Transfer of Thin Film Gallium Phosphide onto Glass for Integrable Optical Filters

Maanav Ganjoo, John A. Carlson, Saoud A. Al-Mulla, James Brown, Brian T. Cunningham, John M. Dallesasse1

1University of Illinois at Urbana-Champaign, Department of Electrical and Computer Engineering, Urbana, Illinois, 61801Phone: (217) 377-9551, Email: ganjoo2@illinois.edu

Keywords: Gallium Phosphide, Anodic Bonding, Smart-Cut, Helium Ion Implantation, Optical Filter

Abstract:

A single-layer optical filter made from thin film gallium phosphide (GaP) is envisioned and a fabrication flow is outlined, with current progress on process development reported. Ion-implantation is simulated and performed on bulk GaP with He+, followed by a field-assisted thermal bonding technique that simultaneously bonds a thin GaP film onto a borofloat glass substrate and removes the GaP substrate. The resulting thin films have consistent thickness, both within and between runs, and RMS surface roughness of < 10 nm. Dry-etch processes that further reduce the thin film material are characterized and designs for etching gratings into them are developed. This process is shown to be a reliable means of creating thin films of consistent thickness and smoothness in GaP, for the purpose of establishing visible wavelength filters for spectroscopic applications.

INTRODUCTION

Numerous advances in micro-fabrication have allowed for microfluidic lab-on-a-chip systems with widespread application in low-volume, small-form factor biomolecular assays [1]. In a previous paper [2], a multi-layer Linear Variable Filter (LVF) was utilized as a compact and versatile optical spectral analyzer in such a system. As a successor to this, a simpler, single-layer LVF is envisioned, to reduce fabrication cost and complexity and improve the manufacturability of this technology. This paper outlines the progress made in fabricating a gallium phosphide (GaP) based optical filter, where the preparation of a thin semiconductor film via field-assisted thermal bonding [3] (FATB) and helium exfoliation are presented, followed by grating definition and preliminary optical characterization.

The single-layer optical filter calls for a transparent, high index contrast thin film material. Gallium Phosphide (GaP) is suitable, owing to its large refractive index and widespread use in LED technologies [4]. To apply the described process to GaP, silicon based techniques, such as FATB and Smart-Cut® [3], [5] were adapted. FATB suits this process since it does not require any additional interfacial layer, which would complicate the filter’s design. Furthermore, successful FATB has been demonstrated at lower

temperatures than those

used by conventional thermal bonding methods [6], alleviating thermal stress and stability concerns

FATB requires a substrate with mobile ions that assist in the formation of an interfacial space-charge region [7]. Thus, borosilicate glass was chosen as the bonding pair for GaP due to its mobile sodium and oxygen ions and the fact that its thermal expansion coefficient of borosilicate (4×10-6/K [8]) is closer to GaP’s (5.8×10-6/K [9]) than that of other glasses.

These adaptations show the potential for new capabilities in fabricating GaP-based devices for spectroscopic and optical filtering applications in microfluidic and other integrated system designs.

FABRICATION

A GaP wafer (Figure 1a) is first ion-implanted with helium. The SRIM [10] software determines that an implant energy of 92 keV and an angle from the normal of 7° would

give a projected range of 503 nm and a straggle of 133 nm (Figure 2).

The implanted GaP (Figure 1b) and the borosilicate glass (Figure 1c) are exposed to an O2 RIE plasma to clean their surfaces and make them hydrophilic before bonding. The surfaces are bonded in a vacuum (~0.075 mTorr) with a bond pressure of 5 MPa and a positive voltage of 1000 V, applied on the side of the GaP. The system is heated to 400°C and allowed to dwell for 1.5 hours, giving time for thermally mobilized ions O- and Na+ ions in the glass to drift under the applied voltage. After the dwell, the system is cooled to 150°C before the voltage is turned off, to prevent ions from diffusing back to their original positions.

When GaP with a high local concentration of ion-implanted helium is heated, the helium de-phases, forming air bubbles that exfoliate the material and significantly weaken it. With the He ion implant, a narrow layer of the substrate is saturated,

causing it to split in this region. This results in a thin layer of GaP fixed to the borosilicate glass post wafer-bonding, with the wafer bulk (Figure 1d) easily separating from the glass. A stylus profilometer is used to measure the thickness of the deposited thin film, which is found to be 707 nm on average, with a standard deviation of 6 nm. This deviation from the expected ~500nm thickness, given the SRIM simulation, is believed to either be a consequence of the actual implant profile not matching the simulation (Figure 2) or of the exact nature of the exfoliation mechanism. AFM is used to measure the absolute surface roughness in a 5μm x 5μm square (Figure 3a), which is found to be ~8nm. The consistency of the thickness, both within and between samples makes this process a promising means of preparing thin film GaP on non-native substrates.

Optical gratings were designed using a finite-element method solver (RSoft) that calculated the transmittance profile for a given set of material dispersion profiles and geometries (Figure 2). The gratings were designed to transmit light at λ0=650 nm and reject light in the surrounding spectra.

0.0 0.2 0.4 0.6 0.8 1.00.0

0.4

0.8

1.2

1.6

2.0

Dose = 5×1016 (Atoms/cm2)

Implant Depth (μm)

He

conc

entra

tion

(1×2

1 A

tom

s/cm

3)

Figure 2: Plot of He+ ion-implant profile simulated in SRIM, for a dose of 5×1016 cm-3, and a tilt of 7°.

Figure 3: Plot of simulated transmission versus wavelength. The inset shows the simulated grating geometry.

(a) (b)

Figure 4: AFM scans showing height maps of exfoliated GaP thin film (a) after bonding to glass and (b) after 3 mins in 3:1:1 H2O:H2SO4:H2O2 solution, at 50°C

The simulated design requires a total thin film height under

300 nm, which necessitates a thinning step to further reduce the film thickness. This thinning is done using a 3:1:1H2O:H2SO4:H2O2 solution, at 50°C (with an etch rate ~60nm/min) to a thickness 300 nm. Further thinning was done using the same solution at a temperature of 25°C (with an etch rate ~ 6nm/min) [11]. Figure 3b shows AFM height data for a thinfilm sample etched at 50°C for 3 minutes. It was found that the etch did not significantly alter surface roughness.

Gratings are then patterned into the substrate with e-beam lithography, followed by a reactive ion etch using 5 sccm of Cl2, 5 sccm of BCl3, 5mT of pressure, 80 W of RF power. The etch is performed with the sample stage at 20°C. Following this, a reflective window of Ti (500 Å)/Al (1500 Å) is evaporated onto the area surrounding the gratings to block stray light during measurement (Figure 1h). The side-wall profile and depth of the fabricated gratings are analyzed using focused-ion beam (FIB) milling (Figure 5). The results of the

FIB-SEM show that, while the etch is sufficiently anisotropic, the etch must be optimized further to improve floor cleanliness and reduce the etch rate.

NEXT STEPS

Optical characterization of the grating transmittance is underway. This data will be compared with modeled results given measured grating geometry.

Preliminary results indicate a deviation in the refractive index between GaP and He-implanted GaP. This is noticeable in comparing Figure 1a versus Figure 1b, where He+

implantation damage has visibly affected the absorption of the GaP, implying a change in the total refractive index due to implant damage. The effect on the dispersion profile of the integrated GaP is currently being investigated.

Further work is being pursued to analyze the effects that the insulating substrate (borofloat) have on the thin film RIE dry-etch conditions. Etch rate acceleration is observed on thin film GaP relative to bulk GaP, owing to the thermally insulating nature of the substrate that does not dissipate heat as readily as native III-V substrates. The dry-etch recipes are currently being optimized to improve end-point control.

CONCLUSION

Helium-implantation followed by FATB and exfoliation is demonstrated as a reliable method of transferring uniform GaP thin films onto glass. Etch recipes for material thinning and optical grating definition in the thin film are established, while simulations of the transmission properties of patterned gratings in these materials have been developed. Characterization of the fabricated filter is commenced to test the efficacy of this grating design by comparing the measured and theoretical transmission profiles.

ACKNOWLEDGEMENTS

Research reported in this paper was supported by the National Institutes of Health under award number R21AI130562. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

Atomic Force Microscopy was carried out in the Frederick Seitz Materials Research Laboratory Central Facilities at the University of Illinois.

REFERENCES

[1] I. Rodríguez-Ruiz, T. N. Ackermann, X. Muñoz-Berbel, and A. Llobera, “Photonic Lab-on-a-Chip: Integration of Optical Spectroscopy in Microfluidic Systems,” Anal. Chem., vol. 88, no. 13, pp. 6630–6637, Jul. 2016, doi: 10.1021/acs.analchem.6b00377.

[2] Y. Wan et al., “Compact characterization of liquid absorption and emission spectra using linear variable filters integrated with a CMOS imaging camera,” Sci Rep, vol. 6, no. 1, pp. 1–9, Jul. 2016, doi: 10.1038/srep29117.

[3] G. Wallis, “Field assisted glass sealing,” Active and Passive Electronic Components, vol. 2, no. 1, pp. 45–53, 1975.

Figure 5: SEM of FIB milled wall in etched gratings, showing the side-wall profile of the grating etch. Inset: bird’s eye-view image of etched gratings.

[4] F. A. Kish et al., “Very high‐efficiency semiconductor wafer‐bonded transparent‐substrate(AlxGa1−x)0.5In0.5P /GaP light‐emitting diodes,” Appl. Phys. Lett., vol. 64, no. 21, pp. 2839–2841, May 1994, doi: 10.1063/1.111442.

[5] K. V. Srikrishnan, “Smart-cut process for the production of thin semiconductor material films,” 5,882,987, 16-Mar-1999.

[6] Z. L. Liau and D. E. Mull, “Wafer fusion: A novel technique for optoelectronic device fabrication and monolithic integration,” Appl. Phys. Lett., vol. 56, no. 8, pp. 737–739, Feb. 1990, doi: 10.1063/1.102697.

[7] Y. Kanda, K. Matsuda, C. Murayama, and J. Sugaya, “The mechanism of field-assisted silicon-glass Bonding,” Sensors and Actuators A: Physical, vol. 23, no. 1, pp. 939–943, Apr. 1990, doi: 10.1016/0924-4247(90)87064-P.

[8] M. M. Lima and R. Monteiro, “Characterisation and thermal behaviour of a borosilicate glass,” Thermochimica Acta, vol. 373, no. 1, pp. 69–74, Jun. 2001, doi: 10.1016/S0040-6031(01)00456-7.

[9] E. D. Pierron, D. L. Parker, and J. B. McNeely, “Coefficient of Expansion of GaAs, GaP, and Ga(As, P) Compounds from −62° to 200°C,” Journal of Applied Physics, vol. 38, no. 12, pp. 4669–4671, Nov. 1967, doi: 10.1063/1.1709201.

[10] J. F. Ziegler, M. D. Ziegler, and J. P. Biersack, “SRIM – The stopping and range of ions in matter (2010),” Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, vol. 268, no. 11, pp. 1818–1823, Jun. 2010, doi: 10.1016/j.nimb.2010.02.091.

[11] D. A. Kudryashov, A. S. Gudovskikh, and A. I. Baranov, “Precision Chemical Etching of GaP(NAs) Epitaxial Layers for the Formation of Monolithic Optoelectronic Devices,” Semiconductors, vol. 52, no. 13, pp. 1775–1781, Dec. 2018, doi: 10.1134/S1063782618130092.

ACRONYM LIST

AFM: Atomic Force Microscopy

FATB: Field-Assisted Thermal Bonding

FIB: Focused Ion-Beam

GaP: Gallium Phosphide

LED: Light Emitting Diode

LVF: Linear Variable Filter

RF: Radio Frequency

RIE: Reactive Ion Etch

RMS: Root Mean Square

SEM: Scanning Electron Microscopy

SRIM: Scattering Range of Ions in Matter