Mechanical property measurement of InP-based MEMS for optical communications

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Abstract

We investigate mechanical properties of indium phosphide (InP) for optical micro-electro-mechanical systems (MEMS) applications. A material system and fabrication process for InP-based beam-type electrostatic actuators is presented. Strain gradient, intrinsic stress, Young’s modulus, and hardness are evaluated by beam profile measurements, nanoindentation, beam bending, and electrostatic testing methods. We measured an average strain gradient of δε0/δt=4.37×10−5 μm−1, with an average intrinsic stress of σ0=−5.4 MPa for [0 1 1] beams. The intrinsic stress results from arsenic contamination during molecular beam epitaxy and (MBE) can be minimized by careful MBE growth and through the use of stress compensating layers. Nanoindentation of (1 0 0) InP resulted in E=106.5 GPa and H=6.2 GPa, while beam bending of [0 1 1] doubly clamped beams resulted in E=80.4 GPa and σ0=−5.6 MPa. We discuss the discrepancy in Young’s modulus between the two measurements. In addition, we present a method for simultaneously measuring Young’s modulus and residual stress using beam bending. Electrostatic actuation in excess of 20 V is demonstrated without breakdown.

Introduction

In time-division multiplexing (TDM), each signal is transmitted for a time interval, after which the next signal is transmitted for the next time interval and so forth. Consequently, the capacity of a TDM communications channel is limited by the data rate available [1]. In wavelength-division multiplexing (WDM) many signals (wavelengths) are transmitted simultaneously over the same communications channel. This increases the capacity of the network significantly without deployment of more optical fibers, an expensive undertaking [1]. In WDM, the capacity of the network is determined by the number of distinct wavelengths that can be transmitted simultaneously. Typically, wavelengths are separated by 10–100 nm in WDM. Future dense-WDM (DWDM) systems may have wavelengths spaced less than 1 nm [2].

The advent of WDM and DWDM has created a need for various micro-optical components including tunable lasers, variable attenuators and equalizers, optical filters and demultiplexers, and optical cross-connect switches. Tunable lasers are significantly cheaper than multiple fixed-wavelength laser sources and are therefore preferred for WDM networks. Variable attenuators can equalize the spectrum and can help compensate for dispersion and other non-linear behavior of the optical channel. Optical filters are needed to separate the wavelengths transmitted over the communications channel, so that they may be directed to individual users. Finally, optical cross-connect switches are needed to direct traffic between various network nodes.

Recently, there has been considerable interest in using micro-electro-mechanical systems (MEMS) technology for optical switches in network applications [1], [3], [4]. The reasons are simple: the required displacements in optical switches and tunable optical filters are of the order of a few wavelengths (micrometers) and are thus well suited to low power MEMS actuators. Also, the potential for batch fabrication enables huge cost savings compared to macro-scale switching networks and enables scaling up to a large number of elements on a single chip.

Typically, MEMS devices are made from silicon, which has an indirect bandgap and is unsuitable for active optoelectronic devices. In contrast, InP is a direct bandgap semiconductor (Eg=1.34 eV [5]), and can be used in active optoelectronic devices, such as lasers and semiconductor optical amplifiers. By incorporating such active devices in a MEMS platform, losses can be compensated on-chip.

Compound semiconductors of the InxGa1−xAsyP1−y family can be grown lattice-matched to InP substrates with tailored bandgaps [5]. Existing fiber optic cables exhibit minimal losses at 1550 nm. Therefore, most modern optical communications systems operate around 1550 nm.

Monolithic integration of existing active optoelectronics with InP-based MEMS actuators will enable novel and versatile WDM optoelectronic devices. However, before InP-based optical MEMS can be realized, the mechanical properties of this material must be characterized to ascertain its applicability to micro-mechanical devices. Few groups have studied the mechanical properties of InP for MEMS applications and very few InP-based MEMS devices have been reported in the literature [6], [7], [8], [9].

The motivation for this research is, therefore, the development of a platform and fabrication process for InP-based MEMS electrostatic actuators and assessment of the mechanical properties of InP for optical MEMS applications. We accomplish this by using InP microbeam electrostatic actuators. Future work will focus on monolithic integration of optoelectronic technology with InP-based MEMS, enabling novel optical components for high-speed WDM networks.

Section snippets

Sample preparation

MEMS mechanical property measurement is performed using InP cantilever and doubly clamped microbeams. We discuss the theory behind each measurement technique in Section 3. The layer structure in Fig. 1 consists of a doped (1 0 0) InP substrate (n=3×1018 cm−3), on which we grow a 0.4 μm thick InP buffer layer (n=5×1018 cm−3), followed by an intrinsic 1.7 μm thick In0.53Ga0.47As sacrificial layer and a 1.7 μm thick p-doped InP beam layer (p=5×1017 cm−3). The dimensions were chosen because they represent

Theory

While some reports of InP-based MEMS can be found in the literature [6], [7], [8], [9], very few efforts have concentrated on studying the mechanical properties of this material for MEMS applications in detail. In this work, we use cantilever curvature and beam buckling measurements [12], [13], [14] for extraction of the strain gradient and intrinsic compressive stress. We chose three methods for micro-mechanical property measurements: nanoindentation [15], [16], [17], beam bending [13], [17],

Strain gradients and intrinsic compressive stress

Out-of-plane curvature of cantilevers is shown in Fig. 10(a), and buckling of doubly clamped beams is shown in Fig. 10(b). We measured the cantilever beam curvature and doubly clamped beam buckling using a confocal microscope (Nikon MM-40 measuring microscope and Lasertec 1LM21 laser microscope) and used curve-fitting to extract the strain gradient (Fig. 10(c)) and compressive strain (Fig. 10(d)). The vertical resolution of the confocal microscope is, ideally, the maximum height profile

Conclusions

We have presented a layer structure and process for fabricating simple InP-based beam-type electrostatic actuators. Our etch process enables low sidewall roughness (<20 nm) with high verticality (89° or better), which is suitable for optical devices. The average strain gradient measured from cantilever curvature is 4.37×10−5 μm−1 and the average intrinsic compressive stress from beam buckling measurements is −5.4 MPa. The compressive stress and the stress gradient are due to arsenic contamination

Acknowledgements

The authors thank Mr. L.C. Olver and the clean room staff at the Laboratory for Physical Sciences (LPS) for assistance with fabrication, and Mr. S.C. Horst and Capt. D. Hinkel for access to diagnostic and processing equipment at LPS. Prof. D. DeVoe, Mr. W.-J. Cheng, and Mr. F. Rosenberger are acknowledged for access to and assistance with the supercritical CO2 dryer.

This research was supported by a National Science Foundation (NSF) CAREER award (Ghodssi) and by the LPS.

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