Method for making a plurality of integrated optical heads

<u>Field Of The Invention</u>

The present invention relates to the field of optics and, more particularly, to a method for making a plurality of integrated optical apparatuses.

<u>Background Of The Invention</u>

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 "<i>Planar Optical Scanning Head having Deficiency-Correcting Grating</i>" 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 <u>Optical Engineering</u> (June 1989) titled "<i>Holographic Optical Head For Compact Disk Applications</i>" by Lee.

Optical heads are known generally as disclosed in EP-A-0510750. Alignment of an optical fiber to a laser is disclosed in "Marker Alignment Method for Passive Laser Coupling on Silicon Waferboard" in IEICE Transactions on Communications, Vol. E79-B, No. 7, July 1996, pp. 939-942 and U.S. Patent No. 5,499,732. The use of a plurality of substrates to mount a plurality of optical elements is disclosed in French patent FR-A-2,725,547. The use of a side emitting laser in an optical head is disclosed in EP-A-0 581 519. However, none of the documents teaches passively aligning an electrical element, e.g., a laser or a detector, whose light is to pass through a substrate on which an optical element is provided.

Unfortunately, these active alignment requirements are complex, time constuming, 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.

<u>Summary Of The Invention</u>

With the foregoing background in mind, it is therefore an object of the present invention to provide a method for making a plurality of integrated optical apparatuses, in particular optical heads, which are more compact and less expensive to manufacture.

This and other objects, advantages, and features of the present invention are provided by the method for making a plurality of integrated optical apparatuses, that comprises providing a first transparent wafer; positioning a plurality of light sources and optical detectors adjacent the first wafer; providing a second wafer having a corresponding plurality of first optical elements; and passively aligning the second wafer with said first wafer so that each of the first optical elements is positioned in an optical path of a corresponding light source or optical detector.

The integrated optical head preferably includes an optically transparent substrate having first an 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 the 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 diftractive 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 beinc required for connecting the first and second substrates.

The method according to the present invention 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 laser 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 be passively aligned using either the contact pads and solder bumps, or the mechanical alignment as discussed above.

The method according to the present invention advantageously provides 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.

<u>Brief Description of The Drawings</u>

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: <ul id="ul0001" list-style="none" compact="compact"><li>FIG. 1 is a schematic view of an integrated optical head according to the present invention;</li><li>FIG. 2 is a fragmentary side perspective view of an integrated optical head according to the present invention;</li><li>FIG. 3 is a side elevational view of an integrated optical head according to the present invention;</li><li>FIG. 4 is side elevational view of the integrated optical head as shown in FIG. 3 rotated ninety degrees.</li><li>FIG. 5 is a plan view of the component side of a first transparent substrate of an integrated optical head according to the present invention;</li><li>FIG. 6 is a plan view of a holographic optical element of a first transparent substrate of an integrated optical head according to the present invention;</li><li>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;</li><li>FIG. 8 is a plan view of diffractive optical elements of an integrated optical head according to the present invention;</li><li>FIG. 9 is an enlarged view of mask portions of FIG. 4 according to the present invention;</li><li>FIG. 10 is a perspective view showing an article including two wafers according to the present invention;</li><li>FIGS. 11A-11D are vertical fragmentary sectional views of example alignment features according to the present invention; and</li><li>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.</li></ul>

<u>Detailed Description of the Preferred Embodiments</u>

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 those skilled in the art. Like numbers refer to 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 in 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 <b>d1/d2</b> 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 demagnified 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 <b>5</b> is mounted, in arm <b>3</b> by known means not shown, for the extension across the various tracks of media <b>31</b>. Head 5 is electrically connected to read and write circuits and tracing control circuits by a flexible printed circuit <b>7</b>. A recess <b>9</b> of approximately two millimeters by one point six millimeters and four and a half or five millimeters deep is provided in head <b>5</b> in which the optical assembly comprising substrate <b>11</b> is mounted and connected to flexible printed circuit <b>7</b>. 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 <b>13</b> of substrate <b>11</b> is approximately 1.6 mm by 2 mm and the substrate <b>11</b> is approximately 0.8 mm thick. A laser chip <b>15</b> is mounted to the surface <b>13</b> by means of some of the mentioned metalized pads. As shown in FIG. 4, laser <b>15</b> 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 <b>15</b> 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 <b>17</b> is also mounted to the component surface of substrate <b>11</b> by means of the metalized pads. A hologram surface <b>19</b> on the opposite side of substrate <b>11</b> 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 <b>19</b> also has the attachment areas <b>23</b> that attach the first transparent substrate <b>11</b> with a second transparent substrate <b>21</b>. These attachment areas and later described alignment areas are shown in detail in FIGS. 11 and 12.

The second substrate <b>21</b> carries the refractive optics in a surface <b>25</b> that provides the second lens of lens pairs or doublets. Light from laser <b>15</b> is shaped and split by a diffractive optical element in hologram surface <b>19</b> 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 <b>15</b>. The other four beams are used for media position or tracking detection. The beams of coherent light are reflected from media <b>31</b> and return through second substrate <b>21</b> and first substrate <b>11</b> to be detected by detector <b>17</b>. 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 <b>19</b>, the order of the optical elements 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 <b>15</b> comes out substantially parallel to the plane of component surface <b>13</b> and must be directed normal to the component surface by the <b>45</b> degree surface of mirror <b>33</b>. The light can then pass through substrate <b>11</b>, a diffractive optical element in surface <b>19</b>, a refractive lens <b>61</b> in surface <b>25</b>, substrate <b>21</b> and be reflected from media <b>31</b> as shown in FIGS. 1 and 3.

FIG. 5 is a plan top view of the component surface <b>13</b> looking down through transparent substrate 11. Electrical contact metalizations <b>39, 41, 43</b> and <b>45</b> provide electrical connections to detecting photo-diodes in detector <b>17</b>. Centered under detector <b>17</b> is a metalized area <b>53</b> having three apertures through which light reflected from media <b>31</b> is received. Solder ball alignment areas <b>47</b> on each side of area <b>53</b> serve both as electrical contacts and as alignment mechanisms in this embodiment. The areas <b>49</b> are also solder ball pads and serve to align and connect the laser <b>15</b> to the first substrate and provide current to laser <b>15.</b> Areas <b>51</b> on the other hand only provide mechanical alignment and mechanical attachment of mirror 33 to first transparent substrate <b>11</b>.

The hologram surface <b>19</b> appears in FIG. 6 in plan view, again looking down onto substrate <b>11.</b> Hologram surface <b>19</b> has metalized area <b>55</b> which acts as a mask to reduce stray light but allow three beams created by diffractive optics from the light from laser <b>15</b> to be directed to media <b>31</b> from which they are reflected to reach detector <b>17</b> through the five apertures shown in metalized areas <b>59.</b> Surrounding metalized area <b>55</b> is a diffraction grating <b>57</b> that scatters stray light from laser <b>15</b> so that it does not adversely affect detector <b>17</b>.

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

Surrounding both surface <b>19</b> and surface <b>25</b> is an attachment area shown generally as area <b>71</b> in FIGS. 6 and 7. Area <b>71</b> contains spacing stand off benches and is the area in which an adhesive is placed in order to join substrate <b>21.</b> 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 <b>71</b> and then after the substrates <b>11</b> and <b>21</b> 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 <b>73, 75</b> with mask <b>55.</b> 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 <b>75</b> provides the power control beam that is reflected from the media and is received at aperture <b>79</b> in mask <b>59</b> as shown in FIG. 8. Elements <b>73</b> and <b>77</b> each provide two beams that interfere at the 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 <b>81, 83</b> and <b>85, 87</b> shown in FIG. 8 to provide the varying light intensity that is used to detect an optical track on the media. The apertures <b>73, 75</b> and <b>77</b> containing diffractive elements are each approximately 100 <i>µ</i>m long and 20 <i>µ</i>m wide.

Referring now to FIG. 9, the apertures of FIG. 8 are shown enlarged. The ends of each aperture <b>73, 75</b> and <b>77</b> 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 <b>11</b> and <b>21</b> 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 <b>11</b>. 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 <b>23</b> is placed in the area <b>71</b> 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 <b>93</b> and <b>95</b> to monitor the relative displacement of the two substrates until they are in alignment with each other. The substrate <b>11</b> can then be lowered onto substrate <b>21</b>, the alignment rechecked, and the adhesive catalyzed by ultraviolet light.

In another embodiment, the two wafers are passively aligned using the alignment means <b>91</b>. Three forms of alignment means are contemplated and shown in FIGS. 11A, 11B and 11C. One, shown in FIG. 11A, takes the form of V-shaped grooves <b>97</b> etched into matching faces of the substrates <b>11</b> and <b>21.</b> These grooves are then aligned with sphere <b>99</b> 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 <b>101</b> which are then connected by reflowing a solder ball <b>103.</b> In a still further embodiment of FIG. 11C, a bench <b>105</b> is raised by etching the surrounding surface and the bench <b>105</b> is indexed into a recess <b>107</b>, also created by photolithographically placed etchant, preferably reactive ion etchant.

In the adhesive area <b>71</b> 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 <b>109 s</b>hown in FIG. 11D. Three or more benches <b>109</b> are located in the area <b>71</b> around each die in an adhesive with high compressive. In another embodiment, the solder bumps or balls and metalizations are used in area <b>23</b> 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 <b>25</b> 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 <b>113.</b> Thereafter, the surface <b>25</b> is etched and a refractive element <b>115</b> having substantially the same shape as the photoresist <b>113</b> is formed by the variable etch rate of the continuously varying thickness of the photoresist <b>113.</b> In the event that a hybrid optical element is desired, the microlens <b>115</b> is further processed by etching or embossing steps. In one embodiment, a layer of photoresist <b>117</b> is placed over the microlens <b>115</b> 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 <b>119.</b> 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 <b>121.</b> 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 scope of the invention as defined in the appended claims.