miércoles, 23 de junio de 2010

Copper micro-coil arrays for the actuation of optical matrix micro-

Electromagnetism is used to actuate optical micro-switches. The micro-switch is composed of arrays of vertical bumicromachined mirrors on bases that move vertically under actuation (1). The displacement of the micro-mirroraround 100 micrometers to ensure the micro-switch operation. Large forces are required to performe such displacemeDifferent type of actuators can be used to move the micro-mirrors with respect to the process of fabrication of micmirror and the required performances of device using the micro-mirror. For example, scratch-drive or comb-dractuators have been reported for the actuation of surface micromachined mirrors (2 and 3 respectively). In the casebulk micromachined mirrors, direct electrostatic actuation of mirrors was also used (4). However electrostatic actuatiis not well suited to get large forces because high voltages are required that is not compatible with the commonly ussupplies in microelectronics. The use of electromagnetic actuation with micro-coil can overcome such disadvantagMiniaturization of the actuator that is required by the array structure is not an obstacle to obtain relative larelectromagnetic forces if the micro-coil is well designed and ferromagnetic materials are employed (1). In the first pof this paper, the structure of the micro-coils will be described in connection with the corresponding optical micswitches in order to explain the operation of the whole device. Electromagnetic modelling will be then presented in second part. Modelling is used to improve the design to get better actuation performance of the micro-switchExperimental results will be discussed in the last part.
The copper micro-coil arrays are dedicated to the actuation of optical matrix micro-switches. The matrix of micswitches is composed of an array of vertical mirror supported by a silicon base (Fig. 1). The bases are supported springs connected to the frame of the matrix. The fixed optical fibers are placed on the outer frame inside the grooves. The well defined 45º angle between the and planes is used to self-align the vertical mirrors athe V-grooves. Due to the (100) plane, the mirror is strictly perpendicular to optical axes. Both this perfect verticaligood quality of mirror and naturally aligned 45º angle allow very low losses between the output of the light from ofiber and its recuperation by another after reflection.
A short description of the operation principle of the switch is done and further details can be found in the literature (1). The matrix of micro-coils should be mounted below the micro-switches matrix (Fig. 2a). The magnetic force acts on magnetic materials deposited just below the moving base. The switch can move vertically between two stable positions. When all the mirrors are down, the laser light goes straight between the input and the output fibers (Fig. 2b). Constant DC field produced by a permanent magnet should allow latching with no required supply. When some mirrors are up, he laser light is switched from input fibers to the output fibers (Fig. 2c). A good alignment between the two matrices should allow each micro-coil to separately actuate the corresponding mirror vertically (see Fig. 3a). The outer electrical contacts of the micro-coils have to be designed in the corners of the matrix in order to allow wire bonding even after the two wafers are associated and the optical fibers in position (Fig. 3b).

Also in order to avoid the electric connections at the center of each micro-coil, a copper back layer should be deposited below the micro-coils. Some large triangular shape pads have been created to easily contact the back layer from the upper side of the wafer (Fig. 3c). The on going improvement of the electromagnetic actuator is to deposit a ferromagnetic layer first in order to increase the available force as it's showed by the following modeling.

Electromagnetic Finite Element Modeling has been used to optimize the parameters of the micro-coils in connection with the micro-switches. In the case of the micro-switch structure previously described, a minimal displacement of 100 µm between two stable positions is required to achieve secure switching operations due to the diameter of the optical beam and some safety margins (1). Electromagnetic actuation complies well with this requirement because bistable systems are easily achieved and electromagnetism can provide large and long range forces though low voltages are needed that is compatible with the common supplies used in Electronics. Axysymmetric simulations of copper micro-coils show the benefits in working with high current and using ferromagnetic materials.

In Fig. 4a, the simulated structure composed by the copper winding is shown. A magnetic core in the center of the winding, a ferromagnetic back layer combined with an external ring also made of ferromagnetic material, can be included into the simulation. An example of results is presented in Fig. 4b. In this simulation, permalloy has been chosen for the material of both the core, the back layer and the external ring. The mirror base is also supposed to be covered by permalloy. The lines through the structure represent the magnetic equiflux lines. The magnetic induction is tangent to these lines. The figure shows clearly if the actuator is surrounded by magnetic materials, the magnetic induction can be enclosed in the region where the mirror base moves. The total force applied on the mirror base is then increased as indicated in the following table.

It should be noticed when the micro-coil is supplied by a pulse current, a higher current is allowed because the heat due to Joule losses can be dissipated between two pulses. As the magnetic force is proportional to the current square, the gain of a factor 10 on current density implies the gain of a factor 100 on forces.Ferromagnetic materials and permanent magnets are the tremendous advantage of electromagnetism. Ferromagnetic materials allow a gain in term of energy, proportional to their permeability that can be up to 106 in the case of Superalloy. Permanent magnets can provide force without contact, electrodes nor power consumption. For a long time, electrostatic actuation has been preferred to electromagnetic actuation because ferromagnetic materials and thick film magnets were not available. However, this starts to change. Ferromagnetic materials are getting to be used frequently (5) and many researchers investigate plated (6), sputtered or bonded magnets (1). This turmoil will boost the electromagnetism for Microsystems.
In order to master the technology for the realization of the electromagnetic actuator, micro-coil arrays in copper were first fabricated without copper back layer for electric connections in the center of the micro-coils. Fig. 5 shows an example of such micro-coil.
These micro-coils have been electrically tested. A low resistance of about 2 Ohms was measured and a DC current up to 0.9 Amperes has been applied before copper melting. The maximum current density supported by such micro-coil is then worth 2.6 kAmperes/mm2 that is relatively high. Also, the micro-coil normally operated in pulse current up to 2 Amperes peak. The micro-coils seem to support much larger current values in pulse regime so that electrical tests above 2 Amperes peak are expected soon. In the new process including the copper back layer, particular attention was turned to obtain large forces. The micro-coils are designed with a larger number of turns that implies a better aspect ratio of the winding in order to reduce the resistance due to the increase in winding length when the number of turns is increased. Fig. 6 represents the technological chart of the new process.

The copper back layer is first deposited and then the chromium seed layer is deposited above an insulation layer (step 1). These two layers are etched to get the through holes between the copper winding and the copper back layer of the future micro-coils (step 2). Then a copper seed layer is deposited on the whole wafer (step 3). The separations between the turns of the micro-coils are realized after etching the copper and chromium seed layers (step 4). At last a thick SU8 mold is formed and serves to get the copper winding by electroplating (step 5 and 6 respectively). Fig. 7 shows the micro-coil array using this new process.
Between the two kinds of realization of micro-coils, the technology was improved. In particular, our efforts were focused on patterning the resin molds to achieve a better aspect ratio of the copper winding. We are looking for a smaller copper width to increase the number of turns and the available electromagnetic force. But in the same time, the copper thickness has to be increase to keep a low resistance. In the following table, the geometric characteristics of the two kinds of realization of micro-coils are summarized.

It should be noticed that the micro-coils obtained with the new process have a diameter smaller than the first realized micro-coils while the number of turns has increased. That shows a great improvement of the technique of realization. According to the favorable geometric characteristics of the micro-coils fabricated with the new process, the on going electric measurements are expected to show improved performance. An electromagnetic actuator composed of copper micro-coil arrays has been presented in close connection with the actuated optical micro-switch system. The description of the whole system shows the electromagnetic actuator needs to be design according to the requirements of the actuated part. Large and long range forces are required that is well fit by electromagnetic actuators. The process of fabrication of the micro-coils has been progressively developed in order to master the technologies and two kinds of micro-coil design were realized. In particular, efforts were focused on patterning the SU8 resin molds to achieve a better aspect ratio of the copper winding. Electrical measurements have been performed on the first design and show encouraging results to go on the realization of the second design. Electromagnetic Finite Element Modeling has been used to optimize the parameters of the micro-coils. These simulations show the benefits in using both permanent magnet and magnetic materials to increase the available force. The next on going step of this research will be to include such materials in the process of fabrication of the micro-coils.
Jorge Polentino

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