Atomic Diffusion Mechanism in BF2+ Implanted and Annealed n-Fz Si junction using Analytical Approach: Comparison with 2-D TCAD Process Simulation Result

Ion implantation controls the diffusion of the dopants inside the n-Fz Si bulk of the p+n Si microstrip detector. In order to understand about the diffusion mechanism of BF2 + molecules/dopants into the n-Fz Si bulk of Double Sided Silicon Strip Detector (back side) for the R3B Silicon Tracker, it is essential to know the precise information about the microscopic defect introduces inside the Si lattice of the detector for the next phase upgrade of the R3B experiment. The purpose of this paper is to present the atomic transport and electrical activation behavior of implanted BF2+ molecules/dopants at an energy of 80 KeV and a dose of 1015 ion/cm2 into the n-Fz Double Sided Silicon Strip Detector for the R3B Silicon Tracker after annealing at 400 °C - 1350 °C. The result shows the amorphous-crystalline interface position and recrystallization temperature using the results revealed from the 2-D TCAD process simulation of the Si microstrip detector.


Introduction
The next phase upgrade of the R3B (Reactions with Radioactive Relativistic Beams) Experiment, a part of NUSTAR scientific collaboration, requires high performance n-Fz (n-type Float zone) DSSSD (Double Sided Silicon Strip Detector).This leads to grow interest of fabricating high performance DSSSD that can provide improved electrical performance of the detector.In detector technology, the main purpose of ion implantation is to improve the electrical properties of the implanted layers.
Ion-implantation is a well-known technique to inject the p-dopants/impurities into the n-Si bulk [1].Ion implantation disrupts the crystallinity of the Si surface.At threshold or critical implant dose, an amorphous layer has been formed into the n-Fz Si bulk of the DSSSD for the R3B Silicon Tracker.Ion implantation gives crystal damage to the silicon lattice.Various factors of the implant, such as: energy, dose, and temperature, as well as post-implantation annealing plays an important role in the various type of defect formation.There are two types of crystal damage: (1) implantation damage and (2) residual damage that occurs in the silicon detector during the ion implantation.The implantation damage includes a high concentration of point defects: interstitials (I) and vacancies (V) develops into the Si surface.This damage can be just below the amorphous/crystalline (a/c) interface.However, residual damage has introduced various types of typical defects of extended defects (type I to type V) in the regrown layer like {113} or rod-like defect, small point defect cluster, dislocation loop (DL), twins, etc [2][3][4][5][6][7].
Ion implantation introduces damages at the vicinity of the p + n junction (back side) of the n-Fz DSSSD, leads to degrade the electrical properties of silicon lattice and makes the resistivity high.Annealing is required to repair crystal damage and to improve the electrical performance of the Si lattice.It electrically activates the 100% dopant atoms, which reside at the substitutional site inside the silicon lattice.At higher temperature, annealing can electrically activate the dopant atoms and boosts the effective charge carrier concentration, which are responsible for electrical conductivity.As a result, the effectiveness of annealing in enhancing the performance of the Si lattice depends on several factors, including the annealing temperature, the type and concentration of dopants, and the presence of impurities [2].
It has been known that implanting molecular ions such as BF2 + can produce more lattice damage than implanting boron atoms at the same dose and same energy in the n-bulk silicon of DSSSD.In BF2 + implantation, the molecules are considered to split into their constituent atoms (BF2 B + F + F) upon impact with the substrate surface with energies in proportion to their atomic masses [8][9][10].It was observed that in BF2 + implantation after annealing, fluorine gas diffuses up to considerable depth in the nsilicon bulk of the DSSSD.Therefore, it was observed that at a particular annealing temperature the Fluorine gas has escaped from the a/c interface and further not diffused into the n-silicon bulk of the DSSSD [8][9].
In this paper, we have shown atomic transport and electrical activation behaviors of BF2 + molecules/dopants just after ion implantation and after low/moderate/high temperature annealing in the n-Fz bulk of the DSSSD for the R3B Silicon Tracker.The results are shown using 2-D process TCAD (Technology Computer Aided Design) simulation [11][12][13][14][15][16].
In section 2 below, the device structure and its device and process parameters have been discussed.TCAD simulation results and discussions have presented in section 3. Section 4 describes the conclusion of the paper.

Device structure
The p + n junction at the backside of the n-Fz DSSSD for the R3B Silicon Tracker was constructed on a phosphorous doped 300 µm thick n-type silicon wafer substrate.The resistivity of the n-Fz Si wafer has taken 4 kΩ-cm, which is equivalent to the effective doping concentration of 1x10 12 cm -3 .During ionimplantation, silicon wafers were oriented along <111> plane and tilted 7 0 of the incident beam direction to minimize the channeling.After BF2 + implantation in the backside of the n-Fz DSSSD at an energy of 80 keV with a dose of 1x10 15 cm -2 , n-Si wafers isochronally annealed in N2 ambient for 180 minutes.The annealing temperature was varied over a wide range from 400 0 C to 1350 0 C. The schematic structure of an abrupt p + n junction side (back side) of the DSSSD used in the present TCAD simulation (see Figure 1).The device and process parameters (XJ-Junction depth, tox-oxide thickness, WN-Device depth) are shown in Figure 1.The critical dose for amorphization in the BF2 + implantation is 1x10 15 ion/cm 2 [3].Ion implantation introduces lattice defects along its path in the crystal lattice, leads to disrupts the crystallinity of the Si surface of the DSSSD.Annealing is required for reestablishing the crystallization and electrically activated the dopant atoms.The temperature at which crystallization restored is known as the recrystallization temperature.After recrystallization, regrown layers are formed just below the a/c interface, and extended defects develop in the regrown layer.It is different in size as well as in density.The old studies have shown that recovery of residual damage is not possible.It can be decreased up to a certain extent because it affects the device characteristics.
A cross section of the BF2 + implanted p + n junction (back side) in the n-Si lattice of DSSSD structure sketched in Figure 2, where the atomic diffusion mechanism of BF2 + dopants has been explained well.Immediately after BF2 + implantation in the n-Fz Si crystalline lattice of the DSSSD at an energy of 80 KeV and a dose of 10 15 ion/cm 2 , a high concentration of interstitial (I) and vacancy (V) point defects are introduced into the amorphized silicon lattice and dopant atoms have been found in electrical inactive sites.As soon as the BF2 + are implanted, the molecules will be fragmented into their component atoms upon impact with the substrate surface, which will produce an energy proportional to their atomic masses.Boron will implant at 17.93 KeV and fluorine at 31 KeV inside the n-Fz Si lattice.As a first step, it is necessary to determine whether fluorine ions are incorporated inside n-Fz Si lattices when BF2 + dopants are implanted.We know that for BF2 + implantation with an atomic concentration greater than 5x10 19 cm -3 , a substantial fraction of implant F + ions at 31 KeV remains in the Si substrate (see in Figure 3).Figure 3 shows the doping profile of BF2 + implantation at an energy of 80 KeV with a dose of 10 15 ion/cm 2 .The doping profile clearly shows that in case of just after BF2 + implantation the boron ions diffuse at a distance of probably 1.46 μm and the F + ions diffuse up to probably 0.76 μm in the p + n junction (back side) of the DSSSD (see in Figure 3) [12].In the process of Ion-implantation, the p-type dopant atom collides with the n-bulk silicon crystal lattice, and it loses its energy via nuclear stopping and electronic stopping processes.It knows that 15 KeV energy is sufficient to alter the position of a Si atom from the n-bulk Si lattice sites [1].After successive collisions, the continuous amorphous layers formed, and the transition of the dopant ions start (see in Figure 4).Figure 4 represents the pictorial view of the atomic diffusion mechanism after thermal treatment.If the migrated atom is a host, then the diffusion by a vacancy is known as self-diffusion.The probability of finding the migrate atom in the Si crystal lattice is three: 1. Boron acceptor ion 2. Phosphorous donor ion 3. Host Si atom.Localized point defect (I/V) are electrical inactive when they accepted or lose electrons (see in Figure 4).We know that both vacancy and interstitial can be neutral, and strongly charged and doubly charged.The diffusivity of F + in silicon is higher than boron so, it diffuses sharply than boron in the n-Si bulk of the DSSSD.The transition of dopant starts, which mean that the dopant tries to occupy the substitutional sites.It will become an electrically active (Bs) [1].
The diffusion mechanism that occurs inside the Si crystal lattice after thermal treatments are as follows; In the equations (1-3), the BI, ISi, and V are boron interstitial, interstitial silicon, vacancies.
In this paper, we have found the original a/c interface position in the BF2 + implanted n-Fz Si DSSSD, that are undergo into annealing process after ion-implantation from 400 0 C -1350 0 C. The F + ion reaches 200 m and B + at 5.95 m at a temperature of 1150 0 C inside the n-Fz DSSSD [12].Most of the old studies revealed that the F + concentrated near the original a/c interface [12].Therefore, based on the simulation result, it can be concluded that the original a/c interface located at 200 m inside the n-Si lattice from the Si-SiO2 interface of the DSSSD.
We know that the regrown layers have formed underneath the original a/c interface [1].Residual damage introduces various types of extended defects (see in Figure 5).Figure 5 shows the regrown layers underneath the a/c interface (manual visualization).The figure is not to the scale.It knows that the F + ion diffuses up to a considerable depth of 200 μm, and after that, it does not further diffuse into the silicon.Therefore, it can be concluded that F + ion escapes from the original a/c interface.It shows that F + ion diffuses up to 1150 0 C, and from 1150 0 C -1250 0 C, it will not diffuse into the silicon [12].Hence, we can say that F + ions in the shape of bubbles will escape at the temperature of 1150 0 C from the original a/c interface.We know that the residual defects diffuse and recombine sharply between the temperature of 1150 0 C -1350 0 C and [2] at a high temperature of annealing, dislocation loops (DL) and twins type defects completely disappear viz., optimization of the temperature is necessary.
It is necessary to find the value of sheet resistance of the annealed layer at a temperature between 1000 0 C-1350 0 C with the help of the TMA SUPREME-4 version 19999.4 for the understanding of recrystallization temperature.Table .1 shows the variations of sheet resistance of p + n diffused layer at very high-temperature annealing with percentage increase in effective charge carrier.It shows that the percentage increase in effective charge carrier decreased up to 1150 0 C and after increasing the annealing temperature up to 1350 0 C, the percentage increase in effective charge carrier increased.At higher temperature, annealing can electrically activate the dopant atoms and boost the effective charge carrier concentration, which are responsible for electrical conductivity.This step has attributed to the recrystallization of a continuous amorphous layer.These layers formed in BF2 + implantation by a critical dose of 1x 10 15 ion/cm 2 at an energy of 80 KeV.
Table 1.Variations of sheet resistance at a very high temperature annealing with % increase in effective charge carrier.

Conclusion
In this paper, an atomic diffusion mechanism of BF2 + implanted n-Fz DSSSD has been explained after annealing at different temperatures.2-D TCAD have been used to draw the pictorial view of the 80 keV 10 15 ions/cm 2 BF2 + molecules/dopant implanted before and after 400 0 C -1350 0 C annealing in the p + n Si junction of the DSSSD.The Fluorine from 80 keV 10 15 ions/cm 2 BF2 + molecules/dopant diffuses into silicon bulk up to a depth of 200 m till 1150 0 C.For further increase in the annealing temperature, fluorine has not diffused further into n-silicon bulk of the DSSSD.Thus, we can claim the position of the amorphous-crystalline (a/c) interface is 200 m from the Si-SiO2 interface of the DSSSD and recrystallization temperature is 1150 0 C.

Figure 1 .
Figure 1.The schematic structure of an abrupt p + n junction side (back side) of the Double Sided Silicon Strip Detector.3. Results and Discussions-Comparison with 2-D TCAD Process Simulation Results In this paper, the behavior of p-type dopants inside the n-Si substrate of a DSSSD using a 2-D process simulation result have been discussed.The T-CAD tool viz; TMA SUPREME-4 version 2000.4 l have been used to determine the sheet resistance of annealed diffused layer of the Si microstrip detector.

Figure 2 .
Figure 2. Atomic diffusion mechanism of BF2 + implanted p + n junction side (back side) of the n-Fz Double Sided Silicon Strip Detector.Figure is not to the scale.

Figure 4 .
Figure 4.After thermal treatment, atomic diffusion mechanism in p + n junction side (back side) of the Double Sided Silicon Strip Detector.Figure is not to the scale.

Figure 5 .
Figure 5. Regrown layers underneath the amorphous/crystalline interface (manual visualization).Figure is not to the scale.