Mater. Res. Soc. Symp. Proc. Vol. 1 © 2011 Materials Research SocietyDOI: 10.1557/opl.2011.322

Deposition and Etching of Conformal Boron Films for Neutron Detector Applications Nicholas LiCausi1, Justin Clinton2, Yaron Danon2, James J.-Q. Lu1 and Ishwara B. Bhat1 1Electrical, Computer and Systems Engineering Department, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180, U.S.A. 2Mechanical, Aerospace and Nuclear Engineering Department, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180, U.S.A.

ABSTRACT

In this work, the deposition of boron using low pressure chemical vapor deposition (LPCVD) has been investigated on planar and deep reactive ion etched (DRIE) Si substrates. Deposition rate and conformal coverage have been studied. Additional studies of “dry” RIE etching and “wet” chemical etching of the deposited boron films are presented. Deposition rates as high as 1 µm/hr and conformal coverage ratios of ~80% have been achieved. Etching rates for various methods studied range widely from 0.35 µm/hr to 1.2 µm/min.

INTRODUCTION

There has been a significant research effort surrounding the detection of thermal neutrons for the discovery of nuclear materials. Typically these detectors work on the principle of converting spontaneously or induced fission thermal neutrons into alpha-particles and 7Li ions, which are then sensed by a Si p-n junction. The conversion of neutrons to these particles is necessary because the neutron is charge-neutral and cannot be sensed by the standard p-n junction. To detect thermal neutron, a converter material with high thermal neutron cross-section is desirable; typically 10B or 6LiF is used [1,2]. 10B has been selected in this work because it has a higher cross-section and therefore can result in higher detection efficiency. Boron is also compatible with the device fabrication process and thus can simplify the fabrication of the detector device.

To maximize the probability that an incident neutron can interact with a 10B atom, the 10B layer must be very thick (45 µm). However, for the generated alpha-particles to reach the p-n junction, the 10B layer must be thin (2-3 µm); otherwise, the alpha-particles can be reabsorbed by the 10B. To meet this contradicting thickness requirement, we proposed to use 10B filled high aspect-ratio holes and trenches with continuous Si p-n junctions on all areas [3,4]. A schematic representation of this detector cross-section is shown in figure 1.

Thorough GEANT4 simulations were carried out in our earlier studies which determined that the optimal detector geometry is comprised of a honeycomb-like structure with hexagonal holes that have a flat-to-flat diameter of 2.8 µm and 1.0 µm Si walls between these holes [3]. Hole-depths as deep as 50 µm were experimentally studied. Due to the very high aspect-ratio of these features and the availability of processing equipment/materials, low pressure chemical vapor deposition (LPCVD) was selected for the boron deposition. It is believed that LPCVD can be tailored to deposit continuous, conformal films unlike other deposition techniques [5]. This is necessary to obtain void-free hole and trench filling. Other deposition techniques, like sputtering or evaporation, typically result in “capped” holes before complete filling. Furthermore, this work

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intends to find a suitable method for “wet” or “dry” etching of the deposited boron films to allow for detector fabrication.

Figure 1. Schematic representation of the thermal neutron detector illustrating the high aspect ratio trenches that must be filled with boron-10. The representation shows 3 boron-10 filled holes, but the actual detector would have thousands or more depending on detector size.

EXPERIMENT

In order to fabricate the neutron detector, both boron deposition and subsequent etching is required. The full fabrication process of the detector has been outlined previously [4]. Boron deposition has been studied on planar (100)Si substrates and DRIE etched substrates. The 10B films were deposited in a “home-built” quartz LPCVD reactor. “Dry” etching studies were conducted with commercially available RIE equipment. “Wet” chemical etching was done in a basic fume-hood using standard glassware and hotplates.

Boron deposition

LPCVD was carried out in a cold-wall horizontal quartz reactor. Samples were placed on an inductively heated SiC-coated graphite susceptor. Pressures ranging from 200–1000 mTorr and temperatures ranging from 400–700 °C were studied. Diborane (B2H6) was used as the boron source, with a typical flow of 50-100 sccm of 1% diborane in hydrogen. Diborane was selected to allow for easy integration with existing semiconductor fabrication equipment. Diborane is a gas at room temperature and allows for easy transport. Furthermore, it is extensively used as a dopant source in epitaxial Si deposition. For obtaining continuous p+-Si layer, the wafers had a ~50 nm thick boron film deposited and were then annealed at 900oC for 25 minutes in the presence of hydrogen gas. In addition to optimizing the LPCVD process for deposition rate and conformal coverage, film stress was also studied using Tencor Flexus 2320 dual wave length thin film stress system. Conformal coverage ratio, defined as the ratio of film thickness on the bottom of the trench to the top of the trench, was determined using scanning electron microscope (SEM).

Al

Ti

n+-Si

Al

Al

p+-Si

B

n--Si

Boron etching techniques

Both “wet” chemical etching and “dry” reactive ion etching (RIE) of boron were studied. RIE and inductively coupled plasma (ICP-RIE) etching employed a PlasmaTherm 73 and Trion Phantom III reactive ion etch system, respectively. A number of RIE chemistries were investigated, including H2, SF6+H2 and SF6+O2. For etching with H2, typical process conditions maintained were: pressures at ~200 mTorr, RIE power of 150 W and H2 flow of 20 sccm. RIE studies with SF6+H2 chemistries utilized similar pressure/RIE power conditions with H2 and SF6 flows of 15 sccm each. All RIE etching experiments were conducted at room temperature.

ICP-RIE was studied using Trion Phantom III system with SF6+O2 plasma chemistries. Similar chemistry has shown promising results for electron cyclotron resonance plasma etching (ECR-RIE) [6]. ICP-RIE was investigated in the hope of finding an etch technique with improved etch-rate and high selectivity over Si etching. Initial baseline processing was done with a reaction pressure of 120 mTorr and SF6 and O2 flow rates of 20 and 40 sccm, respectively. The RIE power was maintained at 200 W and the ICP power was held at 600 W.

In addition to dry etching, “wet” chemical etching was studied. Chemical etching techniques are attractive for their parallel processing capabilities and their high selectivity. Although literature review found no reports of solutions that would etch boron, boron oxide (B2O3) is soluble in ethanol [7]. With this in mind, a mixture of ethanol and hydrogen peroxide in various ratios have been studied. The hydrogen peroxide was intended to oxidize the boron, forming boron oxide which the ethanol would dissolve. Studies utilized mixtures at room temperature and 75 °C.

DISCUSSION

Boron deposition

Growth rates as high as 1 µm/hr were observed with good conformal coverage and surface morphology, especially for pressure lower than 300 mTorr at 600 °C and higher, with B2H6/H2 flow rates of 80 sccm. As the growth temperature is increased above 600 °C, the film surface morphology tended to become rough with a grainy surface.

(a)

(b)

(c)

Figure 2. Figure (a) shows the SEM cross section of trenches with boron deposited at 600oC at 300mTorr. (b) SEM of the top corner and (c) SEM of the trench bottom. Note that conformal coverage ratio as high as 80% is obtained for linear trenches.

Conformal coverage ratios above 80% were demonstrated for linearly trenched structures with aspect ratios above 6:1. A partially filled trench is shown in figure 2. Hole geometries have a slightly lower conformal coverage ratio, typically ~40%, due to the difficulty of delivering

precursors into the holes and/or removing reaction byproducts. Other variations to standard LPCVD are being investigated for future work to further improve conformal coverage.

High film-stress was evident. Stress often leads to spontaneous peeling of the film [8]. It was observed that this delamination can occur within minutes for sufficiently thick films (2-4 µm) or after weeks or months for thinner films (~0.5 µm). Typical stress for a 0.6 µm film on a 2” wafer is ~850 MPa at 300K. This stress may be caused by thermal expansion coefficient (TEC) mismatch between the boron and silicon substrate. The TECs of β-rhombohedral boron and silicon are reported to be 5-7 µm·m-1·K-1 and 2.6 µm·m-1·K-1, respectively [9,10]. This mismatch may be the reason that thicker boron films can delaminate very quickly upon cooling, while thinner films remain intact for much longer.

(a) (b)

Figure 3. If a single bulk film growth is conducted film stress can (a) catastrophically delaminate boron films. (b) With the use of a multi-stage growth technique low stress films can be deposited that remain stable.

To mitigate the film delamination, a multistage deposition and cooling technique was developed. By depositing a sufficiently thin film – typically ~100 nm – then cooling to room temperature the film is able to non-catastrophically relax. The film is able to deform during cooling via micro-crack formation and then subsequent deposition steps are carried out on a relaxed boron film. Depending on the desired final film thickness, 2-5 deposition/cooling cycles may be necessary. Films deposited on 2” wafers (and larger) with this multi-step technique were stable for many weeks with no signs of degradation as seen in figure 3(b).

Boron etching techniques

H2 plasma RIE resulted in an initial etch rate of ~0.8 µm/hr. However, the H2 RIE did not have a linear etch-rate. As etching continued, the etch rate slowly decreased, and the surface of the etched region became rougher and darker in color. Hence the etch rate degradation may be caused by non-volatile byproduct re-deposition. The film roughening was verified with SEM which is shown in figure 4. The H2 plasma etching scheme was used because the underlying Si substrate and Si3N4 etch-mask had significantly lower etching rates in H2; on the order of 50 nm/hr. Although the observed selectivity was ideal, the slow, nonlinear boron etch rate observed makes the H2 RIE scheme undesirable.

Figure 4. Boron films become very rough after H2 RIE. After sufficient etching the etch rate degrades to negligible value most likely due to non-volatile byproduct re-deposition.

RIE etching of boron with SF6+H2 also showed very low etch rates. This is probably due to the low plasma density that is produced by standard RIE. In order to achieve higher etch-rates, the SF6+O2 ICP-RIE scheme was used, resulting in notable increases in etch rate. Etch rate improved from ~13 nm/min for standard RIE schemes to ~500 nm/min for the ICP-RIE method. The improved etch rate is ideal for removing the boron from the underlying silicon in order to form the p+-Si ohmic contact. Unfortunately, the SF6+O2 ICP-RIE process preferentially etches silicon more rapidly than boron, by a ratio of 2.4:1. This is not ideal because it is important to leave the underlying Si un-etched to preserve the p+-Si layer on the top surface as well as minimize any crystal damage from the plasma. Fortunately, the SF6+O2 etch chemistry has strong selectivity over SiO2. The etch rate of SiO2 is only ~60 nm/min resulting in a B:SiO2 selectivity of 8.3:1. The ease of TEOS SiO2 deposition and its high selectivity makes it an ideal candidate as an etch stop material. This SiO2 etch stop layer preserves the crystalline quality of the underlying Si and hence high device performance can be maintained.

Figure 5. Boron etching in a solution of ethanol and hydrogen peroxide increases with the concentration of hydrogen peroxide.

“Wet” chemical etching was also studied. Neither the ethanol nor the hydrogen peroxide used in this etching solution has any effect on silicon or on boron by itself, but the solution

PR masked boron

boron after etching

Si

etches boron but not silicon. Unfortunately, this solution yielded very low etch-rates at room temperature. To achieve measurable etch rates, the solution was maintained at 75 °C. It was observed that the etch rate increases with the concentration of H2O2 as shown in figure 5. At 75 °C with a ratio of hydrogen peroxide to ethanol of 5:1, an etch rate as high as 6.3 nm/min was achieved. This is a significant achievement, allowing for the removal of the remaining boron without damaging the underlying silicon.

CONCLUSIONS

This paper presents the first experimental results of boron thin-film deposition using LPCVD with diborane. Reasonable deposition rates as high as 1 µm/hr and conformal coverage ratios as high as 80% have been observed. Furthermore, these boron films have been etched with both “dry” plasma RIE and ICP-RIE methods and “wet” chemical etching techniques. RIE etching rates as high as 0.5 µm/min have been achieved and chemical etching can achieve complete selectivity over silicon. This is the first known results of room temperature, wet chemical etching of boron thin-films. These film deposition methods and etching techniques are major milestones towards the fabrication of a high-efficiency solid-state neutron detector.

ACKNOWLEDGMENTS

This work is supported by the US Department of Homeland Security under grant award number 2008-DN-077-ARI008-003. Project funding has also been provided by the Nuclear Energy University Program. Additional student support has been provided by the National Science Foundation, program number 0333314 IGERT.

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