In this paper the authors present a new method for making shallow p-type junctions in silicon by molecular ion implantation. Unlike current molecular ion implantation methods which use boron and fluorine molecules, this new method uses an element which is completely miscible in silicon. Note that fluorine is an element that saturates at a very low concentration in silicon. The compounds used in this new method are boron silicides and boron germanium molecules. These compounds have several distinct advantages including the facts that the co-element silicon (or germanium) has a very high saturation value in the silicon matrix, the co-element is massive and therefore creates more damage during implantation, and the co-element has a larger projected range than the boron. Note that the Rp for fluorine is shallower than that of Boron for a BF{sub 2} implant. Recent experiments indicate that BSi ion beams can be generated in a sputter ion source with efficiencies of 0.5% with respect to the generated Si beam. A plan to develop a new ion source that is compatible with current ion implantation systems is presented

Bennett, J.

Doyle, B.

Felch, S.

Larson, L.

Ling, C.H.

Ling, P.

Strathman, M.D.

Walsh, D.

UNT Digital Library

A New Method for Making Shallow p-type Junctions C O N f -  9 8 0 6 7 -  - Peiching Ling, Michael D. Strathman, Chih Hsiang Ling Advanced Materials Engineering Research (AMER) 250 Santa Ana Court, Sunnyvale CA 94086 USA Email: cling@amerinc.com Barney Doyle, David Walsh, Sandia National Labs Larry Larson, Joe Bennett, SEMATECH Susan Felch, Varian Research Center Abstract -- In this paper we will present a new method for making shallow p-type junctions in  silicon by molecular ion implantation. Unlike current molecular ion implantation methods which use boron and fluorine molecules, this new method uses an element which is completely miscible in silicon. Note that fluorine is an element that saturates at a very low concentration in silicon. The compounds used in this new method are boron silicides and boron germanium molecules. These compounds have several distinct advantages including the facts that the co-element silicon (or germanium) has a very high saturation value in the silicon matrix, the co-element is massive and therefore creates more damage during implantation, and the co-element has a larger projected range than the boron. Note that the Rp for fluorine is shallower than that of Boron for a BF2 implant. Recent esperiments indicate that BSi ion beams can be generated in a sputter ion source with efficiencies of 0.5% with respect to the generated Si beam. A plan to develop a new ion source that is compatible with current ion implantation systems wil l  be presented. I. INTRODUCTION As the overall dimensions of semiconductor devices are miniaturized and made ever smaller, the formation of very shallowly doped regions, e.g. those less than a tenth-micron in depth, becomes a major limiting factor in the fabrication process for all miniature devices, including but not limited to MOSFET and CMOS devices. The method used to make these vital CMOS and MOSFET transistors involves the formation of both n-type and p-type doped regions where n-type doped regions can be formed by the ion implantation of n-type elements in Group V A of the Periodic Table, and p-type doped regions can be formed by the ion implantation of elements in Group I11 A of the Periodic Table. Technical difficulties currently hamper the formation of shallowly doped regions. In most semiconductor fabrication processes, the dopant boron is used to form p-type regions, and boron has a very low atomic number (Z=5). Traditional ion implantation techniques pose several problems in implanting low 2 dopants. During the ion implantation process, a low Z dopant such as boron tends to channel through the crystalline structure of the substrate and forms a very undesirable, deep implantation profile tail where the concentration and depth of the dopant easily extends beyond the desired junction depth. This inability to control the junction depth seriously degrades device performance. Thus, it is difficult to use conventional implantation technologies to implant a low atomic dopant to form shallowly doped regions for VLSI circuits or ULSI circuits. Although there are techniques that are currently available to minimize the channeling effect, these techniques require additional processing steps and thus result in higher manufacturing costs. An existing method for implantation of boron ions is to use ultra low energy ion implantation techniques, but these techniques pose other problems. One problem is that it is difficult to manufacture ultra low energy ion implantation equipment, i.e. equipment generating energy less than 5 keV. Another problem is that using low energy ion implantation (energy less than 5 keV) can result in sputtering and Transient Enhanced Diffision (TED). There are several possible p-type dopants including B, Al, Ga, In, and T1. It is required that the dopant have enough solubility to create a substitutional site in the substrate. Only boron has a solid solubility limit which is sufficient to obtain the doping levels needed for ULSI semiconductors. Boron is the only choice. When the concentration of a dopant exceeds the solid solubility limit in a substrate it dramatically effects the annealing behavior of the implant. A much longer annealing cycle is needed to recrystalize the amorphous region and the residual damage is very hard to anneal'. Several techniques are currently used to overcome the technical difficulties associated with ion implantation of low z dopants and low energy ion implantation. In one case, the shallow p-region is formed using a heavier ion compound, i.e. BF,. Due to the higher mass of the compound, the constituent atoms of the BF, ion provide a shallower penetration depth for a given ion energy, thus enabling the formation of shallower p-type regions. The BF, ions provide another key advantage in that they reduce the channeling effect and thus the associated problems. This improvement is accomplished by the increased crystalline structural damage caused by the heavier fluorine atoms of the ion compound. Furthermore, since fluorine is neither a p-type nor an n-type dopant, the fluorine atoms that are introduced from the BF, DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal l i a b ~ t y  or responsibility for the accuracy, completeness, or use- fulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spe- cific commercial product, proc+ss, or service by trade name, trademark, manufac- turer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation. or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. DISCLAIMER Portions of this document may be illegible electronic image products. images are produced from the best avaiiable original document. ions do not directly contribute to the electrical performance of is implanted with Si,B ion compounds to create a shallowly the semiconductor device. p-doped region. In another implementation, a silicon However, the introduction of fluorine generates a substrate is implanted with &,Ed, ion compounds. In yet new set of problems2. Due to their low solubility in silicon, another implementation, a germanium substrate is implanted the fluorine atoms tend to migrate, particularly if the substrate with SixEd, or Ge,Edy ion compound. is heated. After a BF, ion implantation process, any In another embodiment, the ion compound subsequent process which requires elevation of temperature implanted is represented by the symbol E,,E,,E, where E, will tend to cause the implanted fluorine to migrate to the and E, represent two elements having high solubility in the silicon surface, i.e. silicon-oxide interface. In some cases, this substrate material with minimal detrimental chemical, migration cause the fluorine to coalesce and form a gap or electrical, or physical effects, and one of the elements can be bubbles at the interface which in turn causes contact problems the same element as that of the substrate material, E, such as poor contact reliability, high contact resistance, and represents an element which is an electron acceptor or donor unstable electrical performance. in the substrate, and x, y, and z indicate the number of For all the above reasons, BF, ion implantation is respective El, E,, and Ed atoms in the ion compound. For not a desirable solution for the fabrication of shallowly doped example, a silicon substrate is implanted with Si,Ge,Edjon regions. Therefore, there is a profound need in the art of compound. semiconductor device fabrication, particularly for devices Note that it is within the scope of the present method requiring shallowly doped regions, for a fabrication process to use ion compounds comprised of EnEd elements where En that will resolve these difficulties and limitations. represents two or more elements in various atomic ratios and Boron-hydrogen clusters have also been suggested as Ed represents the doping element. a possible method for introducing boron into a silicon The present method can also be applied to a substrate3. This method does not have enough mass to previously doped region to alter the doping in that region. produce fully amorphized layers at a depth greater than the For example, the present method can be applied to a Rp of the implant. The use of BSi or BGe solves this previously n-doped region where a p-type dopant ion difficulty. compound of the present invention is implemented to change 11. SUMMARY OF THE PROCESS The object of the present process is to provide a fabrication process for forming shallowly doped regions in semiconductor devices. This process provides a method for introducing a dopant in the fabrication of semiconductor devices where a doped region is formed that has a well- defined, reproducible shallow-depth and an uniform diffusion profile. This method provides a method for introducing a dopant in the fabrication of semiconductor devices where the contacts to the substrate surfaces of the resultant semiconductor devices have low contact resistance. This new method also has the advantage that it provides a method for introducing a dopant in the fabrication of semiconductor devices having shallow-depth regions which minimizes channeling effects in the implantation process. Briefly, the new method for manufacturing shallowly doped semiconductor devices is as follows: (a) a substrate where the substrate material is represented by the symbol Es (element of the substrate); and (b) implanting the substrate with an ion compound represented by the symbol E,.$,, where E, represents an element having high solubility in the substrate material with minimal detrimental chemical or electrical effects and can be the same element as the substrate element, E, (dopant element) represents an element which is an electron acceptor or donor having high solubility limit in the substrate material, and x and y indicate the number of respective E, and Ed atoms in the ion compound. For example, in implementing the method, a silicon substrate the total active dopant of the n-doped region, and vice versa. The method can also be used in conjunction with preamorphized substrate materials such as Si or Ge implanted silicon substrates. Note that the use of the term “ion compound” in this specification refers to both positively and negatively charged ion compounds which may have one or more charges and various extra hydrogen atoms attached to the ion compound. Depth 0 100 200 t I Boron @ 4.48keV + . -  _ _  Fluorine @ 7.75keV ‘ (BFd Fig 1 TRIM4 Rp for BF2 @ 20keV Depth 0 100 200 300 Boron @ 4.48keV Silicon @ 1 1 BkeV (BSi & BSi2 ) Fig 2 TRIM4 Rp for BSi @15.9keV BSi2 @ 27.25keV I Species I KV I Element I Ratio I Energy I Projected 1 Range BF2 20 B 11/49 4.48 201 F 19/49 7.75 187 BSi2 27.25 B 11/67 4.47 20 1 Si 28/67 11.4 209 BSi 15.9 B 11/39 4.48 201 Si 28/39 11.4 209 ~~ BGe2 65 B 11/59 4.48 20 1 Ge 741159 30.2s 280 BGe 35 B 11/85 4.48 201 Ge 74/85 30.9 285 Table 1. Projected Range Table for 5 keV equivalent boron 111. BENEFITS OF THE NEW PROCESS OVER BF2 IMPLANTS One of the current methods for making shallow p- type junctions in silicon is to use BF, ions. This method has several problems including the following: (a) the solubility of fluorine in silicon is low, (b) fluorine is not a dopant in silicon, (e )  fluorine precipitates and forms bubbles, (d) all of the fluorine must be removed before residual defects can be annealed out of the silicon. Finally, the projected range for fluorine (using a BF, molecule) is less than that for the boron. In the case of both silicon (BSi and BS,) and for germanium (BGe and BGe,) the boron projected range is less than that of that the profile shows evidence of channeling during the implant process. This is probably due to the zero degree tilt used during the implantation step. The dose of each species and the junction depth (Xj) are listed in Table 2. l o ' 5 ~ " " " " " " " " " " " " " " ' a  Em lam 15m 2mo Po0 DwH w . 1  Fig. 3 20 keV BSE Implant profiles I SDecies I DosefatomsIcmZ) I Xi*(nm) I I .,, I 1OB I 1.58e14 I 104 11B I 1.36e15 I 143 I I Table 2. Dose and Junction Depth 0 the other element in the molecule. Thus the amorphous zone created by the implant will be substantially deeper than the boron. See Fig 1 and Table 1. IV. CURRENT RESEARCH A. Molecular Ion Implantation A General Ionex Model 860 Cesium Sputtering ion source was used to generate molecular BSi ions. The sputtering target for the source was made by cold pressing SiB, powder with copper filings. The BSL beam current was 10% the intensity of the SC generated by this source configuration. The ions were extracted from the source at 2OkeV and analyzed in a 15 degree magnet. A slice of <loo> Silicon was placed in the Faraday Cup with a zero degree tilt and implanted at room temperature was performed at SEMATECH On Note that the TEM Micrograph of the unannealed sample the implanted silicon on a Cameca IMS4E 10B and 11B were shows an amorphous layer from fie surface to near the B S ~  monitored as positive secondary ions under 02+ Rp and a damage layer in the region close to the channelling bombardment at an impact energy of3 keV- Secondary ions tail in the SIMS profile. By using conventional techniques were from the center lo% Of a 170 we believe that we can elminate these channeling effects. rastered area. The atomic concentrations of 10B and 11B were calculated using a relative sensitivity factor (RSF) B. varian Associates determined from implanted standard. Stylus profilometry Varian Associates has begun an internal program to was used to determine the depth scale for the profiles. Note develop a production-worthy BSix ion source capable of high Fig 4. A I7O beam currents. The first phase of this project is the design and assembly of a proof-of-concept ion source. The ion species output and general performance of this source will be evaluated on a test stand. If these results look promising, the Phase 1 design will be modified to produce a prototype source for use in ion implanters. This prototype source would be installed on a Varian implanter and enable the BSix shallow junction process to be studied with silicon wafers. Finally, if Phase 2 is successful and the technology appears to meet the needs of the marketplace, then a commercial product will be developed. C. SIMS analysis of SiB, In an experiment designed to determine if BSi molecules made better positive or negative ions a SIMS analysis was performed on the SiB, powder. The powders were pressed into In foil. A minimum amount of powder was loaded so that charging would not be a problem during sputtering. The powders were sputtered with approximately 20OnA of 8 keV 02+ (incidence angle about 40 deg) and 6OnA of 14.5 keV Cs+ (angle about 25 deg). Analysis areas were chosen such that several small particles of the powders SaMTECn FILE: 212UoB SiB4 powder 14.5keV Cs+ si 10 3  lo^^.."""'."'"'"""''''. rm zm 301 40 M -(-I Figure 5. Cs+ Bombardment of Si4 Powder would be overscanned by the prima6 ion beams (about 170 center 20% of the sputtered area. Medium mass resolution We have presented a new method for the formation was used to reduce isobaric interferences. ne o2 of shallow p-type JUnCtiOnS Using molecular ion imp1antatiOn. bombardment showed a region around dZ 39 in the positive w e  have demonstrated this method by the implantation Of 20 ion mass spectrum with a significant interference of SiB’ keV BSi- into silicon. Considerable Work needs to be from 39K’. This interference was not present in the negative performed to further demonstrate the application of this new ion mass spectrum under Cs+ bombardment. Figure 4 shows method to the f~briCation Of Sub micron ULsl the results of the 0, bombardment. Note that the BSi yield is semiconductors. considerably less than the Boron yield. Figure 5 shows the results of the Cs+ bombardment. Note that the BSi‘ yield is greater than the boron yield. um x 170 um). Secondary ions were collected from the v. S W Y  ACKNOWLEDGMENT The authors would like to acknowledge the many discussions on diffusion mechanisms in silicon with Professor m S E L l  M T A  -- Jack Washburn, University of California, Berkeley. ems  a FILE 2 1 w  10 REFERENCES [l] “Regrowth of Implanted-Amorphous Si”, P. Ling, N.R. 107:  Wu and J. Washbum, Materials Research Society [2] ‘7s Fluorine Good or Bad for SourceDrains and Gate 1997, page 38. SiB4 Powder 8KeV 02+ E :  c -  0 N -  ,- Si Symposium, Proc. Vo17 1 (1 986) p. 179 - -B Y -  I 0 .  Oxides?” P. Singer, Semiconductor International, April 0 .  N -  [3] Cluster Ion Implantation for Shallow Junction T -  u i  1 0 4 +  B+Si Formation”, J.Matsuo, D.Takeuchi, T.Aoki and I. E Yamada, Proceedings of the Eleventh International Conference on Ion Implantation Technology (1 996) [4] “TRIM-90, The Stopping and Range of Ions in Solids”, J.F. Ziegler, J.P. Biersack, Pergaman Press, New York 10 3 rm zm 3m 40 ga 1985  lo^^'"^"'"""'"^'"'^''' Seconds Fig 4. Oxygen Bombardment of Si4 powder Smdia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company. for the United Sates Depar?mrnr of Energy mder contract DE-AC(N-94AL85000. 

A New Method for Making Shallow P-Type Junctions

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A New Method for Making Shallow P-Type Junctions

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