High numerical aperture objective lens manufacturable in wafer form

A composite micro-lens for use in an optical or magneto-optical information storage system, made up of one or more lens elements which, when coupled in assembly, provide a desired numerical aperture. The design may provide, if necessary, a photo resist lens, a shaped ball lens, or one or more contoured surfaces within the composite construction to correct for aberrations. The composite lens designed in this manner allowing for the use of wafer-level assembly processes to provide high volume production capabilities. It is further intended that this micro-lens design support integration in an optical or magneto-optical head design.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to manufacture and use of wafer 
level optics with optical heads and more particularly to manufacture and 
use of a high numerical aperture (NA) objective micro-lens through an 
assembly of subcomponents that can be produced in an array format using 
wafer-level techniques. 
2. Background Art 
Prior to this invention, the manufacture of high numerical aperture 
objective lens involved a molding and a polishing of an aspheric surface 
using a high-index (of refraction) glass, techniques not capable of 
producing micro-lenses at high production rates. Current methods of 
producing a micro-part makes use of wafer-level processes, which can 
include an etch (ion milling), or a photoresist reflow technique. However, 
these processes are generally limited to a low-index glass (typically 
silica) with a spherical or near-spherical surface, or to a diffractive 
surface, preventing their use in the fabrication of high-quality, high 
numerical aperture lens. 
Information storage technology and the storage capacity available therefrom 
has historically been limited by a number of factors. A typical prior art 
Winchester magnetic storage system includes a magnetic head that has a 
body and a magnetic read/write element and is coupled to a rotary actuator 
magnet and coil assembly by a suspension and actuator arm so as to be 
positioned over a surface of a spinning magnetic disk. In operation, lift 
forces are generated by aerodynamic interactions between the magnetic head 
and the spinning magnetic disk. The lift forces are opposed by equal and 
opposite spring forces applied by the suspension such that a predetermined 
flying height is maintained over a full radial stroke of the rotary 
actuator assembly above the surface of the spinning magnetic disk. 
Head designs are being used with other storage technologies, in particular, 
magneto-optical (MO) storage technology. In one type of MO storage system, 
a magneto-optical head assembly is located on an actuator that moves the 
head along a radial direction of the disk to position the optical head 
assembly over data tracks during recording and readout. A magnetic coil is 
placed on a separate assembly on the head assembly to create a magnetic 
field that has a magnetic component in a direction perpendicular to the 
disk surface. A vertical magnetization of polarity, opposite to the 
surrounding material of the medium, is recorded as a mark indicating zero 
or a one by first focusing a beam of laser light to form an optical spot 
on the disk. The optical spot functions to heat the magneto-optical 
material to a temperature near or above a Curie point (i.e. a temperature 
at which the magnetization may be readily altered with an applied magnetic 
field). A current, passed through the magnetic coil, orients the 
spontaneous magnetization either up or down. This orientation process 
occurs only in the region of the optical spot where the temperature is 
suitably high. The orientation of the magnetization mark is preserved 
after the laser beam is removed. The mark is erased or overwritten if it 
is locally reheated to the Curie point by the laser beam while the 
magnetic coil creates a magnetic field in the opposite direction. 
Information is read back from a particular mark on the disk by taking 
advantage of the magnetic Kerr effect to detect a Kerr rotation of the 
optical polarization that is imposed on a reflected light beam by the 
magnetization at the mark of interest, the magnitude of the Kerr rotation 
being determined by the material's properties (embodied in the Kerr 
coefficient). The sense of the rotation is measured by established 
differential detection schemes as being clockwise or counter-clockwise 
depending on the direction of the spontaneous magnetization at the mark of 
interest. 
Conventional magneto-optical heads tend to be based on relatively large 
optical assemblies which make the physical size of the head rather bulky. 
Consequently, the speed at which conventional MO heads are mechanically 
moved to access new data tracks on a MO storage disk, known as a `seek 
time` is slow. In addition, due to the large size of these optical 
assemblies, most commercially available MO disk drives use only one MO 
head to enable reads and writes to one side of a MO disk at a time. 
Magneto-optical information access requires the use of polarized laser 
light for reading and writing information on an MO disk. In the case of 
reading information, MO technology makes use of the magneto-optical effect 
("Kerr" effect) to detect a modulation of polarization rotation imposed on 
the linearly polarized incident laser beam by the recorded domain marks in 
the recording layer. The polarization rotation (representing the 
information stored at recorded marks or in the edges of the recorded 
marks) is embodied in a reflection of the linearly polarized laser beam 
and is converted by optics and electronics for readout. Magneto-optical 
technology allows for increased storage capacity with drives through the 
ability to store information on the particular storage disk with an 
increased areal density. 
Larger capacity MO drives can be designed by increasing the number of disk 
platters and attendant read/write MO heads (over the current MO 
convention); this requires the provision of a MO head for each side of 
each disk platter. In these designs an increase in the rate of information 
transfer from the hard drive is desirable. The factors which effect 
overall information transfer from a hard drive include: a data-transfer 
rate, the head `seek` time (a function of a rate of movement of the 
actuator-arm-head assembly), and a drive rotation rate. Therefore, as the 
areal density increases, and the number of MO heads per drive increases, 
it is a corollary that the MO head size will need to be decreased for 
packaging reasons as well as to improve performance. 
To this end and others, what is needed is a method for manufacturing 
micro-lenses, alone or in combination with an optical head, with high 
numerical aperture, and/or by using the large volume processes of wafer 
production. 
OBJECTS AND ADVANTAGES 
The present invention provides several objects and advantages, including: 
(a) to provide for a micro-lens with a high numerical aperture; 
(b) to manufacture the lens using a wafer-level manufacturing technique; 
(c) to meet the constraints of (a) and/or (b) through a design of a 
composite lens having one or more lens elements; 
(d) to place aberration correcting contoured surfaces within the composite 
lens as necessary; 
(e) to provide the micro-lens design to be compatible with wafer-level 
processes applicable to total MO or optical head fabrication; and 
(f) to provide a head design with a small mass and size. 
SUMMARY OF THE INVENTION 
A numerical aperture is a measure of a resolving power of a lens, which is 
a function of the lens geometry and a refractive index of the lens-space 
medium. With a present invention, a high NA micro-lens is achieved through 
the use of a single lens or multiple converging lenses placed in series. 
In the present invention, a multi-component or composite lens is designed 
where each lens element and lens subassembly either is manufactured by, or 
can be used in, wafer-level processes in an array format. These available 
processes include; reflow of photoresist, and etching (including ion 
milling) the lens from a silica substrate. Because the individual lens 
elements can have the relatively low NA, these processes can be used to 
manufacture the various elements of the composite lens. 
The invention may construct a single or a stack of two or more 
lens-substrate subassemblies. For a two-lens design, this construction 
begins a first subassembly by placing a first element on a flat silica 
(SiO2) wafer by a combination of any of the mentioned processes; etching a 
curved surface into the silica wafer, the photoresist addition of a lens 
onto the wafer, or secondarily fixing a separate lens element to the 
wafer. A second subassembly is constructed in the manner similar to any of 
the approaches mentioned for the first subassembly. For subassemblies not 
having the wafer substrate micro-etched, high NA materials can be 
considered for the substrate to further `boost` the overall lens NA. 
Another approach is to provide a lens element from a sphere (ball) with a 
well-controlled diameter, which is readily obtained in a variety of 
materials having an index of refraction &gt;1.6 including a high-index glass 
or crystalline material. These spheres can be inserted into an opening 
etched into the wafer. The spheres may be glued or soldered in place and 
the assembly polished back to an appropriate thickness creating a flat 
surface on the sphere. The second wafer subassembly can be bonded to the 
first and individual composite lenses are obtained by dicing the resulting 
final wafer assembly. If needed, even more subassemblies can be added, or 
stacked, to achieve the desired numerical aperture. In addition, any wafer 
surface may be contoured to correct for aberrations introduced by the 
lenses. The lenses and other refractive surfaces may make use of 
anti-reflective (AR) coatings to improve light transmission. 
The composite micro-lens can then be placed into a magneto-optical (MO) or 
optical head to provide light convergence to a spot on a read or 
read/write surface. Further, the MO or optical head may be completely 
fabricated through wafer-level processes in which the micro-lens may be 
manufactured as part of a micro-head construction. In this case, rather 
than placing the lens in the head, the micro-lens could be fabricated as 
part of an integral construction of the overall head. The final head 
design is preferably compact and low mass to provide improved performance 
in the optical drive.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1 there is seen a cross-section view depicting a set of 
basic assembly steps for a composite lens of a preferred embodiment. Here 
a first subassembly 113 is made using a wafer substrate 108 made from, for 
example, silica, which may be polished to achieve a thickness and then, if 
desired, etched to obtain an aspheric contour 104. The aspheric contour, 
by proper design of the surface, acts as a lens element to provide 
correction to aberrations placed in the light path by other media such as 
the lenses and/or substrates. To the substrate 108 is applied a first 
photoresist reflow lens element 114 on a side opposite and centered to the 
aspheric contour 104. A second subassembly 117 wafer substrate 102 is 
polished flat to a thickness. A second lens element 118 is placed by photo 
resist on the second substrate surface 102. The second substrate 102 can 
be of a high numerical aperture (NA) material. An anti-reflective (AR) 
coating (not shown) may be applied to both first and second wafer 
subassemblies. The optical axis of the lenses 114/118 are aligned, and the 
subassemblies 113/117 bonded, such as with a UV cure adhesive 112 or the 
like, to form a composite lens 101. In an alternative embodiment (not 
shown) the aspheric contour 104 and the lens element 114 could be placed 
on opposite sides of one substrate. 
The lenses 114/118 may comprise a photo resist, for example, a phenol 
formaldehyde class of resin that functions effectively at a nominal 
operating temperature of 23.degree. C. and a 660 nm laser frequency used 
in an optical system. The phenol formaldehyde resin has a relatively high 
thermal expansion and a refractive index, which varies with a temperature, 
which should be considered in any design. This design reduces complexity 
of fabrication and leads to a higher yield with lower production risks. It 
is understood that the present invention is not limited to a phenol 
formaldehyde class of resins, as other optical resins currently exist and 
may be used by those skilled in the art. 
Referring now to FIG. 3, there is seen a cross-section of an exemplary 
embodiment. This design comprises two 100 .mu.m thick substrates, the 
first 108 of SiO2 and the second 102 of Schott SF56A glass (Schott Glass 
Technologies Inc., Durea, Pa.). SF56A provides the higher index medium 
through which a `boost` in the NA maybe obtained. As with the photoresist, 
it is to be understood that SF56A glass is only one of a number of 
materials that can be used in this application. As discussed above, each 
of the substrates has the custom designed photoresist micro-lens 
fabricated on one side. However, it should be kept in mind that a design 
may also place a lens on both sides of a single substrate to increase 
converging power. The first assembly 113 photoresist lens 114 has a radius 
of curvature of 0.253 mm on a 0.100 mm thick SiO2 substrate 108. The back 
surface of the SiO2 substrate 108 is etched to comprise an aspheric 
profile 104, which may be designed to correct for an aberration placed in 
a light path by the lenses and/or substrates. In other embodiments it is 
understood that an aspheric profile 104 may not necessarily be required. 
The second assembly 117 has the photoresist lens 118 with a 0.120 mm 
radius deposited onto a 0.100 thick substrate 102 of the SF56A glass. 
Referring now to FIGS. 2 and 4, there is seen an alternate embodiment 
including a third subassembly 120 comprising a lens element 115 with 
construction similar to subassemblies 113 and 117 of FIG. 1. A fourth 
subassembly 119 is manufactured to include a holder or patterned opening 
103 in a silica wafer substrate 106. A sphere or ball lens 107 is placed 
into the opening 103 and fixed in place using any effective means. Such 
means could include but are not limited to bonding with an adhesive such 
as an epoxy, or soldering with a metal film 110. One side of the wafer 
substrate 106 is then polished to provide the sphere 107 with a 
substantially flat surface 109 on and provides an exemplary 0.030 mm 
lens-to-image working distance. The two subassemblies are then aligned 116 
and bonded with a UV cure adhesive 112 to obtain a final assembly 100. In 
other embodiments the substantially flat surface 109 could further be 
etched or formed to comprise a contour or aspheric surface. 
In an exemplary embodiment, the third subassembly 120 photoresist lens 115 
comprises a radius of curvature of 0.248 mm on the 0.100 mm thick SiO2 
substrate 108 with a back surface of the substrate 108 having an aspheric 
profile 104. The fourth subassembly 119 comprises a 0.120 mm radius lens 
constructed by fixing the sphere 107 into the opening 103 in the 0.100 
thick substrate 106 of SiO2. In applications where the maintenance of a 
polarization is not required, materials such as a sapphire ball can be 
considered for use as the sphere lens 107, which is beneficial because 
they are readily obtained and inexpensive. Where the polarization state of 
light must be maintained throughout the light path, as in a MO head 
environment, various other optical glasses well known in the art, may be 
used for the ball lens. The two substrate assemblies are aligned 116 and 
bonded 112 such that the aspheric surface 104 and the sphere lens 108 are 
in optical alignment. 
Referring now to FIG. 5, there is seen a second alternate embodiment 
assembly in which a fifth subassembly 123 is constructed similar to the 
fourth subassembly 119 shown in FIG. 2. In this embodiment, a single piece 
lens-substrate subassembly 122 is constructed by etching a lens in a 
silica wafer (or a lens element may be etched or otherwise fabricated 
separately and secondarily bonded to a wafer). As with the other 
embodiments, an aspheric surface within a light path, may be contoured for 
aberration correction if required. Bonding of the two subassemblies 
122/123 completes a final assembly 124. Subsequent dicing may be used to 
produce an individual composite lens. 
Referring now to FIG. 6, there is seen a micro MO head 126 which includes 
components and subassemblies produced with, or in part by, wafer-level 
assembly methods, which may be subsequently assembled. Within FIG. 6 is 
seen a body 128, into which is positioned an optical fiber 130 to send and 
receive light to and from a mirror 132. The mirror 132 includes a moveable 
portion (not shown) to steer a light through the composite micro-lens 125 
and to focus the light to a spot (not shown) on the recording medium (not 
shown). Also included in the head 126 is a magnetic coil 136 to magnetize 
the recording medium. If a polarization light is used, a quarter wave 
plate 134 may be placed in the light path. 
Referring now to FIG. 7, there is seen another embodiment of a micro MO 
head, entailing a micro-composite lens 156 assembly 160 placed into a 
micro-head 170, which is producible in an array format using integrated 
wafer-level manufacturing techniques. This embodiment comprises a head 
body 140 with an aperture 144 etched to provide for placement of the 
optical fiber 142. Deposited on a surface of the body 140 is a 
micro-mirror 138. The 1/4 wave plate 146 may be added if polarized light 
is used such as with a MO head, but may not be required for other designs 
such as optical drives or applications where non-polarized light is used. 
Between the wave plate 146 and the micro-lens 156 is placed a spacer 152, 
or the space may be maintained by an equivalent built-up feature on the 
micro-lens body 156, or a design may not require a spacer at all. In a 
magneto-optical embodiment, the design includes a wafer 162 in which is 
manufactured a coil 164. In addition, a SiN `window` 166 may be placed on 
the opening of wafer assembly 162 to prevent debris from contacting the 
lens 156 surface. All of the aforementioned subcomponents are then bonded 
together (148, 150, 154, 158, & 168) into an array stack and diced to 
provide the completed individual heads. The heads may further be a flying 
head embodiment. 
Along with providing a lens with a high NA that can be manufactured with 
existing micropart techniques, there are additional benefits obtained with 
the present invention. The ease of manufacture and assembly when using 
automated wafer-level manufacturing techniques provides lower production 
costs through; higher throughput rates, the reduction of human `hands on 
the part` labor, and the compatibility with known manufacturing processes. 
This invention allows for a lens design that is tailorable through 
material selection, lens element design, and the `stacking` feature, to a 
wide range of requirements. Another benefit is this design uses low cost 
materials that are readily available now and in the near future. Also, the 
wide range of material and construction options allow for a design 
adaptable to lens shapes not easily produced in other ways, to changing 
material costs, changing product needs, and to material availability. 
Exemplary specification requirements and performance data are provided in a 
Table 1. For all designs, a total focused wavefront formed by a 
manufactured lens is compared to an ideal lens and the comparison is 
referred to as a RMS wavefront error. This difference is created by any 
improper surface contours as well as any radial misalignment of all lenses 
and any aspheric surface(s) to a common axis. A result is then calculated 
as a sum-tolerance value known as a `root-mean-square`. An Image FOV is 
the image-space field-of-view, an area over which a focused spot in image 
space can be moved while still maintaining the acceptable RMS wavefront 
error. An Object Space NA and an Image Space NA are, respectively, a sine 
of an angle of an outermost ray on an object side of the lens and an image 
side of the lens. Finally, a `Total Track` is a distance from an end of an 
optical fiber to a read or read/write disk surface (object-to-image 
distance for the lens). 
TABLE 1 
__________________________________________________________________________ 
Requirements and Design Specifications 
Preferred Alternate 
Requirement 
Embodiment (Photoresist Lenses) 
Embodiment (Ball-Photoresist Lenses) 
Parameter 
Value As-Designed Value 
As-Designed Value 
__________________________________________________________________________ 
Object Space NA 
0.12 0.12 0.12 
Image Space NA 
0.80 0.80 0.80 
Focused Spot Size 
&lt;0.48 .mu.m 
0.48 .mu.m &lt;0.41 .mu.m 
Object-to-lens 
&gt;0.5 mm 0.953 mm 0.878 mm 
distance 
Working distance 
&gt;0.03 mm 
0.03 mm 0.03 mm 
(lens-to-image) 
Total track 
&lt;1.3 mm 1.284 mm 1.194 mm 
(object-to-image) 
Lens diameter 
&lt;0.4 mm 0.230 mm 0.4 mm 
Lens thickness 
&lt;0.4 mm 0.301 mm 0.286 mm 
Wavelength 
660.0 .+-. 10.0 nm 
660 .+-. 10.0 mm 
660 .+-. 10.0 nm 
Operating 
10-60.degree. C. 
10-60.degree. C. 
10-60.degree. C. 
temperature 
RMS wavefront 
&lt;0.08 waves 
&lt;0.079 waves &lt;0.064 waves 
error 
Image FOV 
.+-.3.0 .mu.m 
.+-.3.0 .mu.m .+-.3.0 .mu.m 
Transmission 
&gt;87% &gt;87% &gt;87% 
__________________________________________________________________________ 
It should be noted that the present invention is applicable to an MO or 
optical disk drive used to record and reproduce information and in 
addition to an optical disk drive used explicitly for reproducing 
information. Further, the present invention is not limited to the 
embodiments described heretofore, but variations, alternate combinations, 
and modifications may be made for use in other optical applications 
without departing from the scope of the invention.