Microlens frames for laser diode arrays

Monolithic microlens frames enable the fabrication of monolithic laser diode arrays and are manufactured inexpensively with high registration, and with inherent focal length compensation for any lens diameter variation. A monolithic substrate is used to fabricate a low-cost microlens array. The substrate is wet-etched or sawed with a series of v-grooves. The v-grooves can be created by wet-etching, by exploiting the large etch-rate selectivity of different crystal planes. The v-grooves provide a support frame for either cylindrical or custom-shaped microlenses. Because the microlens frames are formed by photolithographic semiconductor batch-processing techniques, they can be formed inexpensively over large areas with precise lateral and vertical registration. The v-groove has an important advantage for preserving the correct focus for lenses of varying diameter.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to laser diode arrays, and more specifically, 
it relates to microlens frames for laser diode arrays. 
2. Description of Related Art 
Laser diode arrays are used in a wide range of commercial, medical and 
military applications: materials processing (soldering, cutting, metal 
hardening), display technology/graphics, medical imaging (MRI) and 
surgical procedures (corneal shaping, tissue fusion, dermatology, 
photodynamic therapy), satellite communication, remote sensing, and laser 
isotope separation. In certain solid-state laser applications, it is 
desirable to use laser diode arrays to optically excite, i.e. "pump," the 
crystal hosts. Diodes offer a narrow band of emission (reducing thermal 
lensing), compactness, high electrical efficiency and higher reliability 
as compared to flash lamps. Despite these numerous advantages, however, 
diode-pumped solid-state lasers (DPSSLs) have gained slow market 
acceptance due to the high cost associated with the laser diode array 
pumps. Significant diode array cost reductions would enable wide 
deployment of DPSSLs and new architectures to be realized that were 
previously cost prohibitive. In particular, low-cost diode arrays would 
bolster the inertial confinement fusion (ICF) and inertial fusion energy 
(IFE) programs that require laser diode arrays in very high volumes. 
Historically, much of the research and development in this area was devoted 
to solving diode material and fabrication issues in order to improve the 
yield and reliability of laser diodes. High quality InAlGaAs and InGaAsP 
laser diodes are now commercially available for pumping Nd:YAG at 
.about.810 nm. As much as 100 W/cm of peak power is possible under pulsed 
operation, and over 10,000 hours of continuous wave operation in 
commercial systems has been demonstrated at .about.30 W/cm. Although these 
types of performance improvements have led to cost reductions in the past, 
there has not been a complementary improvement in the packaging 
technology, which is now limiting further cost reductions from being 
achieved. 
To date, most packaging/heatsink schemes use a "rack and stack" 
architecture. In this method, individual laser bars are fabricated into 
sub-assemblies, and the sub-assemblies are then bonded together to produce 
larger two-dimensional arrays. Labor intensive steps associated with 
handling individual components prevents the production of arrays in large 
volume and in high yield. To reduce manufacturing costs it is important to 
utilize a monolithic approach for mounting laser diode bars. Similarly, it 
is advantageous to be able to microlens a large number of laser bars 
simultaneously. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide monolithic microlens 
frames that will enable the fabrication of monolithic laser diode arrays. 
It is another object of the present invention to manufacture monolithic 
lens frames inexpensively. 
It is another object of the present invention to manufacture monolithic 
lens frames with high registration (horizontally and vertically). 
It is another object of the present invention to manufacture monolithic 
lens frames with high registration with inherent focal length compensation 
(approximate and exact) for any lens diameter variation. 
It is another object of the present invention to manufacture monolithic 
lens frames that will accommodate cylindrical or irregular, custom-shaped 
lenses. 
A monolithic substrate is used to fabricate a low-cost microlens array. The 
substrate is wet-etched or sawed with a series of v-grooves. The v-grooves 
can be created by wet-etching, by exploiting the large etch-rate 
selectivity of different crystal planes. The v-grooves provide a support 
frame for either cylindrical or custom-shaped microlenses. Because the 
microlens frames are formed by photolithographic semiconductor 
batch-processing techniques, they can be formed inexpensively over large 
areas with precise lateral and vertical registration. The v-groove has an 
important advantage for preserving the correct focus for lenses of varying 
diameter. 
A variety of embodiments of the microlens frame are possible. A slot-frame 
structure resembles a ladder. The v-grooves are etched through the 
substrate. The length of the grooves corresponds to the length of the 
diode emission. The individual lens frames are then separated. Afterwards, 
the individual microlenses are bonded to the frame. Another slot-frame 
structure is etched from both sides to also resemble a ladder. In this 
case, the v-grooves are etched through the substrate from both sides 
simultaneously. The rest of the fabrication steps are equivalent to that 
described above. This embodiment increases the total light emission that 
passes through the lens frame. A third embodiment resembles a rails-like 
structure. Once the v-grooves are formed, the substrate is sawed into 
series of "rails." The rails are fastened on either side of the laser 
diode array, and the microlenses are supported by their edges to the 
rails. The advantage of this approach is that there is no substrate to 
scatter the laser diode output light. A forth embodiment resembles a 
picture frame so that the rails are fully supported even without any 
microlenses present. This structure is more robust than the former, but is 
slightly more complicated to produce. Similar to the first case, the 
microlenses are supported at their edges and fastened to the sides of the 
frame.

DETAILED DESCRIPTION OF THE INVENTION 
The invention is a monolithic substrate that is machined to form a 
microlens frame. A variety of embodiments are possible as shown below. 
Since there is no requirement of thermal conductivity of the frame, it is 
believed that Si makes an excellent candidate since it can be wet-etched 
with precision features (v-grooves) well suited for this application, and 
can also be easily sawed. The most common choices are described here. 
Referring to FIGS. 1A-D, an etch mask (e.g., SiN having a thickness of a 
few thousand angstroms) is deposited over the entire substrate 10. 
Photolithography is used to define a series of stripes. The stripe width 
corresponds to the opening width of the v-groove 12 to be formed, and the 
pitch of the stripes will correspond to the pitch of the laser bar array. 
After the photoresist is patterned onto the SiN, the remaining SiN is 
removed (e.g., by CF.sub.4 plasma) from the areas not protected by the 
photoresist. 
The substrate is then wet-etched. The wet etchant (e.g., KOH-based) 
provides a high etch-rate selectivity between different crystal planes; 
the {111} planes etch much slower than all other crystal planes 
(.about.200-600 times), so for prolonged etching, a v-groove 12 is formed 
by these {111} crystal planes. An advantage of this is that the v-groove 
feature will retain its approximate shape even if it is "over etched" 
(i.e., etched past completion). An additional advantage is that the 
v-groove to v-groove spacing remains invariant to etch depth or etch time. 
Individual lens arrays 14 can then be separated and individual microlenses 
can be bonded thereto. FIG. 1D shows a side view of a completed embodiment 
having a substrate 10 with cylindrical microlenses 24 or a custom shaped 
microlens 25. 
A "splay" pattern is sometimes first etched into the substrate to determine 
the exact orientation of the substrate, since the flats of the substrates 
are not usually better than a degree (which would otherwise lead to 
roughness along the edges of the grooves). The patterning and etching 
steps of the splay pattern are analogous to those used for the v-groove 
formation. Unless extremely high registration is required, etching a splay 
pattern will be unnecessary. 
Silicon substrates with either &lt;100&gt; or &lt;110&gt; surface orientation can be 
used to form v-grooves. In the case of the (100) surface plane, v-grooves 
can be formed along two orthogonal directions 01-1! or 011!. The full 
angle of the v-groove is approximately 70.5.degree.. In the case of the 
(110) surface plane, the stripes should lie along the 1-10! direction in 
order to form a v-groove. The full angle of the v-groove is approximately 
109.5.degree. in this case. It is often desirable to specify that the 
major flat edge (which is used for course alignment) should lie normal to 
the 1-10! direction so that v-grooves will also run normal to the flat 
regardless of which side of the wafer is wet etched. Several types of lens 
frames can be fabricated using this wet etching technique. In each of the 
embodiments, it is assumed that once the desired features are formed in 
the substrate, the individual lens frames will be separated from each 
other using a dicing saw. In each case, many lens frames can be fabricated 
from a single Si substrate depending on the array size. Afterwards, the 
individual microlenses 24 are bonded to the frame (e.g., by epoxy or resin 
that cures upon exposure to UV radiation). The entire lens frame assembly 
can then be optically coated if desired (typically anti-reflection 
coatings are applied to reduce optical losses). Note that the precise 
angled edges also allows custom-shaped (non-cylindrical) microlenses to be 
oriented correctly. Custom-shaped microlenses could be used to improve 
collimation by reducing spherical aberrations. Examples of microlenses and 
methods of fabricating microlenses are shown in U.S. Pat. No. 5,155,631, 
filed Jan. 13, 1992, issued Oct. 13, 1992 and titled "Method of 
Fabrication of Cylindrical Microlenses of Selected Shape" and U.S. Pat. 
No. 5,081,639, filed Oct. 1, 1990, issued Jan. 14, 1992 and titled "Laser 
Diode Assembly Including a Cylindrical Lens," which is incorporated herein 
by reference. 
A slot-frame structure resembles a ladder and is shown in FIG. 2. In this 
case, the v-grooves 20 are etched through the substrate 22 (the backside 
of the wafer is protected by the SiN mask). The length of the grooves 20 
corresponds to the length of the emission (typically 1 cm for most laser 
bars). The SiN mask on the backside of the wafer would produce optical 
reflective losses from the underlying laser diode array. Therefore, it is 
usually removed (e.g., by wet etching in HF-based solution). The 
individual lens frames are then separated using a dicing saw. 
A slot-frame structure etched from both sides, shown in FIG. 3, resembles a 
ladder. In this case, the v-grooves are etched through the substrate 32 
from both sides simultaneously (using backside photolithographic 
alignment). In this case, the thickness of the substrate must be 
correlated with the width of the v-groove as well as the acceptance angle 
of the v-groove (which depends on which substrates are used, {100} or 
{110}). The v-grooves are etched until an opening breaks through. 
Prolonged etching will begin to attack the non-{111} planes that have been 
exposed, and will destroy the desired shape. The present embodiment 
increases the total light emission that passes through the lens frame (at 
the expense of having to pattern both sides, and to not allow excessive 
etching). The rest of the fabrication steps are equivalent to that 
described in the previous embodiments. 
A rails-like structure is shown in FIGS. 4A-C. The thickness of the 
substrate is such that the v-grooves will remain terminated in the Si 
substrate, so that "over" etching is inconsequential to the desired 
feature (the backside is protected by the SiN mask). Once the v-grooves 
are formed, the substrate 42 is sawed into series of "rails." The rails 
are located on either side of the laser diode array, and the microlenses 
44 are supported by their edges to the rails 46. They are then fastened to 
the rails. The advantage of this approach is that there is no Si substrate 
to scatter the laser diode output light, and a single substrate can 
produce many lens frames. A fixture for properly locating the rails can be 
formed using the same etched substrate (i.e., a "rail" whose width 
corresponds to the desired spacing between the two rails that will be used 
to form the lens frame). The rails will straddle the laser diode array 
that is located underneath. The lens frame assembly should be fastened to 
the laser diode array so that the lens frame will be protected from 
mechanical damage. FIG. 4C shows a perspective view of the rails 
embodiment. 
A picture frame structure, shown in FIGS. 5A and 5B, resembles a picture 
frame so that the rails 50 are fully supported even without any 
microlenses present. This structure is more robust than the former, but 
the etch mask is slightly more complicated. In this case, the etch mask 
will be removed in the center of the picture frame, so that the etching 
creates an opening window in the picture frame. Two sides of the picture 
frame (that will later be used to hold the lenses, will be patterned with 
a series of stripes, as before. The opposite two sides of the picture 
frame and backside of the wafer will be masked. However, for thick 
substrates, it will be desirable to pattern a window in the backside (that 
aligns with the frontside), to minimize etching time. Only a single etch 
step is needed to create the structure. Similar to the previous case, the 
microlenses 52 are supported at their edges and fastened to the sides of 
the frame 54. There is no Si support in the central region which might 
scatter the laser diode output light, as in the previous embodiment. 
For all embodiments, the Si thickness can be chosen to correspond to the 
working distance of the microlenses, in such a way that the lens frame can 
lay flush against the laser diode array while maintaining proper output 
collimation. This reduces the degrees of freedom for lens alignment so as 
to simplify positioning of the lens assembly. Typically the lenses will be 
located at the focus corresponding to the circle of least confusion as to 
minimize the divergence (however, in certain applications greater 
divergence is desirable). 
A critical factor for monolithic lensing is having precise registration of 
the v-grooves that locate the lenses. Lateral variation in the v-groove 
pitch will introduce pointing errors among the various laser bars, and 
vertical variations will prevent all laser diodes from being 
simultaneously focused. Using lithographic processes, both the lateral and 
vertical variations will be extremely accurate (within .about.1 .mu.m 
tolerances). Furthermore, any variations in lens diameter which alters the 
focal length, can be compensated by the v-groove. This occurs because both 
the v-groove and the focal length of the lens are proportional to the 
radius of the lens. For example, the "working distance" of the (110) lens 
frame is 1.225.times.R, and the (100) lens frame is 1.731.times.R, 
respectively (where R=lens radius). For a standard glass fiber lens 
(n.about.1.5), the "working distance" of the lens is .about.1.6.times.R. 
Thus for a standard cylindrical fiber lens, a (100) lens frame would 
remain approximately in focus even for varying lens diameters. For 
cylindrical lenses with shorter focal lengths (e.g., graded index lenses), 
the (110) Si v-grooves would be more suitable. 
FIG. 5 shows a side-view of the V-groove geometry created on (110) and 
(100) substrates. These orientations refer to the top surface of the Si 
substrate where the mask resides. In either orientation, the v-groove 
sidewalls are formed from the {111} crystal planes 50. 
For cases in which the focal length of the lens frame must be even more 
precisely controlled, a v-groove can be sawed into the substrate with a 
v-shaped wheel that has a cutting angle that provides the exact matching 
focal length of the lens. The radius of the saw tip is not critical in 
this case because the cylindrical lens rests on the sides of the v-groove 
and not at the bottom. The simplest embodiment to fabricate is vitually 
identical to the lens frame shown in FIG. 4 (rails) except that the 
v-groove is formed by sawing. In this case the substrate material is not 
limited to Si, but any easily machinable material. High precision saws 
have vertical and lateral resolution tolerances that approach 1 .mu.m, 
which is comparable to lithographic-type tolerances. 
Changes and modifications in the specifically described embodiments can be 
carried out without departing from the scope of the invention, which is 
intended to be limited by the scope of the appended claims.