Integrated optical head apparatus and associated methods

Description of equivalent WO 9813825 A2

INTEGRATED OPTICAL HEAD APPARATUS AND ASSOCIATED METHODS

Field Of The Invention

The present invention relates to the field of optics and, more particularly, to an integrated optical head, such as for use in a disk drive.

Background Of The Invention Many typical computer systems include a disk drive cooperating with storage media to permit storage and retrieval of data. A typical optical disk drive includes an optical head that conventionally uses a laser to transmit light to the optical disk. Light reflected from the surface of the disk is detected by an optical detector and processed to read data from the disk. An example of such an optical head is disclosed, for example, in U.S. Patent No. 5,204,516 titled "Planar Optical Scanning Head Having Defi ciency- Correcting Gra ting" by Opheij . The size of the various optical head components, however, are often too large for many desired applications and many market demands . Also, as densities of integrated circuits and system boards increase, the demand for smaller components increases. Additionally, the production process for a conventional optical head requires that the laser be excited or turned-on (i.e., "active alignment") for alignment of the laser, the detector, and the optical elements. An example of active alignment processes is illustrated and described in an article published in Optical Engineering (June 1989) titled "Holographi c Op ti cal Head For Compact Di sk Applica tions " by Lee.

Unfortunately, these active alignment requirements are complex, time consuming, and relatively expensive. Further, the level of size reduction in the vertical direction of an optical head is limited. In addition, the relatively large size of the elements of an optical head which can be manipulated is determined by the need for active alignment.

Summary Of The Invention

With the foregoing background in mind, it is therefore an object of the present invention to provide an optical head, such as for a disk drive, and related methods which is more compact and less expensive to manufacture .

This and other objects, advantages, and features of the present invention are provided by an integrated optical head that relies on passive alignment of the various components. The integrated optical head preferably includes an optically transparent substrate having first and second faces. The substrate may include a diffractive optical element formed on the second face of the substrate. An optical light source, such as a laser, is positioned adjacent the first face of the substrate to transmit light through the substrate, through the diffractive optical element, and toward a target, such as optical storage media. An optical detector is positioned adjacent the first surface of the substrate to detect light reflected from the storage media, through tha diffractive optical element, and through the substrate. Passive alignment means is positioned between the first surface of the substrate and at least one of the laser and the optical detector for passively aligning the laser or the optical detector with respect to the substrate. Accordingly, the laser and detector may also be aligned with the optical elements on the second surface of the substrate.

An optical head according to the present invention may provide a size reduction of more than three times compared to the prior art, in part, based upon photolithographically shaped and placed refractive optical elements, as well as diffractive optical elements. Further, the laser and detector are preferably also accurately and passively aligned by means of photolithography. More particularly, in one embodiment, passive alignment is achieved by the wetted area and volume of solder in opposing alignment areas provided by contact pads. In another embodiment, a second transparent substrate is aligned and joined to the first substrate. The second substrate may carry one or more optical elements. According to this aspect of the invention, alignment areas in the form of benches or other mechanical features may be formed in one surface and mating recesses, for example, may be formed in the other surface. Adhesive attachment areas, which may overlap the alignment areas, hold the substrates together. Alignment may also be accomplished at the wafer level by having the elements of each die accurately placed using photolithography to accurately align the two wafers. The assembled dies can then be diced without the individual alignment means or steps being required for connecting the first and second substrates.

Methods of forming an optical head are also provided according to the present invention. A method of forming an optical head preferably includes forming at least one optical element on a first face of a transparent substrate and positioning a laser adjacent the first face of the substrate so as to emit light through the substrate, through the at least one optical element, and toward the data storage media. An optical detector preferably is positioned adjacent the first face of the transparent substrate adjacent the ]aser to detect light reflected from the data storage media, through the substrate, and to the optical detector. The laser, the optical detector, and/or the at least one optical element on the substrate may passively aligned using either the contact pads and solder bumps, or the mechanical alignment as discussed above. An integrated optical head and the related methods according to the present invention advantageously provide a significantly smaller optical head for fabrication without the need for exciting or turning on the laser light source to actively align the components. The integrated optical head according to the present invention overcomes the disadvantages of the prior art so that the wavelength of the laser is left as the predominant limiting factor in size reduction instead of other considerations.

Brief Description Of The Drawings Some of the objects and advantages of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings in which: FIG. 1 is a schematic view of an integrated optical head according to the present invention;

FIG. 2 is a fragmentary side perspective view of an integrated optical head according to the present invention;

FIG. 3 is a side elevational view of an integrated optical head according to the present invention;

FIG. 4 is side elevational view of the integrated optical head as shown in FIG. 3 rotated ninety degrees . FIG. 5 is a plan view of the component side of a first transparent substrate of an integrated optical head according t o t ho piesont- invon on; FIG. 6 is a plan view oi a holographic optical element of a first transparent substrate of an integrated optical head according to the present invention;

FIG. 7 is a plan view of a refractive lens surface of a second transparent substrate of an integrated optical head according to the present invention;

FIG. 8 is a plan view of diffractive optical elements of an integrated optical head according to the present invention; FIG. 9 is an enlarged view of mask portions of FIG. 4 according to the present invention;

FIG. 10 is a perspective view showing an article including two wafers according to the present invention; FIGS. 11A-11D are vertical fragmentary sectional views of example alignment features according to the present invention; and

FIG. 12 is a vertical sectional view of a substrate showing a method of creating a hybrid microlens for an integrated optical head according to the invention.

Detailed Description of the Preferred Embodiments

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein. Rather, these illustrated embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to t ose skilled in the art . Like numbers refer lo like elements throughout, and prime and double prime notation are used to indicate similar elements in alternative embodiments.

FIG. 1 is an optical design schematic of an assembly according to the invention for use Ln detecting an optical track on a storage media. A light source 10 directs coherent light, with a dispersion angle of fifteen degrees, upward through an object distance dl through a diffractive element (DOE) not shown and to a refractive lens 12. The DOE divides the light into a number of beams, only three of which are shown as a plurality of rays in FIG. 1. The beams are focused on surface 14 located at image distance d2 from the lens 12. The spot size and spacing of the light on the image surface 14 determines the tracking accuracy and therefore the amount of information that can be stored on the media. The size to which the spot can be reduced is in the instant design, approximately 0.020 mm. In the design of FIG. 1, the refractive lens 12 must have a significant curvature in order to focus the light to 0.020 mm spots on the media. The spots of light are spaced approximately 0.100 mm from each other on the media to limit crosstalk noise. As would be readily understood by those skilled in the art the optical head can be positioned by the illustrated positioning means 29.

If a design were attempted using a single lens as taught in the prior art, the lens curvature required to focus the laser light to 0.020 mm spots in this compact architecture would control the dimensions of the single lens. Thus the use of a single lens as taught in the prior art for reducing the size of optical heads, is a limiting factor in size reduction of the entire optical head assembly. This factor is one of the reasons that multiple lenses are employed in the instant invention instead of a single lens.

The distance dl is used to advantage to provide an adequately wide beam at the DOE as shown later with respect to FIG. 7. The distance d2 is chosen to achieve adequate spot size modulation depth and depth of focus at the media surface.

The ratio of the distances dl/d2 determines the amount of demagnification of the image reflected from the media that occurs in a lens. In a single lens design, this demagnification affects not only spot size but spot spacing. A demagnification of 1/4 gives a spot size of 0.005 mm which because of aberration is spread to an area 0.025 mm. If a single lens design had been used, the spacing of the spots would also have been demagnif ied to 0.025 mm and significant crosstalk noise would result. By using individual lenses, spaced approximately 0.200 mm, the detectors can be spaced at about 0.220 mm and thereby eliminate crosstalk noise using the 0.025 mm light spots.

FIG. 2 is a side view of a magnetic floppy disk head 5 with an optical tracking assembly according to a preferred embodiment of the invention. Head 5 is mounted, in arm 3 by known means not shown, for the extension across the various tracks of media 31. Head 5 is electrically connected to read and write circuits and tracing control circuits by a flexible printed circuit 7. A recess 9 of approximately two millimeters by one point six millimeters and four and a half or five millimeters deep is provided in head 5 in which the optical assembly comprising substrate 11 is mounted and connected to flexible printed circuit 7. It will be appreciated that the same assembly techniques and methods of the invention may be used to assemble optical disk read heads, as well as magnetic disk heads with optical tracking. Referring now to FIG. 3, a first transparent substrate 11 comprising fused silica or other optical material has component mounting metalized pads or contact pads placed on its bottom surface 13, such as using substrate fiducial marks or indicia and accurately aligned photolithographic masks and metal deposition steps known in the art of microelectronic circuit manufacture. In this preferred embodiment, surface 13 of substrate 11 is approximately 1.6 mm by 2 mm and the substrate 11 is approximately 0.8 mm thick. A laser chip 15 is mounted to the surface 13 by means of some of the mentioned metalized pads. As shown in FIG. 4, laser 15 is an edge emitting laser with the laser light directed upwards through means of a precision mirror 33 as shown in FIG. 4. It will by understood that the edge emitting laser 15 can be replaced with a vertical cavity surface emitting laser and thereby obviate the need for the precision mirror in order to direct the laser beam normal to the substrate surface.

An optical detector chip 17 is also mounted to the component surface of substrate 11 by means of the metalized pads. A hologram surface 19 on the opposite side of substrate 11 carries the diffractive optical elements shown in detail in FIG. 7. The diffractive optical element phase profiles are designed using the computer calculations and manufactured using techniques taught by Swanson et al . in U.S. Patent 5,161,059, the entire disclosure of which is incorporated herein by reference.

The optical elements are created photolithographically using the same fiducial marks or indicia used to place the metalized pads. Alternately second fiducial marks that have been aligned with the first marks may be used to align the masks that are also used to create the optical elements. In this way-, when the light source, mirror and detector are mounted on their metalized pads, the optical paths among the devices and through the optical elements are in optical alignment as shown more clearly in FIGS. 3 and 4. The precision mirror, if needed for redirecting light from an edge emitting laser, is considered to be a device for the purposes of this description only because of the way it is mounted using metalized pads and solder as a silicon chip would be mounted. The hologram surface 19 also has the attachment areas 23 that attach the first transparent substrate 11 with a second transparent substrate 21. These attachment areas and later described alignment areas are shown in detail in FIGS. 11 and 12.

The second substrate 21 carries the refractive optics in a surface 25 that provides the second lens of lens pairs or doublets. Light from laser 15 is shaped and split by a diffractive optical element in hologram surface 19 into five separate beams of light that are directed through substrate and travel approximately 2.4 mm to the media. Only the chief ray of each beam is shown in FIG. 3 for clarity of the description. One beam is used for intensity feedback to control the electrical power to laser 15. The other four beams are used for media position or tracking detection. The beams of coherent light are reflected from media 31 and return through second substrate 21 and first substrate 11 to be detected by detector 17. Since the elements are all in their designed optical alignment by virtue of the placement of the metalization pads, there is no need to energize the laser and move the elements relative to each other to bring them into optical alignment. In other words, passive alignment is used rather than the active alignment requiring operation of the laser as in the prior art. It will be recognized that although the beams preferably pass first through the diffractive optical element in surface 19, the order of the optical ele ents in the light path could be changed or the elements could be combined into one more complex element without departing from the scope of the invention . FIG. 4 is another side view of the assembly of FIG. 3. As shown in FIG. 4, the light emitted by edge emitting laser 15 comes out substantially parallel to the plane of component surface 13 and must be directed normal to the component surface by the 45 degree surface of mirror 33. The light can then pass through substrate 11, a diffractive optical element in surface 19, a refractive lens 61 in surface 25, substrate 21 and be reflected from media 31 as shown in FIGS . 1 and 3. FIG. 5 is a plan top view of the component surface 13 looking down through transparent substrate 11. Electrical contact metalizations 39, 41, 43 and 45 provide electrical connections to detecting photo-diodes in detector 17. Centered under detector 17 is a metalized area 53 having three apertures through which light reflected from media 31 is received. Solder ball alignment areas 47 on each side of area 53 serve both as electrical contacts and as alignment mechanisms in this embodiment. The areas 49 are also solder ball pads and serve to align and connect the laser 15 to the first substrate and provide current to laser 15. Areas 51 on the other hand only provide mechanical alignment and mechanical attachment of mirror 33 to first transparent substrate 11. The hologram surface 19 appears in FIG. 6 in plan view, again looking down onto substrate 11. Hologram surface 19 has metalized area 55 which acts as a mask to reduce stray light but allow three beams created by diffractive optics from the light from laser 15 to be directed to media 31 from which they are reflected to reach detector 17 through the five apertures shown in metalized areas 59. Surrounding metalized area 55 is a diffraction grating 57 that scatters stray light from laser 15 so that it does not adversely affect detector 17.

FIG. 7 shows the refractive lens surface 25, again in plan view looking down, this time through substrate 21. Lens 61 in combination with the diffractive optical elements in mask 55 shape and focus the laser light into three spots of approximately 20 μm diameter and spaced at approximately 100 μm onto media 31. Lenses 63 and 65 focus the light reflected from media 31 through mask 59 to detector 17 for position control and/or reading. Lens 67 focuses reflected light to the photo-diode of detector 17 that provides an intensity level signal to the power control circuits which control the electrical power provided to laser 15.

Surrounding both surface 19 and surface 25 is an attachment area shown generally as area 71 in FIGS. 6 and 7. Area 71 contains spacing stand off benches and is the area in which an adhesive is placed in order to join substrate 21. The standoff benches passively define a proper or desired vertical spacing or alignment. Preferably the adhesive is ultraviolet light cured adhesive that can be laid down without concern for time to harden. The adhesive is placed in areas 71 and then after the substrates 11 and 21 are aligned, the assembly is flooded with ultra-violet light to catalyze the adhesive.

In an alternate embodiment, the adhesive is replaced with photolithographically placed metalization pads and the two substrates are joined using solder ball technology.

FIG. 8 shows three diffractive optical elements 73, 75 with mask 55. These three elements provide the five beams of light to be reflected from the media, the three main rays of which are shown in FIG. 3. Element 75 provides the power control beam that is reflected from the media and is received at aperture 79 in mask 59 as shown in FIG. 8. Elements 73 and 77 each provide two beams that interfere at th<=> media surface to create a dark band with two Light bands on either side of the dark bands. The light bands are reflected back down to the pairs of apertures 81, 83 and 85, 87 shown in FIG. 8 to provide the varying light intensity that is used to detect an optical track on the media. The apertures 73,, 75 and 77 containing diffractive elements are each approximately 100 μm long and 20 μm wide.

Referring now to FIG. 9, the apertures of FIG. 8 are shown enlarged. The ends of each aperture 73, 75 and 77 are provided with an irregular boundary that change the orientation of the interference fringes so that they are not parallel with the optical track being detected and accuracy is improved.

FIG. 10 shows the two substrates 11 and 21 prior to their being assembled into optical assemblies and diced. Because each element has been accurately placed on each substrate using photolithography, the entire wafers can be aligned and joined prior to being diced into chips without the need to energize any of the laser devices on the substrate 11. FIG. 10 shows the substrates inverted from the way they are shown in FIGS. 2, 3 and 4 in order to show the lasers, mirrors and detectors in place on top of each die.

Prior to putting the wafers together, the adhesive material 23 is placed In the area 71 of each die on at least one of the wafers. After the adhesive is placed, the two wafers are placed one above the other and aligned. In one embodiment of the invention, a known photolithographic mask aligning tool is used with vernier fiduciary marks 93 and 95 to monitor the relative displacement of the two substrates until they are in alignment with each other. The substrate 11 can then be lowered onto substrate 21, the alignment rechecked, and the adhesive catalyzed by ultraviolet 1 ight .

In another embodiment, the two wafers are passively aligned using the alignment means 91. Three forms of alignment means are contemplated and shown in FIGS. 11A, 11B and llC. One, shown in FIG. 11A, takes the form of V-shaped grooves 97 etched into matching faces of the substrates 11 and 21. These grooves are then aligned with sphere 99 to index the two wafers into alignment. Note that only a few grooves and spheres are needed to align all of the dies while they are still together as a wafer. Another embodiment of the alignment means, shown in FIG. 11B, comprises photolithographically placed metalization pads 101 which are then connected by reflowing a solder ball 103. In a still further embodiment of FIG. 11C, a bench 105 is raised by etching the surrounding surface and the bench 105 is indexed into a recess 107, also created by photolithographically placed etchant, preferably reactive ion etchant.

In the adhesive area 71 of each die, means may be needed to accurately space the two substrates from each other. Spacing is accomplished in one embodiment by means of a bench 109 shown in FIG. 11D. Three or more benches 109 are located in the area 71 around each die in an adhesive with high compressive. In another embodiment, the solder bumps or balls and metalizations are used in area 23 accomplishing both attachment and alignment as shown in FIG. 11B. Alternately, when an adhesive with high compressive strength is chosen, only three or more such benches are needed for spacing the entire wafers and after the adhesive has set, the joined wafers can be diced without substrate spacing. Referring now to FIG. 12, a method of photolithographically placing an optical element on a substrate surface 25 in alignment with diffractive elements and/or electrical devices is shown. A refractive optical element in the form of a microlens 115 is formed by placing a circular layer of photoresist 111 on a surface of optical material using a mask. The photoresist is then partially flowed using controlled heat so that the photoresist assumes a partially spherical shape 113. Thereafter, the surface 25 is etched and a refractive element 115 having substantially the same shape as the photoresist 113 is formed by the variable etch rate of the continuously varying thickness of the photoresist 113. In the event that a hybrid optical element is desired, the microlens 115 is further processed by etching or embossing steps. In one embodiment, a layer of photoresist 117 is placed over the microlens 115 and exposed through a photolithographic mask with the phase pattern of a diffractive optical element. When the exposed photoresist is then developed, the surface of the microlens can be further etched with the diffractive optical element pattern to produce a hybrid optical element 119. In another embodiment, a polymer is placed over the microlens in place of the photoresist and the phase pattern is embossed into the polymer as shown at 121. It also will be understood that although a convex element has been shown, the same technique can be used to create a concave microlens .

Having described the invention in terms of preferred embodiments thereof, it will be recognized by those skilled in the art of optical system design that various further changes in the structure and detail of the implementations described can be made without departing from the spirit and scope of the invention. By way of example, the diffractive optical elements may be placed on the same surface of a substrate on which the electronic components are accurately placed with these diffractive optical elements using photolithography. Likewise refractive optical elements may be placed using photolithography in alignment on the other surface of the same substrate thereby allowing an entire optical assembly to be fabricated using but one substrate without the need for actively energizing a light source in the assembly to accomplish alignment .

In the drawings and specification, there have been disclosed illustrated preferred embodiments of the invention, and although specific terms are employed, the terms are used in a descriptive sense only and not for purposes of limitation. The invention has been described in considerable detail with specific reference to these illustrated embodiments. It will be apparent, however, that various modifications and changes can be made within the spirit and scope of the invention as described in the foregoing specification and as defined in the appended claims.