Procedia Chemistry Procedia Chemistry 1 (2009) 437– 440 www.elsevier.com/locate/procedia
Proceedings of the Eurosensors XXIII conference
Silicon microstructure for bringing closer parallel beams of laser diodes I. Zubela, * M. Kramkowskaa a
Faculty of Microsystem Electronics and Photonics, Wrocław University of Technology, Wrocław, Poland
Abstract
A spatial silicon structure containing parallel {111} faces forming two pairs of micromirrors used for making close two parallel optical beams at micrometric distance has been designed. The structure was fabricated in a single (113) oriented Si wafer. The fabrication process involved one stage double-side anisotropic etching in KOH solution. The structure employs natural arrangement of crystallographic planes in silicon, which assures an ideal parallelism of beam axes of reflected light. The reduction of the distance between the laser beams is possible without the need of setting up and adjusting a complex optical system. Keywords: MOEMS, micromirrors, silicon anisotropic etching, (hkl) planes
1. Introduction Modern MOEMS systems connecting micromechanical devices and optical phenomena, more frequently use reflection of laser beam from a complex system of mirrors fabricated in monocrystalline silicon. A connection in a single microsystem of two laser beams with different wavelengths enables considerable enhancement of density of information, what can be used in DVD/CD drives, HVD holographic drives or fast laser printers. The main problem is the distance between two light sources, which is usually much more extended than currently available sizes of semiconductor devices. Typical distances between two lasers diodes, arranged parallel to each other in a single chip, amount to ca 300 µm. To extend the possibilities of using the laser light and open new fields of applications for integrated optics, a new system consisting of micromirrors, enabling bringing closer or overlaying of two parallel laser beams at the smallest possible distance is necessary. There are different ways of accomplishment of such micromirrors and different materials are used for this purpose. They demand, however setting up and adjusting of a complex system, consisting of some single components made of silicon or glass1. 2. Idea of the structure In realization of the structure, the fact that during wet anisotropic etching of silicon, the planes {111} with the lowest etching rate develop on the sidewalls of etched hole was employed2. The planes are characterized by
* Corresponding author. Tel.: +48-(0)71-355-97-95, E-mail:
[email protected]
1876-6196/09 © 2009 Published by Elsevier B.V. Open access under CC BY-NC-ND license. doi:10.1016/j.proche.2009.07.109
I. Zubel and M. Kramkowska / Procedia Chemistry 1 (2009) 437–440
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relatively smooth surfaces and can be used as mirrors, reflecting light beams. To design the structure, a silicon substrate with appropriate crystallographic orientation, enabling fabrication of two pairs of parallel {111} planes, inclined at the angles of β and 180°-β is necessary. In result of double reflection of a light beam from the pair of two parallel faces, a displacement of optical axis occurs without a change of its direction. In such way, two parallel beams, reflected from two pairs of such faces can be brought closer at a required distance. The inclination of {111} faces in relation to some successive planes from [110] crystallographic zone is shown in table 1. The {111} faces are arranged in the substrates in two sections A-A and B-B. The sections and the angle ϕ between [110] direction and crystallographic zone where the {111} planes are located, are explained in Fig. 1a. The planes from the upper and lower hemispheres are indicated by dots and circles, respectively. Table 1.Angles of inclination of {111} planes toward Si(hkl) substrate (hkl) (100)
β1 ( A-A) 54.73°
β2 (A-A) 54.73°
β (B-B) 54.73°
ϕ 0°
(115)
38,94°
70,53°
56,25°
10,89°
(113)
29.50°
79.97°
58.52°
16.76°
(112)
19.47°
90°
61.87°
22.20°
Design of the structure and its arrangement on the substrate is shown in Fig. 1 b, where also the run of light beans across the etched structure is indicated. It is obvious that the light beams will be brought at the closer distance when the reflection occurs at the vertex X.
a)
β2
B-B β
180°-β
(-11-1)
(-111)
A-A
(-1-11)
β
(1-11)
180°- β
[110 ]
ϕ
β
(111)
(1-1-1)
B-B
β2
B-B
β1
b)
B
β1
SiO2 mask
2ϕ
X
180°-β
optical beam
B
B-B
A-A β
Fig. 1. (a) stereographic projection with indicated {111} planes and the cross-sections of structures restricted by the faces, (b) design of the mask enabling fabrication of the structure for bringing close two parallel light beams and its arrangement on the substrate
The following criteria of selection of substrates for accomplishment of such structure were employed: • The angle of inclination of {111} planes in B-B section (β) should be relatively low to ensure double reflection of the light beam and its sufficient parallel displacement. The influence of the angle of inclination of such {111} mirrors on the reflection of light beams is shown in Fig. 2a. • As the beams should be brought at the closest distance toward the vertex X (Fig. 1), it is important to choose the angle ϕ as low as possible. Then, the distance of reflected beams p-p at the distance f from the vertex X will be the smallest one (Fig. 2b). • The {111} faces, whose projection on substrate plane amounts to b, inclined at a low angle β1 should not hinder reflection of the light beam near the vertex X (Fig. 2b). Thus, the angle β1 should not be too low. Plane (100) does not have any parallel {111} which would enable an accomplishment of such structure (Table 1). The planes (112) and successive planes, inclined at the higher angles toward (100) (not shown in Table 1) do not fulfill the above criteria. The accomplishment of such structure is possible on the planes inclined at low angles towards (100), like (115) and (113).
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I. Zubel and M. Kramkowska / Procedia Chemistry 1 (2009) 437–440
B-B
p
B-B 2φ β 1
SiO2 mask p X
SiO2 mask
X
2φ
p
p
f
β
b
a)
f
b
b)
Fig. 2. (a) cross-sections of the structures with low and high inclination angles of (111) planes, (b) top view of the structure with low and high value of ϕ angle.
3. Structure design The main problem in designing the mask for etching the structure in a substrate with selected crystallographic orientation (defined by β and ϕ) is determining the width of a gap, confined with two parallel {111} planes. If Si substrate used for the design has the thickness d, then based on simple geometrical relationships, the relation between the substrate thickness and the gap width s can be determined (Fig. 3). The angle γ between the reflected beam and substrate (at the assumption that the incident angle 90°-β equals reflection angle) amounts to γ = 2β - 90°. tg γ = d/(s+c) where: c = d/tg β (1) then: s =d (1/ tg γ - 1/ tg β) (2) The biggest gap width smax on the substarte with thickness d at which the double reflection of light beam would take place is shown in Fig. 3a. Further increase in the gap width would disable the double reflection (Fig. 3b). In practice, the gap should be narrower than smax to ensure a margin in position of both incident and reflected beam. Then, the substrate thickness d1 is effectively used (Fig. 3c) and the gap width amounts to: (3) s1 = d1 (1/ tg γ - 1/ tg β) B-B
B-B
B-B 90°- β
90°- β γ
d
β
γ smax
a)
c
d
90°- β
β
90°- β
c
s b)
90°- β
d
γ
d1
β
s1
c1 c)
Fig. 3. Determining of gap thickness s (a) the biggest gap width at which a double reflection of the light beam takes place, (b) too wide gap, (c) optimal gap
The largest distance from the light source to the midpoint of the structure in B-B section amounts to w = c + smax, but the optimal one should be a little smaller, i.e. w1 = c1+s1 = d1⋅tg γ. The actual distance between the light sources a can be determined based on ϕ angle (Fig. 4a): a/2 = w⋅ sin(90° - ϕ) (4) Since the beams are brought the closest near the vertex X, a compensation structure, protecting the vertex against underetching should be designed. The compensation structure, based on {111} planes would be the most effective, but the plane inclined at low angle β1 toward the substrate, whose projection amounts to b=d/tgβ1, may hinder a free transition of the beam through the gap if the gap was too narrow (Fig. 4b). The condition b
