1. Field of the Invention
The invention is in the field of manufacturing integrated optical devices with two or more optical elements, e.g. refractive and/or diffractive lenses, in a well defined spatial arrangement on wafer scale by means of a replication process. Such integrated optical devices are, for example, camera devices, optics for camera devices, or collimating optics for flash lights, especially for camera mobile phones. More concretely, the invention relates to a wafer scale package comprising two or more substrates (wafers) that are stacked in an axial direction and have a plurality of replicated optical elements. The invention further relates to an optical device, e.g. a camera or a collimating optics therefor, comprising two or more replicated optical elements and optionally also electro-optical components, to a method for production of such a wafer scale package, and to a method of manufacturing a plurality of optical elements.
2. Description of Related Art
Manufacture of optical elements by replication techniques, such as embossing or molding, is known. Of special interest for a cost effective mass production are wafer-scale manufacturing processes where an array of optical elements, e.g. lenses, is fabricated on a disk-like structure (wafer) by means of replication. In most cases, two or more wafers with optical elements attached thereto are stacked in order to form a wafer scale package where optical elements attached to different substrates are aligned. Subsequent to replication, this wafer structure can be separated into individual optical devices (dicing).
Replication techniques include injection molding, roller hot embossing, flat-bed hot embossing, UV embossing. As an example, in the UV embossing process, the surface topology of a master structure is replicated into a thin film of a UV-curable replication material such as an UV curable epoxy resin on top of a substrate. The replicated surface topology can be a refractive or a diffractive optically effective structure, or a combination of both. For replicating, a replication tool bearing a plurality of replication sections that are a negative copy of the optical structures to be manufactured is prepared, e.g. from a master. The tool is then used to UV-emboss the epoxy resin. The master can be a lithographically fabricated structure in fused silica or silicon, a laser or e-beam written structure, a diamond turned structure or any other type of structure. The master may also be a submaster produced in a multi stage generation process by replication from a (super) master.
A wafer or substrate in the meaning used in this text is a disc or a rectangular plate or a plate of any other shape of any dimensionally stable, often transparent material. The diameter of a wafer disk is typically between 5 cm and 40 cm, for example between 10 cm and 31 cm. Often it is cylindrical with a diameter of either 2, 4, 6, 8 or 12 inches, one inch being about 2.54 cm. The wafer thickness is for example between 0.2 mm and 10 mm, typically between 0.4 mm and 6 mm.
If light needs to travel through the wafer, the wafer is at least partially transparent. Otherwise, the wafer can be nontransparent as well. It can also be a wafer bearing electro-optical components, e.g. a silicon or GaAs or other semiconductor based wafer; it may for example be a CMOS wafer or a wafer carrying CCD arrays or an array of Position Sensitive Detectors, a wafer carrying light sources such as LEDs or VECSELs, etc.
The wafer-scale replication allows the fabrication of several hundreds of generally identical devices with a single step, e.g. a single or double-sided UV-embossing process. The subsequent separating (dicing) step of the wafer then yields the individual optical devices.
Integrated optical devices include functional elements, at least one of which is an optical element, stacked together along the general direction of light propagation. Thus, light travelling through the device passes through the multiple elements sequentially. These functional elements are arranged in a predetermined spatial relationship with respect to one another (integrated device) such that further alignment with themselves is not needed, leaving only the optical device as such to be aligned with other systems.
Such optical devices can be manufactured by stacking wafers that comprise functional, e.g. optical, elements in a well defined spatial arrangement on the wafer. Such a wafer scale package (wafer stack) comprises at least two wafers that are stacked along the axis corresponding to the direction of the smallest wafer dimension (axial direction) and attached to one another. At least one of the wafers bears replicated optical elements, and the other can comprise or can be intended to receive optical elements or other functional elements, such as electro-optical elements. The wafer stack thus comprises a plurality of generally identical integrated optical devices arranged side by side. Precise positioning of the optical/functional elements on the different wafers, but also within the same wafer, is essential for the performance of the individual integrated devices. Subsequent dicing of the stack then yields the individual integrated optical devices.
By spacer means, e.g. a plurality of separated spacers or an interconnected spacer matrix as disclosed in US 2003/0010431 or WO 2004/027880, the wafers can be spaced from one another, and optical elements can also be arranged between the wafers on a wafer surface facing another wafer.
Wafer scale packages as presently known generally comprise two or more substrates that have optical elements arranged on both of their main surfaces. Such substrates are also designated as double-sided wafers/substrates. The optical elements are, for example, convex or concave structures, each forming a classical refractive (half-) lens. For optical design purposes, each pair of such structures/half-lenses on both sides of the wafer can be treated as a single classical lens with two convex/concave surfaces, for example. Generally, when trying to fulfill given performance requirements, the aim is to keep the optical design as simple as possible by reducing the number of lenses and to keep manufacture as simple and well-priced as possible by reducing the number of substrates. As a consequence, all designs actually employed in integrated devices use double-sided wafers, wherein empty surfaces are generally avoided.
An example for an optical device 1 manufactured from such a package according to the prior art is shown in
The following problems arise when manufacturing or handling such packages or devices:
The freely accessible optical elements on the end faces of the package are subject to damage or contamination by dust or an adhesive, especially during the dicing step and/or when further components like a camera or a flash light or other (opto-) electronic components are attached to the wafer scale package or the individual optical device. Protective hoods or cover plates or additional spacer means as described with respect to
Another problem is associated with the manufacture of double-sided wafers in a replication process. In a double sided substrate with optical structures on both main surfaces, it is necessary that the optical structures on both sides are precisely aligned with respect to one another. Consequently, the substrate has to be precisely aligned two times with respect to the replication tool, in a first step for replication of the structures on one surface and in a second step for replication of the structures on the second surface. Alignment in the second step is especially difficult, because of the structures already present on the other surface.
A further problem is that the substrates need a certain thickness to ensure stability during replication. Especially when replicating on the second surface, the substrate cannot be supported over its entire area due to the structures on the first surface.
There are further limitations associated with the present design. As discussed above, the optical structures on a double sided substrate can be considered as a single (double sided) lens. The optical parameters of this lens are influenced by the thickness of the substrate, and this thickness generally cannot be changed. Further, an aperture stop of common packages or devices, if any, normally coincides with the plane of one of the lenses. This is a restriction of the design possibilities and may also lead to unwanted collection of stray light into the device.
It is therefore an objective of the present invention to provide a wafer scale package as well as an optical device that overcomes the above mentioned problems and is easier to manufacture than known packages or devices having the same functionality. It is a further objective of the invention to provide a wafer scale package as well as an optical device ensuring protection of all optical elements from damage or contamination. It is a further objective of the invention to provide a wafer scale package as well as an optical device that is easy to manufacture and provides more freedom of design.
The wafer scale package according to the invention comprises at least two outer substrates and optionally one or more intermediate substrates stacked in an axial direction (perpendicular to the main surfaces of the substrates). A plurality of preferably closed cavities is arranged in between the substrates. In case of two substrates, there is one layer or group of cavities, in case of n substrates, there are generally n-1 or less groups or layers of cavities. Replicated optical elements, e.g. classical convex/concave lenses or diffractive/refractive microstructures, attached to the inner surfaces of the substrates, are arranged within the cavities. At least one pair of neighboring substrates of the package has optical elements on each of the surfaces which face one another. In other words, each cavity located in between this pair of substrates comprises two optical elements. Preferably, these optical elements are axially aligned.
The minimum wafer stack consists of two single sided substrates, i.e. substrates with replicated optical elements only on one of their main surfaces. The substrates are arranged such that the optical elements face one another, and the distance between them is defined by spacer means which may be a separate element or an integral part of one or both of the substrates. The outer surfaces of the substrates, i.e. the end faces of the package/stack do not comprise any replicated optical elements. Typically, there is also at least one intermediate substrate, again spacer separated. This intermediate substrate is preferably, but not necessarily double sided, i.e. comprises optical elements on both of its surfaces. The top substrate is typically a transparent wafer with optical elements on its inner surface. The bottom substrate may be a transparent substrate with or without optical elements, or it may be a substrate carrying an array of electro-optical components, in particular imaging elements (cameras, CCD, Position Sensitive Detectors) or light sources (LEDs or VECSELs etc.); for this purpose, silicon or GaAs or other semiconductor based (e.g. CMOS) wafers can be used.
According to the invention, the outer surfaces of the outer substrates and thus the end faces of the package and the optical device do not comprise any replicated optical elements. Thus, no replicated optical elements are exposed to the exterior. All optical elements are arranged in between the outer surfaces of the outer substrates, as seen in the axial direction. The end faces of the wafer stack are generally unstructured and planar. They may, however, contain apertures and/or alignment marks that leave the generally planar surface unchanged. They may also contain a coating such as an IR cutoff filter or an anti reflection coating. Such elements may be applied at a later stage after replication and stacking is completed.
The invention uses a completely different approach than in state-of-the-art designs as discussed in the introduction:
According to the invention, the lens of conventional designs—a double sided lens formed by optical structures on both surfaces of a transparent substrate (double sided substrate)—is split in two “halves” by having two substrates with optical structures on only one surface and a planar other surface. There are, thus, two single sided substrates instead of one double sided substrate, and the order of the “halves” is reversed. This means that the individual thickness of the two “halves” as well as their distance can be chosen individually and, thus, opens up new design degrees of freedom. The optical elements are shaped and arranged in such a way that the same optical performance as with the double sided lens is achieved. As there are no limitations with respect to the shape, thickness and distance of the optical elements, an even better performance can be achieved. This splitting generally concerns the outermost lens as seen in the axial direction. Intermediate substrates, if present, may be double-sided.
The invention makes possible that in integrated optical devices, especially, no lens is present on the outermost surface, i.e. the surface that is most remote from the active (e.g. CMOS) device. This is in contrast to the prior art, where the total number of wafers is minimized by using as many double-sided substrates as possible. Here, the outer substrates are single-sided or do not comprise any optical elements at all, e.g. in case of a CMOS wafer as bottom substrate of the stack. In other words, in contrast to the state of the art, the invention dispenses with an especially shaped refractive (or possibly diffractive) surface on the outermost layer that according to the state of the art was considered essential for achieving the best performance. This has the advantage that all optical elements are arranged in between the unstructured end faces of the system as seen in axial direction. They are, thus, protected from damage or contamination during manufacture and handling. The planar end faces simplify manufacture and handling as well as the optical design of the package. Nevertheless, not much additional space/additional elements is/are required. For example, in contrast to state-of-the art solutions, both, the lowermost and the topmost element of the assembly have a flat surface and may be assembled to rest directly against a surface of an other part—thus no additional, external spacers are required, sometimes even space saving and part saving solutions are possible. The latter is especially (also) suited for cases where passive and active optical components are manufactured at different places, as the stack with just passive optical components comprises no outermost lenses, it can be shipped without any sophisticated packaging protection (the protection is an intrinsical property of the wafer scale package and of the individual optical devices), and it can nevertheless be, in the final assembly, no more spacious than prior art assemblies.
In general, the inventive wafer scale package ensures a well defined spatial arrangement of the replicated optical elements and, optionally, by means of integrating a semiconductor substrate into the package further electro-optical components, as well as the simultaneous production of a plurality of identical optical devices with very small dimensions and at low cost. All optical elements are well protected during manufacture and handling, especially during the step of dicing the package into individual optical devices.
These and further beneficial effects will be described in more detail below.
Preferably, the cavities are closed such that all optical elements are fully encapsulated by the substrate and/or the spacer means also in a lateral direction. This can be achieved by using spacer means or recesses having an appropriate shape, e.g. through-holes in an otherwise continuous substrate.
The cavities are formed by connecting two neighboring substrates via spacer means, e.g. a plurality of separated spacers or an interconnected spacer matrix as disclosed in US 2003/0010431 or WO 2004/027880, and/or by using one or more preshaped substrates with a plurality of recesses.
The claimed optical device may be manufactured by dicing the wafer scale package described above. It is thus suited for mass production. It comprises at least two outer substrate portions stacked in an axial direction with at least one preferably closed cavity in between the substrate portions. The cavity is formed e.g. by using spacer means or a preshaped substrate, as described above. The device further comprises two optical elements, arranged in the at least one cavity. The optical device comprises two essentially planar end faces that are constituted by outer surfaces of the outer substrate portions. All optical elements are, thus, well protected.
In a preferred embodiment, the optical device is made of a wafer scale package with three or more substrates and, thus, comprises at least one intermediate substrate portion arranged in between the outer substrate portions, and two or more preferably axially aligned cavities spaced from one another by the intermediate substrate portion(s). The intermediate substrate portion(s) is/are preferably double sided, i.e. comprise(s) optical elements on both surfaces, while the outer substrate portions are single sided. The bottom substrate may be a substrate with electro-optical components, like an imaging device or light sources, on its inner surface. These components are also arranged within a preferably closed cavity and thus well protected. For example, the optical device may be a camera with integrated optics which can be mass produced at low cost, e.g. for the use in mobile phones.
The method for producing a wafer scale package comprises the following steps: Providing at least two substrates; providing said at least two substrates with a plurality of optical elements by means of a replication technique; stacking the at least two substrates in an axial direction; and connecting the at least two substrates in such a way that cavities enclosing the optical elements are formed, wherein end faces of the package are essentially planar and are constituted by outer surfaces of outer substrates of the package.
The method for producing an optical element, in particular a camera, comprises the steps of the method for producing a wafer scale package and further the step of dicing the package along planes running in axial direction in order to separate the package into individual optical elements. Preferably, dicing takes place along planes running through the spacer means such that the cavities in the individual devices remain closed and the optical elements arranged therein fully encapsulated.
The Invention has the Following Advantages:
Optical Design:
Mechanical design, especially if the optical device is used in a camera module:
Stack Manufacturing & Module Assembly:
One preferred application of the inventive optical device is for CMOS cameras, including CMOS cameras for mobile phones. Here, one of the flat and unstructured end faces could be directly used as the cover window of the camera, of a module within the camera, or even of the phone cover instead of a separate cover window. This leads to both simplified assembly and lower material cost.
The axial walls 42, 44, i.e. in
Optical elements 62, 64 are attached to the inner surfaces 24, 34 of the substrates 20, 30 at places corresponding to the locations of the cavities 40, and more particular, at places corresponding to the bottom and top walls 42, 44 of the cavities 40. The outer surfaces 22, 32 of the top and bottom substrate 20, 30 do not comprise optical elements. Consequently, each cavity 40 houses two optical elements 62, 64 such that they are encapsulated as seen in the axial direction. Preferably, the spacer means are shaped such that the optical elements 62, 64 are also encapsulated as seen in the lateral direction, such that all optical elements 62, 64 present are fully encapsulated and protected.
In this example, the optical elements 62 attached to the top substrate 20 are aligned with respect to the optical elements 64 from the bottom substrate 30 in the same cavity 40; other embodiments include also off-axis arrangements.
The package 10 shown in
The package 10 shown in
Individual optical devices 100 are manufactured by dicing the wafer scale package 10 along axial planes P. An example for an optical device 100 manufactured from a package as shown in
The individual optical device 100 may optionally be attached to a further substrate 80, e.g. a CMOS wafer carrying electronic components like an optical sensor, or a cover glass in case of a packaged sensor. Since the bottom end face 32′ of the bottom substrate portion 30′ is planar, attachment of the further substrate 80 is particularly easy, and there is also no danger of exposing the optical elements 62, 64 to any substances that might damage them when attaching the further substrate 80.
Instead of attaching the further substrate to the diced optical element 100, it may also be attached to the wafer package 10 prior to the dicing step, e.g. as disclosed in WO 2005/083789 which is incorporated herein by reference. This further simplifies manufacture.
An aperture 70 may be attached to or manufactured on the top end face 22′ of the optical device 100 or already on the top end face 22 of the package 10. As shown in
Like in
Dicing of stack 110 along the planes P again yields the individual optical devices (not shown).
As with the embodiments described above, the top and bottom substrates 220, 230 are single sided and comprise optical elements 262, 264 only on their inner surfaces 224, 234, while the outer surfaces 222, 232 and, thus, the end faces of the stack 210 are planar and without optical elements. The intermediate substrate 290 is double sided and comprises optical elements 266, 268 on both of its main surfaces 292, 294. The cavities 240, 240′ of the two layers are axially aligned with respect to one another. Within the cavities, the optical elements are also axially aligned; off-axis arrangements (not shown) are possible. Again, all optical elements are fully encapsulated. The individual optical devices 2100 are produced by dicing along the planes P.
Though one double sided substrate 290 is present in the embodiment of
For more complicated optical devices, further single or double sided intermediate substrates and corresponding spacer means may be incorporated into the stack.
The end faces 222′, 232′ do not comprise replicated optical structures, however. they may receive some sort of finishing treatment, e.g. polishing, attachment of apertures, attachment of a further substrate 280, like a CMOS wafer or a cover class. The further substrate 280 can be attached prior to or after dicing.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/CH2008/000487 | 11/18/2008 | WO | 00 | 10/22/2010 |
Number | Date | Country | |
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60990451 | Nov 2007 | US |