantially crystalline semiconductor substrate; holding the substantially crystalline semiconductor substrate on said substrate holder within said reduced pressure vacuum chamber; selecting an energy for gas cluster ion processing; generating a gas cluster ion beam within said reduced pressure vacuum chamber; accelerating the gas cluster ion beam to said selected energy; and directing the accelerated gas cluster ion beam onto and within at least a portion ofthe doped semiconductor thin film so as to electrically activate at least a portion ofthe dopant atoms.

7. The method of claim 6, further comprising the step of receiving the semiconductor substrate from a molecular or atomic ion implantation process for doping the semiconductor thin film with the dopant atoms.

8. The method of claim 6, wherein the step of selecting an energy further comprises the step of determining a depth for electrically activating said at least a portion of the dopant atoms, said depth being less than about 200 angstroms.

9. The method of claim 6, wherein the directing step further comprises controllably scanning the accelerated cluster ion beam.

10. A method of forming a doped ultra-shallow subsurface region of a semiconductor substrate, comprising the steps of: providing a reduced pressure vacuum chamber containing a substrate holder for holding the semiconductor substrate; holding the semiconductor substrate on said substrate holder within said reduced pressure vacuum chamber; selecting a ratio of dopant to non-dopant gas molecules or atoms; selecting an energy for gas cluster ion processing; selecting a dose for gas cluster ion processing; generating, within said reduced pressure vacuum chamber, a gas cluster ion beam comprising dopant and non- dopant molecules or atoms in approximately the selected ratio; accelerating the gas cluster ion beam to said selected energy; and directing the accelerated gas cluster ion beam onto a portion ofthe surface ofthe semiconductor substrate so as to irradiate the portion ofthe surface ofthe semiconductor substrate with approximately the selected dose of dopant molecules or atoms to form a doped ultra-shallow subsurface region.

11. The method of claim 10, wherein the step of selecting an energy further comprises the step of determining a depth for the ultra- shallow subsurface region, said depth being less than about 200 angstroms.

12. The method of claim 10, wherein the selected ratio is in the range of from about 1 : 10,000 to about 1:1.

13. The method of claim 10, wherein the dopant in the irradiated portion of the semiconductor substrate is at least partially electrically activated.

14. The method of claim 10, wherein the directing step further comprises controllably scanning the accelerated cluster ion beam.

15. A semiconductor device formed in part by the method of claim 1.

16. A semiconductor device formed in part by the method of claim 6.

17. A semiconductor device formed in part by the method of claim 10.