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Micro/Nano Lithography

MOEMS vertical-air-cavity filters are widely and continuously tunable around 1.55 microns

Micromachined vertical-resonator-based filters that are capable of wide, continuous, and kink-free tuning at 1.55μm using a single control parameter have been designed and implemented.
7 February 2006, SPIE Newsroom. DOI: 10.1117/2.1200601.0005

Tunable devices based on micro-opto-electro-mechanical systems (MOEMS), such as vertical-cavity filters and surface-emitting lasers (VCSELs), have attracted much interest because of their unique features: a wide continuous tuning range and two-dimensional array integration. Tunable devices with a wide tuning range and high spectral purity have many applications: these include dense wavelength division multiplexing (DWDM) for optical communications, sensing, analytic instruments, confocal microscopy, process control applications, as well as medical diagnostics. We are particularly interested in the former.

DWDM revolutionized data transmission technology by increasing the signal capacity of embedded fiber. Incoming optical signals are encoded at specific wavelengths within a designated frequency band, and then multiplexed onto a single fiber. This process allows for multiple video, audio, and data channels to be transmitted over one fiber while maintaining system performance and enhancing transport systems. The ability to provide potentially unlimited transmission capacity is the most obvious advantage of DWDM.

Here we present our recent results on tunable vertical cavity filters, which show a tuning rage of 221nm. Our filters consist of top and bottom In(GaAs)P/air DBR mirrors that provide high reflectivity and wide stopbands: for a three-period DBR, the theoretical reflectance is 0.997 and the filter's stopband is 1000–1500nm. The DBRs are separated by a cavity. Electrostatic actuation is enabled by n-doping the bottom DBR mirror and p-doping the top mirror. By reverse biasing this pn-junction and varying the voltage, the cavity length can be controlled. Depending on the cavity length, the filter can be adjusted to be transparent to only one of the wavelength channels while blocking the others.

Growing the structures requires epitaxy to deposit interleaved binary—In(GaAs)P membrane—and ternary (InGaAs sacrificial) layers.1 Arsenic carryover keeps the interface from being abrupt. Due to this intermixing at the interface of membrane-sacrificial layers, underetching produces an inhomogeneous surface on both sides of each membrane. Thus, within the InP-membranes, the material composition is most likely slightly inhomogeneous in the vertical direction. Suspended multiple membrane structures (Figures 1 and 2) are sensitive even to extremely small vertical compositional variations, since they cause slight bending in the membrane after release. As a consequence, the filter properties can be varied widely, including the spectral tuning range, filter line transmission, filter linewidth, and lateral mode structure.

Figure. 1. Left: Top view of the vertical-air-gap-resonator filter design. Right: Cross section of the vertical layer structure of the electrostatically-actuated filter. DBR: distributed Bragg reflector.
Figure. 2. Scanning-electron micrographs of multiple air-gap filters with membranes 40μ m in diameter and four suspensions. (a) An In(GaAs)P/air-gap filter with 30μ m-long suspensions. (b) A closer view of the layers (c) A spiral-shaped suspension InP/air-gap filter. (d) An InP/air-gap filter with 10μ m-long suspensions.

We fabricated two types of filters. Filter type 1 was grown with several minutes of epitaxial growth interruption prior to each InP deposition, to reduce the carry-over that allows arsenic to incorporate into the InP layers. Filter type 2 was grown with short growth interruptions and tensile-strained GaInAs sacrificial layers to generate a compensating interface layer between the GaInAs and InP layers that remains after the underetching process. Type 1 causes less net strain accumulation and thus less membrane bending but type 2 has the potential for larger wavelength tunability.

The micromachined fabrication2 is based on three main steps: defining the multiple-layer structure by epitaxy or other deposition methods; dry etching to define the lateral structure (vertical patterning of the mesa) and removing the sacrificial layers by selective wet-chemical underetching to define the air-gaps. The surface-micromachining fabrication process requires no micro-mounting since the entire structure is fabricated in a batch process: the filter can only be low-cost if many devices are batch-fabricated simultaneously. Our process is therefore compatible with mass production.

Figure 3(a) displays the experimental results obtained for a type-1 filter suspended from four beams, each 40μm long. The membrane is 40μm in diameter. The filter has three 3λ/4 InP membranes separated by λ/4 air-gaps, a λ/2 cavity, and an air-gap of 637.5nm to the InP substrate. Because this filter has almost no strain, the membranes are nearly flat when the applied voltage is 0V. The transmission dip of the device is located at λ = 1.599μm for non-actuated membranes (U=0V) and at λ = 1.457μm in the case of actuation by U = -3.2V, covering a tuning range of 142nm. The type-2 filters have four 30μm-long suspensions and a 20μm membrane diameter. Three 3λ/4 InP membranes are separated by λ/4 air-gaps, and this filter has a λ-cavity and a spacing of 465nm to the InP substrate. We obtained a larger tuning range. The filter is weakly strained and thus at U=0V has curved membranes. Using an electrostatic tuning voltage of 28V, we obtained a 221nm tuning range: see Figure 3(b). To the best of our knowledge, this is a record value for air-gap-based micromachined DBR filters demonstrating fast tuning.

Figure 3. (a) Measured tuning of an InP/air multiple membrane (type 1) filter under electrostatic actuation (nearly-flat membranes in the unactuated condition at U=0V). (b) Measured tuning and corresponding reflectance spectra (inset) of an InP/air multiple membrane (type 2) filter under electrostatic actuation (strained and bent membranes in the unactuated condition at U=0V).

Our experimental and theoretical calculations indicate that the tuning range and the required voltage range are sensitive to a large number of parameters such as suspension lengths, number of suspensions, suspension widths, membrane diameter, membrane and suspension thickness, as well as cavity length. Less evident is the fact that we found a strong dependence on the layer strain and thus the membrane and suspension pre-bending in the unactuated state, i.e. at U=0V. The images show a much stronger bending of the top membrane and the corresponding suspensions for type 2. Due to the miniaturization of the filters, the efficiency of the electrostatic force increases considerably. Table 1 compares a selection of DBR-based tunable filters.

Table 1. (Click to expand)

Our filters (the 10th and 14th lines in Table 1 are batch-process compatible and, thus, can potentially be produced at low-cost. They show very-fast tuning, low actuation voltages, and extremely-wide tuning ranges. Due to the very-high mechanical resonance frequencies, the tuning speed is very high: more than high enough for today's DWDM system requirements.

Support by the German BMBF and DFG funding is gratefully acknowledged. The authors wish to thank K. Streubel, D. Gutermuth, H. Schröter-Hohmann, I. Kommallein, I. Wensch, W. Scholz for technical support and stimulating discussions.

Hartmut Hillmer, Sören Irmer, Friedhard Römer
Institute of Nanostructure Technologies and Analytics INA, University of Kassel
Kassel, Germany

Prof. Dr. Hartmut Hillmer is co-director of the Institute of Nanostructure Technologies and Analytics. He has published anumber of papers in SPIE journals and given invited talks at SPIE conferences.

Andreas Hasse
IPAG Innovative Processing AG
Darmstadt, Germany
M. Strassner
Marcoussis, France
J. Daleiden
Infineon Technologies
Dresden, Germany

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