Optical initiation of nanoporous energetic silicon for

Invited Paper

Optical Initiation of Nanoporous Energetic Silicon for Safing and Arming Technologies Wayne A. Churaman*a, Collin R. Beckerab, Grace D. Metcalfea, Brendan M. Hanrahanac, Luke J. Curranoa, and Conrad R. Stoldtb a U.S. Army Research Laboratory, 2800 Powder Mill Road, Adelphi, MD, USA 20783; b Dept. of Mechanical Engineering, University of Colorado, Boulder, CO, USA 80309; c Dept. of Material Science, University of Maryland, College Park, MD, USA 20783 ABSTRACT Nanoporous silicon, commonly recognized for its photoluminescent properties, has gained attention as a new energetic material capable of energy density more than twice that of TNT. The addition of an oxidizer solution to inert nanoporous silicon results in an exothermic reaction when heat, friction, or focused light is supplied to the system. The energetic material can be integrated alongside microelectronics and micro-electro-mechanical systems (MEMS) for on-chip applications. This integration capability, along with the potential for large energetic yield, makes nanoporous energetic silicon a viable material for developing novel MEMS Safing and Arming (S&A) technologies. While ignition of nanoporous energetic silicon has been demonstrated for the purpose of propagation velocity measurements using a YAG laser, in this paper we show optical ignition for potential integration of the energetic with a miniaturized S&A device. Ignition is demonstrated using a 514nm laser at 37.7mW and a power density of 2.7kW/cm2 at a stand-off distance of 23cm. Raman spectroscopy verifies that significant stress in porous silicon is produced by a laser operating near the power density observed to ignite porous silicon. Lastly, we integrate the nanoporous energetic silicon with a MEMS S&A, and demonstrate transfer to a firetrain consisting of one primary and one secondary explosive using a thermal initiator to ignite the nanoporous energetic silicon. Keywords: Raman spectroscopy, metastable intermolecular composite, nanoenergetic, porous silicon, explosive

1. INTRODUCTION Nanoporous silicon, referred to as porous silicon (PS) in this paper, has emerged within the last decade as a new class of energetic material, capable of energy output more than twice that of TNT1. With the accidental discovery of a nanoenergetic reaction in PS at room temperature2, great potential has been shown for applications requiring the rapid release of energy, such as airbag initiators3. By itself, the PS is inert, and only becomes energetic once it has been filled with an oxidizer. A qualitative analysis of oxidizers is presented by Clement, et al3. The resulting exothermic reaction can be triggered via heat, friction, or focused light1. Figure 1 shows the flame produced as a result of igniting the PS with a spark after it has been infused with sodium perchlorate (NaClO4) oxidizer. Ignition of the energetic with a spark results in localized heating of the PS, which supplies energy to the system needed to trigger the exothermic reaction. Thermal initiation of the PS is demonstrated by Clement et al using an electrical pulse-driven heating bridge connected to an electrical circuit3. We have previously reported ignition using a hotwire monolithically integrated onto the PS using conventional lithographic techniques4. Initiation via friction has been demonstrated with frictional forces ranging from 1.5N down to 0.5N. The force is generated by moving a calibrated weight across the top of the oxidized PS to scratch the surface. These results are based on sensitivity experiments performed by the U.S. Army Research Lab in collaboration with the Armament Research, Development and Engineering Center. The frictional force causes the PS layer to crack and separate from the bulk silicon substrate. This induced stress causes the fuel and oxidizer to interact, such that an exothermic reaction is achieved. *[email protected]; phone 1 301 394-0952; fax 1 301 394-4562; arl.army.mil

Optical Technologies for Arming, Safing, Fuzing, and Firing VI, edited by Fred M. Dickey, Richard A. Beyer, Proc. of SPIE Vol. 7795, 779506 · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.860778

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Optical ignition of PS with a single pulse YAG-laser has been demonstrated for the purpose of measuring propagation speed3, although the power density and pulse duration are not reported. The variety of ignition methods, along with the ability to integrate the energetic on a silicon chip using bulk micromachining techniques, makes PS a viable technology for miniaturized S&A devices. Of these ignition techniques, optical ignition eliminates the need to interface the energetic material with on-board power or mechanical actuators, therefore providing an isolated triggering system. Integrating the optical system on chip as a standalone component provides an additional layer of protection to prevent accidental arming of the fuzing system.

Fig. 1. Reaction flame after igniting energetic PS with a spark

2. FABRICATION The PS is fabricated on a double-side polished, 1-20 ohm-cm, boron doped, oriented wafer. Before electrochemically etching the silicon wafer to form the PS layer, surface oxide is removed from the wafer using a 2 min buffered oxide etch (BOE) in dilute hydrofluoric acid. The wafer is then rinsed with water and dried with a stream of nitrogen. After removing the native oxide, the surface of the wafer is roughened by submerging it in a heated mixture of sodium hydroxide and water. Thick PS films of high porosity are typically susceptible to cracking as they dry after the etch process; however, using this texturing process5, PS films ~130 µm thick and with porosity ~74% (determined by a gravimetric method), can be fabricated. A thick PS film is favorable for enhanced energetic reactions. A layer of 20nmTi/85nm-Pt is deposited on the backside of the wafer to provide electrical contact. The contact resistance of the Ti/Pt layer is reduced by annealing the wafer in nitrogen at 700°C for 60 seconds. The sample is then placed in a Teflon etch cell, where electrical contact is made to the Ti/Pt backside. While protecting the backside with an o-ring seal, the topside of the wafer is exposed to a 2:1 mixture of HF:ethanol etchant that is added to the etch cell. With the electrolyte solution in contact with the silicon substrate, a gold wire, serving as the cathode, is submerged in the solution. Electrical contact is made between the cathode and Ti/Pt anode. Applying a current from an external power supply causes the formation of nanopores with a hydrogen passivated surface. The samples are allowed to dry in pentane which further reduces the chances of cracking because pentane has very low surface tension. The fabrication process is similar to that presented by Churaman4 et al, except the wafer is textured with sodium hydroxide and dried in pentane. Figure 2A shows the pyramids at the surface that result from the texturing process and Figure 2 B shows the full thickness of the film.


Fig. 2. SEM M images of PS. A. Cross sectioon near surface of PS, B. Full cross-section c of PS film.

3 EXPERIM 3. MENTAL RESULTS R Pulsed and coontinuous wav ve laser configuurations were tested to determ mine the speciffic wavelength at which the PS P would absorb sufficcient energy to ignite. Two wavelengths werre analyzed to determine whiich was more readily r absorbeed by the PS. In the first f test, a fem mtosecond Ti::Sapphire laserr beam at 8000nm wavelengtth, incident poower of 50mW W, and a repetition ratte of 250kHz was w used to opttically trigger the t sample. The laser beam was w focused to a spot size diaameter of approximatelly 50µm. Ign nition was nott achieved at a power denssity of 2.5kW//cm2. After tryying the IR laaser, the Ti:Sapphire laser l was frequ uency doubled (400nm wavellength) and atttempts to ignitee the sample were w made withh 0.1mW and 10mW. At a correspo onding power density d of 0.0005kW/cm2 andd 0.5kW/cm2, respectively, r thhe laser was unable u to ignite the PS. The PS samp ple was prepareed using the fabbrication proceess outlined aboove. Failure too ignite the PS with the 800nm light could be the result of inaddequate spectraal absorption as a well as the femtosecond pulses being too fast, resulting in sampling s cooliing. While bettter spectral abbsorption may have h occurred using the 400nm frequency doubled Ti:Sapphire laser, the pow wer density waas too low to trigger an ignnition event. These T tests werre performed in a dry environment. After attempting to ignite samples s with a femtosecond Ti:Sapphire T lasser, several diffferent PS sampples ~ 10 mm x 10mm in dimensionn were prepared d to determine if a continuous wave laser would w ignite thee energetic matterial. A 12µm m layer of parylene wass deposited ontto several of thhe PS samples after applying 3.2M NaClO4. Because igniition of the sam mple was attempted in humid air, thee parylene acted as a moisturee barrier to preevent the oxidiizer from absorrbing water moolecules. It was shownn that the hygrroscopic naturee of the oxidizeer led to a reduuction in the ennergetic outpuut. Since parylene may inhibit heat transfer t to the porous siliconn, the other saamples were noot packaged with w parylene. The continuoous wave laser used in this test was a Model 177G Series S air-cooled argon laser designed by Spectra Physicss. The continuoous wave laser outputss at 514nm. Th he laser can ouutput a maximuum power of 4220mW, and haas a beam diam meter of approxximately 0.82mm. Thee PS was placeed in direct linee of sight of thee laser, approximately 23cm away from thee beam. Figure 3 shows a test setup, where the beam was confineed on five sidees and the sam mple was vertically positionedd in the enclosure. The enclosure waas designed with a small aperrture to allow the t beam to ennter the box. For F each powerr level tested, the laser beam shutterr was first clossed, and the beeam allowed too come up to power. p The shhutter was thenn opened and thhe beam immediately illuminated th he PS.


Fig. 3 Test T set-up used d to ignite the nanoporous siliccon

Using the mounted m paryleene coated eneergetic silicon sample, the 0.82mm 0 beam was positioneed at the centeer of the sample. Atttempts to ignitte the parylenee coated PS saamples were made m at power densities of 18.9-56.8W/cm 1 m2 for 50 seconds, butt the samples failed to ignite. Failure to ignite i that paryylene coated sample s was pootentially the result r of inadequate thhermal transferr to the PS. The texturedd PS sample wiithout parylenee was ignited after a 5 minutess of being directly bombardeed by the laser beam at 420mW (79.5W/cm2). How wever, these reesults were inconsistent and some s of the sam mples did not ignite. i Figure 4 shows a damaged sppot caused by the t laser beam on a PS layer which failed too ignite. The time t needed foor the laser to iggnite the sample mighht be dependentt on the amounnt of moisture absorbed a by thhe oxidizer, whhich is experiennced when trannsporting the PS samplle from the dry y box to the lasser test enclosuure. As the poroous layer beginns to take on moisture, m it appears that more energyy must be supplied to the system to achieve ignition. An A ignition tim me greater thaan 5 minutes was not practical for applications th hat require rapiid response.

Fig. 4. Daamage (blue-greeen spot near ceenter of figure) to PS layer resulting from bom mbarded by lasser beam. The clear c particles are residual r NaClO4 crystals.

t power den nsity incident on o the energetiic PS, the laseer beam was foocused using a 50mm opticaal lens to To increase the reduce the beeam diameter to t 42µm. Usinng the set-up shhown in Figuree 3, three sampples without thhe parylene were tested with the focuused beam. Sam mple 1 was dried for 10 minuutes in a nitrogen dry box afteer the oxidizer was applied too the PS.


The sample was then removed from the dry box and transferred to the laser testing area, while trying to minimize exposure to moisture. Similar to the samples attempted without the 50mm lens, the porous layer began to show signs of cracking when exposed to ambient air. The sample was mounted and the laser tuned to 420mW (30.3kW/cm2). Instantaneous ignition of the nanoporous energetic silicon took place once the shutter was opened. The power was then reduced to 100mW (7.2kW/cm2) to determine if ignition could be achieved. At 100mW, an identical sample to that ignited at 420mW was instantaneously ignited. A final attempt was done to show ignition below 100mW with a similar sample. The sample was dried for 6.5 minutes in ambient air, rather than in nitrogen, and was successfully ignited instantaneously at 37.7mW, or a power density of 2.7kW/cm2. This was the lowest output power available for the laser. Focusing the beam creates a 400x increase in power density, and results in successful ignition at the lowest available power. Figure 5 shows successive frames of optically initiated energetic PS. The power density was comparable to the femtosecond Ti:Sapphire laser beam at 800nm, which may indicate either that the PS showed a greater affinity for absorption at 514nm or pulsing the beam prevents the reaction from being triggered.

Fig. 5. Successive frames showing optical initiation of nanoporous energetic silicon

4. PS MATERIAL CHARACTERIZATION Our results indicated that PS-NaClO4 is ignited at a critical laser power density of 2.71kW/cm2. Using Raman spectroscopy, we examine the effect the laser heating has on the PS structure. The crystallite sizes of silicon in PS, strain in the structure, and strain effects induced by heating can be characterized using Raman spectroscopy6-12. The peak in the Raman spectrum of PS typically is red shifted and broader than that of the standard Si peak at ~520 cm-1 and FWHM of ~4 cm1. When heated, this peak is further shifted and broader. This shift and broadening is a result of stress in the sample. Figure 6 shows Raman spectroscopy (Renishaw inVia, 514nm laser wavelength, 100x objective lens) results of a single crystal Si sample and of a PS sample prepared under the same conditions as the PS used for ignition. The pores are not filled with NaClO4 in these measurements as the laser could ignite the sample and damage the instrument. The literature reports that power densities between 0.3 and 1kW/cm2 will not induce stress from heating9-10. Additionally, we find that long signal accumulation times can induce heating effects as the sample slowly heats from the laser illumination. The lowest power setting, 0.16kW/cm2, produces very similar Raman spectra for accumulation times up to 250s. However, as the accumulation times increases to 750s, we see a peak broadening and shift. At 1.62kW/cm2 with an accumulation time of 150s, which illuminates the sample with the same energy (243kJ/cm2) as 0.16kW/cm2 for 750s, very similar spectra are produced. The spectrum is also very similar at 8.1kW/cm2 for 30s (243kJ/cm2). At 8.1kW/cm2 for 10s (810 kJ/cm2), the spectrum shifts even further.


The results in Figure 6 demonstrate that as laser power increases, the time to induce a shift in the PS spectrum, which corresponds to a stress, decreases. An exact numerical comparison of Raman results to the laser ignition results is difficult because a Raman spectrum requires at least several seconds to collect; however, in the ignition tests, at several kW/cm2 of power, the laser instantaneously ignites the sample. A good estimate of the laser power necessary for ignition is made by comparing the results at 0.16kW/cm2 and at 1.62kW/cm2 with calculated Raman spectra shown as dotted curves in Figure 6. The calculated spectra, which are reproduced from a model proposed by Sui et al6, equation 1, and is similar to the model by Campbell and Fauchet7 fit the data well for the Raman spectra without heat induced effects. In equation 1, I(ω) is the intensity, q is expressed in units of 2π/a, L is the crystallite size in units of a, a is the lattice constant of Si of 0.54nm, Γ is the natural linewidth for crystalline Si (~4 cm-1), and ω(q) = A - Bq2 is the dispersion relation for LO phonons along [001] in Si where A = 520.5cm-1 and B = 120cm-1.

.

(1)

The modeled data assume crystallite sizes of either 2 or 3nm, and do not account for stress in the sample. The measured spectrum lies between the curves for 2 and 3 nm, indicating the sample probably has a crystallite size between those two values. For power densities greater than 0.16kW/cm2, or for long accumulation times, the spectrum is shifted and broadened and the model no longer fits the data. The error in the fit results from stress in the PS structure induced by heating effects.

Fig. 6. Raman spectroscopy results of PS samples without NaClO4. The solid curves are spectra collected at unique power densities and accumulation times. The dotted curves represent predicted spectra6. The vertical bars aid in visualizing the shift of each sample.

The Raman peak shift, Δω, can be used to predict the stress, σ, in the PS structure11, equation 2. ∆

.

(2)

In equation 2, Sij are the strain tensor components and vary depending on the porosity of the sample, p and q are the phonon deformation potentials, and ω0 (~520cm-1) is the Raman peak shift for single crystal Si 11-12. For porosities ranging from 70-80%, the stress values of the peaks are calculated in Table 1. These values reveal the sensitivity to


ignition of the PS sample. The intrinsic stress of the PS lies in the range of 35-83MPa, and the stress of the PS under laser heating that leads to ignition is in the range of 43-123MPa. These values are quite similar and explain why only a small mechanical force can ignite the PS. Table 1: Raman spectroscopy results and predicted stress in the sample at several laser power densities and accumulation times.

laser power density (kW/cm2)

duration (s)

energy density (kJ/cm2)

peak position (cm-1)

Δω (cm-1)

σ (Mpa)

8.10 8.10 1.62 0.16

100 30 150 750

810 243 243 120

510.8 512.5 511.7 512.5

-10.1 -8.4 -9.3 -8.4

52-123 43-102 48-113 43-102

0.16

250

40

514.2

-6.8

35-83

5. APPLICATIONS Optical ignition of energetic PS is an enabling technology for safing, arming, and fuzing, where the energetic material can be used in an explosive train to trigger a much stronger secondary charge once a specific event is detected. On-chip integration of energetic PS with micro-electro-mechanical systems (MEMS) allows for incorporation of the energetic with a suite of sensors and actuators. These sensor suites can be directly coupled with the energetic output of the energetic PS for an additional level of intelligence for a more effective safing and arming (S&A) mechanism. Because the energetic material is directly fabricated on a silicon chip, it offers a unique alternative to slurry based explosives. In a collaborative effort between the U.S. Army Research Laboratory and the U.S. Army Armament Research, Development and Engineering Center, work was conducted to integrate the energetic PS with a MEMS based S&A system for small munitions. A conceptual rendering of the MEMS S&A is shown in Figure 7.

Fig. 7. Diagram showing energetic stack, where PS is integrated with an S&A

A primary explosive was positioned on top of the energetic PS, which was packaged in a dual in-line package (DIP). An additional layer of secondary explosive was then placed in direct contact with the primary explosive. The chain of energetic transfer is shown in Figure 8. The energetic output of the energetic PS was transferred to the primary, which then triggered detonation of the secondary explosive.


Fig. 8. Ch hain of charge trransfer from prrimary to secon ndary

Preliminary results r showed d that when ignnited with an electro-therma e l initiator, the energetic outpput of the enerrgetic PS could ignite another a primarry explosive which w was then able to ignite a secondary exxplosive. A 4m mm diameter PS S sample with an integgrated thermal initiator was placed p underneath a metal covver that was paacked with a primary. p The seecondary explosive waas placed on top p of the primarry. Finally an aluminum a blocck was placed on o top of the sttack. Ignition of o the PS resulted in thhe direct ignitiion of the prim mary. Because a visible indenntation was prresent in the alluminum blockk, it was highly possibble that detonation of the seecondary was achieved. Thee results indiccated that the energetic e PS could c be directly integgrated with an S&A and trannsferred to prim mary and seconndary charges.. Figure 9 show ws a slider meechanism that was desttroyed after traansferring chargge between thee energetic PS, primary and secondary chargge.

Fig. 9. S& &A destroyed after a secondary explosive deton nate

6. CO ONCLUSION NS Optical ignittion of a nano oporous energeetic silicon sam mple is demonnstrated using a 514nm conntinuous wave laser at 37.7mW outtput power, wiith an effectivee laser power density of 2.771kW/cm2. Raaman spectrosccopy data veriifies that significant sttress in PS is produced by a laser operatiing near the power p density observed to iggnite PS. Thee Raman spectra also reveals that fo or a given sam mple collectionn time, the PS Raman peak will shift furthher (indicatingg greater stress in the sample) s as laseer power is increased. Transsfer of the enerrgetic reaction to a primary annd secondary charge c is also separateely achieved ussing an electro-thermal initiator for the eneergetic PS. Com mbining the abbility to opticallly ignite the PS with the t energy tran nsfer to anotherr energetic proovides enablingg technology foor a fully integrrated S&A meechanism that uses PS S as a low-inp put energy, onn-chip initiatorr. With the abbility to integrrate the siliconn energetic with w bulk micromachinning techniques, the nanoporrous energetic silicon providdes a platform to integrate opptics and sensoors for a fully self-conntained device for intelligent munitions.


7. ACKNOWLEDGEMENTS The authors would like to acknowledge Dr Charles Robinson and Mr. Thinh Hoang of the U.S. Army Armaments Research, Development, and Engineering Center for their assistance in transfer testing. The authors also thank Dr. Paul Shen and Mr. Matthew Chin of the U.S. Army Research Lab for help modeling the Raman spectra. Funding for C.R. Becker is provided by a Department of Defense SMART scholarship.

8. REFERENCES [1] Currano, L.J., Churaman, W. and Becker, C., “Nanoporous silicon as a bulk energetic material,” Proc. Transducers, 2172 (2009). [2] Mikulec, F.V., Kirtland, J.D., and Sailor, M.J., “Explosive nanocrystalline porous silicon and its use in atomic emission spectroscopy,” Adv. Mater., 14, 38-41 (2002). [3] Clement, D., Kiener, J., Gross, E., Kunzner, N., Timoshenko, V., and Kovalev, D., “Highly explosive nanosiliconbased composite materials,” Phys. Status Solidi (a), 202, 1357-1364 (2005). [4] Churaman, W., Currano, L., Becker, C.; “Initiation and reaction tuning of nanoporous energetic silicon,” J. Phys. and Chem. of Solids, 71(2), 69-74 (2010). [5] Shailesh N. Sharma, R.K. Sharma, G. Bhagavannarayana, S.B. Samanta, K.N. Sood, S.T. Lakshmikumar. “Demonstration of the formation of porous silicon films with superior mechanical properties, morphology and stability.” Mater. Lett., 60, 1166-1169 (2006). [6] Sui Z., Leong P.P, Herman I.P., Higashi G.S., and Temkin H., “Raman Analysis of Light-Emitting Porous Silicon,” Appl. Phys. Lett., 60, 2086-2088 (1992). [7] Campbell I.H., and Fauchet P.M., “The Effects of Microcrystal Size and Shape on the One Phonon Raman-Spectra of Crystalline Semiconductors,” Solid State Comm, 58, 739-741(1986). [8] Yang M., Huang D.M., Hao P.H., Zhang F.L., Hou X.Y., and Wang X., “Study of the Raman Peak Shift and the Linewidth of Light-Emitting Porous Silicon,” J. Appl. Phys., 75, 651-653 (1994). [9] Manotas S., Agullo-Rueda F., Moreno J.D., Ben-Hander F., Guerrero-Lemus R., and Martinez-Duart J.M., “Determination of stress in porous silicon by micro-Raman spectroscopy,” Phys. Status Solidi A-Appl.Res., 182, 245248 (2000). [10] Kozlowski F., and Lang W., “Spatially Resolved Raman Measurements at Electroluminescent Porous N-Silicon,” J. Appl. Phys., 72, 5401-5408 (1992). [11] Lei Z.K., Kang Y.L., Cen H., and Ming H., “Variability on Raman shift to stress coefficient of porous silicon,” Chinese Phys. Lett., 23, 1623-1626 (2006). [12] Bellet D., Lamagnere P., Vincent A., and Brechet Y., “Nanoindentation investigation of the Young's modulus of porous silicon,” J. Appl. Phys., 80, 3772-3776 (1996).
