WO2003069389A1

WO2003069389A1 - Three dimensional alignment method and system

Info

Publication number: WO2003069389A1
Application number: PCT/SE2003/000253
Authority: WO
Inventors: Edvard KÄLVESTEN; Thorbjörn EBEFORS; Patrick Griss; Sjoerd Haasl; Hans SOHLSTRÖM; Göran Stemme
Original assignee: Silex Microsystems Ab
Priority date: 2002-02-14
Filing date: 2003-02-14
Publication date: 2003-08-21
Also published as: AU2003206553A1

Abstract

ABSTRACTA system for achieving fixation of one or more moveable micro machined platforms for passively aligned optical components, comprising - a support structure containing structures for passive alignment of optical components- platform(s) containing structures for passive alignment of optical components- actuators for actively aligning the already passively aligned components on the platform(s) and the support structure to each other, where the actuators are strong enough while actuating to keep the platform(s) steady during fixation and weak enough while not actuating so as not to impede the fixation- a fixation mechanism to fixate the moveable platform(s) to the support structure.

Description

THREE DIMENSIONAL ALIGNMENT METHOD AND SYSTEM

Technical field

The present invention relates to MEMS-based techniques for active in-package optical alignment and maintenance re-alignment of semiconductor lasers and other micro-optical devices to fibers. In particular it relates to micromechanical polymer thermal actuators for two and three directions of freedom, electrically controllable solder fixation methods and mounting techniques for optical components.

Background of the invention

Semiconductor lasers and other micro-optical components need to be well aligned to the components that are used to couple light into optical fibers or other components.

Laser chips are often mounted on a carrier which in turn is mounted in the laser package together with one or two lenses, the fiber, and in many cases an isolator module consisting of polarizers, a YIG crystal rod and a permanent magnet. At present, two basic approaches are used for laser module alignment.

Passive alignment on a MEMS carrier using self-aligning solder technology or special mounting features is used with low or medium cost laser components were a slight increase in optical loss due to misalignment can be tolerated. At best, an alignment tolerance of about 2 μm can be achieved.

For more advanced laser types, e.g. tunable lasers and lasers with external modulators, active alignment and fixation using an external positioning and welding equipment is the standard procedure. This is however costly, time-consuming and requires well-trained technicians. Many other optical components, such as optical semiconductor amplifiers and some types of optical switches, have similar alignment and mounting requirements. There are very few publications dealing with on-chip alignment. What we have found is aligners without fixation and devices that seem less applicable to laser alignment.

The following publications, hereby incorporated by reference in their entirety, disclose various methods applicable for carrying out certain procedures not explicitly disclosed herein.

[1] Thorbjόrn Ebefors, Polyiinide V-groυe joints for Three- Dimensional Silicon Transducers, PhD Thesis, TRITA-ILA-0001, ISSN 0281-2878, ISBN 91-7170-568-6,

Royal Institute of Technology, Department of Signals Sensors and Systems, Stockholm 2000.

[1] J. M. Haake, "In-package microaligner speed fiberoptic packaging", Laser Focus World, October 1998, pp. 169-171, http : //lfw.pennwellnet. com/Articles/Article_Display . cfm?ARTICLE_ID =26376

[1] L. Fan and M. C. Wu, "Self-assembled micro-xyz with micro-ball lens for optical scanning and alignment", presented at International Conference on Optical MEMS and their Applications, Nara, Japan, 1997. [Or see page 18 in NR.L report Optics&MEMS by S. J. Walker and D. J. Nagel, 1999.] Summary of the invention

A technology allowing on-chip alignment using MEMS carriers including actuators and fixation features, eliminating the use of expensive machines, would have a great impact even if the laser carrier would be more complex and thus slightly more costly.

In-package alignment, i.e. when alignment is done after the assembly and packaging procedure has been completed, would also simplify the production flow.

The present invention concerns silicon carriers which contain features for passive alignment and mounting of all the optical components as well as one or more movable platforms, driven by micromechanical actuators, for active alignment of lenses or other components. Also, the carrier should allow electrically controlled fixation of the platform so that the alignment could be maintained during the life of the component. The actual requirements on the platform in terms of degrees of freedom and movement range /fixation accuracy differs for different application cases, the present invention produce a "toolbox" of techniques for active alignment and fixation. A preliminary study that we have made however indicates that one movable platform with three degrees of freedom would in most cases be satisfactory, provided that the components are passively aligned using state of the art technology, i.e. MEMS technology.

The technological solution allows all other features of a modern laser carrier to be integrated, e.g. efficient heat transfer, integrated temperature sensor, integrated passive components, conductor pattern, component mounting features, and low- loss microwave waveguide for modulation signals.

Thus, the object of the present invention is to provided methods and means for enabling on-chip optical alignment in three directions, where the process is based on an optimization of the optical efficiency, i.e. minimizing losses in the transmission of light through the components on the chip.

This object is achieved with a method as defined in claim 1, and an active alignment system as defined in claim 5.

Brief Description of the Drawings

The invention will be described in detail below with reference to the drawings, in which

Fig. 1 shows schematically a structure embodying the invention;

Fig. 2 shows a cross section (A-A^') of Fig. 1 ;

Fig. 3 shows a cross section (B-ET) of Fig. 1;

Fig. 4 illustrates the leverage principle; Fig. 5 shows actuation in z-direction;

Fig. 6 shows an alternative actuation in the z-direction;

Fig. 7 illustrates the fixation procedure;

Fig. 8 illustrates the fixation procedure using localized heating

Fig. 9 an optional variation of the fixation.

Detailed description of the invention

The present invention has general applicability for alignment of components where the relative position of individual components can be detected, e.g. by optical means, and feed-back (optical) signals can be processed by a computer employing some optimization algorithm. Therefore, the example given herein is only one embodiment and is in no way limiting on the scope of the invention.

Fig. 1 is a top view of a system for active optical alignment. It comprises a lens for focusing e.g. laser radiation (not shown) onto an optical fiber (not shown). The lens is located on a platform in a square depression etched in the platform. After fixation the platform is held to a bottom wafer with a solder "bump". If melted, the solder will of course allow the platform to move with respect to the bottom wafer, and when solidified, the solder bump will act as a position fixation for the platform.

In the illustrated embodiment, the platform is supported by an arm with integrated V-groove like expansion actuators that extends parallel to one side of the platform. The three different polymer thermal actuators are all provided with heat operable hinge elements (to be described below in detail). Thus, when heat is applied to said hinges, they will cause the arms and the platform respectively, to deflect in different directions, i.e. in the x-, y and z-directions (fist arm, second arm and beam, respectively) . The principle for the heat operable hinges or actuators is the following:

If a V-groove like structure is made in an elongated member, such as an arm shown in Figs. 1 and 3, and filled with a material that has a (thermal) coefficient of expansion different from that of the material in the arm, the elongated member will be forced to deflect because the material in the wider part of the groove will expand more than at the bottom of the groove. This is schematically illustrated in Fig. 4. Suitable materials for this purpose are for example BCB, polyimide for thermal expansion or piezo electric polymers or shape memory polymers (SMP). Other possible materials are any material having a coefficient of expansion that is different from that of the material from which the structure in which the V-groove is formed.

Fig. 4 also illustrates an important aspect, namely that the longer the arm (or lever) is the larger will the leverage become. A side effect that is greatly advantageous in the present application, is that the movement will be more linear the longer the lever is, i.e. anomalous movement in the "wrong" direction is reduced.

Figure 1 is not to scale concerning the relative length of the leverage arms and the platform. The actuated movement is so much smaller relative to the large optical components so that the leverage arms do not need to be longer than the optical component arms, resulting in a very space effective system.

In the system shown in Figs. 1 - 2 the principle is used to create hinges for enabling deflection of the arms. However, the applicability is more general, and it is appropriate to talk about "actuators" as a general term for the structures involved.

The actuators (or hinges) must be able to move the platform in a range of typically ±10 μm, which is large enough to compensate for residual alignment errors after the passive alignment. The force must be large enough to overcome the inertia of the platform and to hold the platform virtually stable during the fixation process, cf. below and weak enough when not actuating so as not to impede the fixation. For a structure as the one illustrated in Figs. 1 -2, the movement caused when the actuators (hinges) are energized will be a slight rotation, i.e. it will not be a perfect translation in any of the x-, y- or z-directions. Such a situation is illustrated in Fig. 6, This can be handled in different ways. As shown in Fig. 5, the movement in the z- direction can be forced to be a true translation by providing two arm structures, one on each side of the object to be moved (i.e. the platform as discussed above).

The upper state is when the polymer filled in the V-grooves is uncured, wherein the structure will be fiat. As illustrated by the middle state, the platform is elevated where the polymer is cured but cold, i.e. no heat applied by the heating elements. Finally, in the bottom state the platform is deflected downwards when heat is applied to the polymer in the V-grooves, by virtue of their (thermal) expansion being larger than that of the material in the arms or beams carrying the platform.

Alternatively, the V-grooves can be made and filled with uncured polymer before they are "free etched", i.e. before the lithographic process. In this case the lever arms will be planar in the cold state, and upon application of heat they will deflect downwards.

For movements in the x- and y-directions, there can be provided on the respective arm, V-grooves on the opposite side of the first groove or grooves. In this way a deviation from linear translation can (at least partially) be compensated for by activating the actuators on the other side which then cause a deflection back.

In order to heat the material (polymer) in the grooves, there can be arranged heating elements on the wall surfaces of the grooves or in the bulk material close to the grooves. These heating elements can preferably consist of resistive elements heated by applying a voltage. In order to control the lever movements, the applied heat can be varied under a computer control according to the optimization algorithm, such that position signals can be used to correct or adjust the level of heat applied, thereby causing the arms to move to a new position. Fixation

When the platform has been moved to the correct position it should be permanently fixed. According to claim 1 and 2 of the invention this fixation is achieved by melting and solidify solder bumps which joins the platform to the supporting structure which allows a general alignment structures. Allowing in-package alignment requires electrically controlled local soldering which could be used according to claim 3 in the present invention. . The method is illustrated in Fig. 7. Thus, a solder metal (e.g. Ni/Au) is deposited over a spot of a wetting film of a suitable material. This is suitably made by depositing a layer of solder over the entire wafer, masking and etching away solder leaving a spot of the desired size, i.e. larger than the wetting spot. The solder deposit will extend beyond the borders of said spot. Surrounding the spot of wetting material there is deposited non-wetting material such as an oxide film. When heat is applied to the solder, it will melt and because of the differing wetting properties just mentioned, the droplet of molten solder will form a sphere only contacting the wetting spot. Thus, when the platform has been moved to a correct position during e.g. an alignment operation, as illustrated in the lower left hand corner of Fig. 7, the solder is melted by applying heat, and will then form a droplet contacting the platform from beneath, as indicated in the lower right hand corner of Fig. 7. If heat supply is terminated the solder will solidify and the platform is fixed in its position.

There are two possibilities: either the solder is melted and kept in a molten state during the alignment procedure, or the solder is melted only when the correct alignment is achieved. Both methods are feasible and the appropriate method should be selected according to circumstances.

It is possible to heat the surface which the solder bump hits to improve the solder strength. This heating can be done by an integrated heater on this contacting surface. This heater can be a resistor where the current is provided from a voltage source on the platform or via a voltage difference between two solder bumps which each contact one end of the integrated heater resistor. This is illustrated in Fig. 8. In instances where the platform is located slightly out of range of the solder droplet, as illustrated in Fig. 9, an electrostatic effect can be used to stretch the droplet to contact the platform. This can be done by applying a voltage across the gap between solder drop and platform bottom.

Expected performance

We estimate that the range of the movable platform could be at least ±10 μm and that the position error after fixation will be less than about 0.2 μm.

Although the invention has been described with reference to an optical alignment system, it is to be understood that it has more general applicability for any application where very small objects need to be moved in three dimensions for the purpose of adjusting or optimizing their position.

Claims

CLAIMS:

1. A system for achieving fixation of one or more moveable micro machined platforms for passively aligned optical components, comprising - a support structure containing structures for passive alignment of optical components

- platform(s) containing structures for passive alignment of optical components

- actuators for actively aligning the already passively aligned components on the platform(s) and the support structure to each other, where the actuators are strong enough while actuating to keep the platform(s) steady during fixation and weak enough while not actuating so as not to impede the fixation

- a fixation mechanism to fixate the moveable platform(s) to the support structure

2. A method of fixating two objects in a desired position with respect to each other, comprising the following steps: subjecting the first of the objects to a positioning procedure for accurate positioning opposite and in close vicinity to and with respect to the second of said objects; melting one or more solder bumps provided on one of said first and second objects, said solder bump(s) being deposited so as to a wetting spot each, said bump(s) being larger than said wetting spot(s), said wetting spot(s) surrounded by non-wetting material; applying heat to melt said bump(s), whereby the solder bump(s) will contract and form a droplet thereby contacting only said wetting spot(s), and grow towards the surface of the opposite object, and be brought in contact with the second object; and terminating heating so as to solidify the solder.

3. A fixation as claimed in claim 2, where the heat to melt said bump(s), I produced electrically controlled integrated local heater(s)

4. A fixation as claimed in claim 2, where the object initially without solder bump(s) is also heated before or during contact with the solder bump(s).

5. A fixation as claimed in claim 4, where the heating of the object initially without solder bump(s) is achieved by integrated heater in the said object.

6. A fixation as claimed in claim 5, where the integrated heater in the object initially without solder bumps is activated by electrical current caused by a voltage difference between the two bumps contacting the object.

7. The method as claimed in claim 2, optionally comprising applying a voltage between the melted solder and the opposite object so as to elongate the solder droplet electrostatically, in a case where the droplet does not reach the second object.

8. A V-groove like structure filled with an expandable material with different expansion at the wide and narrow part of the V-groove like structure, where the V-groove expansion force is leveraged and transformed into a near-linear movement.

9. An actuator as claimed in claim 8, where the expandable material is a polymer.

10. An actuator as claimed in claim 8, where the expansion is achieved by thermal expansion

11. An actuator as claimed in claim 10, where the heating is achieved using electrical heating of resistors.

12. An actuator as claimed in claim 10, where the heating is achieved using heating by laser.

13. An actuator as claimed in claim 5, where the expansion is achieved by piezo electrical expansion.

14. An actuator as claimed in claim 5, where the expansion is achieved by heating.

15. An actuator as claimed in claim 5, where the expansion is achieved by a shape memory material.

16. An actuator as claimed in claim 8, where the arm material is a polymer, with a different coefficient of expansion.

17. An actuator as claimed in claim 9, where the polymer is piezo electrically expanded.

18. An actuator as claimed in claim 9, where the polymer is thermally expanded

19. An actuator as claimed in claim 9, where the polymer is a shape memory polymer (SMP)

20. An actuator as claimed in claim 8, where said V-groove like structure is fabricated by wet etching for out-of-plane actuation.

21. An actuator as claimed in claim 8, where said V-groove like structure is fabricated by deep reactive ion etching for in-plane actuation.

22. An actuator as claimed in claim 8, where said V-groove like structure is provided with hook- like structures which allow actuating materials with low or no adhesion to the surrounding structure to grip it at the hook-like structure.