Connecting a terahertz imaging device to a lens to provide an optimal interface

Description

technical field

[0001] The invention relates to optical systems used in terahertz imaging. Background

[0002] A terahertz imager can comprise an array of sensors fabricated on a silicon chip using semiconductor manufacturing techniques. Such a chip is typically assembled into a module where it is coupled to a silicon collimation lens. The lens is often hyperhemispherical, meaning it is a truncated sphere, retaining more than half of its original shape. The chip is preferably mounted on the truncated face of the lens and centered on the lens's optical axis.

[0003] This type of assembly presents some difficulties, particularly with regard to the automation of the assembly and the optical quality of the interface between the chip and the lens. Summary

[0004] The invention is defined by the claims. The documents "An Ultrawideband Leaky Lens Antenna for Broadband Spectroscopie Imaging Applications", Hahnle et al., XP011797869, and "A 288-GHz Lens-Integrated Balanced Triple-Push Source in a 65-nm CMOS Technology", Grzyb et al, XP011515898, present examples of the prior art.

[0005] A terahertz optical module is generally designed to include a support structure with a recess, typically cylindrical in shape, opening onto one face of the support, and an aperture centered on the cylindrical wall and extending through the bottom of the recess to a second face of the support; a lens arranged within the recess, the lens having a circular cross-section corresponding to the cylindrical wall of the recess and a flat face resting on the bottom of the recess; and a chip comprising terahertz components manufactured using semiconductor technology, the chip being centered in the aperture and fixed to the flat face of the lens. The chip can be attached to the lens by an adhesive applied to the interface between the chip and the lens.The adhesive may have the following characteristics: a) hot-curing epoxy resin; b) Brookfield CP51 viscosity, 25°C, speed 5 rpm: 50000 mPa•s at ± 20%; and c) absence of fillers to make the adhesive electrically or thermally conductive.

[0006] The adhesive may be referred to commercially as Ablebond 84-3 or Loctite™< Ablestik 84-3.

[0007] The chip may include, on one face opposite the lens, contacts connected by conductive wires to corresponding contacts of the substrate at the edge of the orifice, the contacts at the edge of the orifice being themselves connected to contacts on the periphery of the substrate by conductive tracks.

[0008] The second side of the support may include a cup at the bottom of which is located the orifice with its peripheral contacts, the cup being filled with a product to protect the conductive wires and the chip.

[0009] A method for assembling a terahertz optical module is also provided, comprising the following steps: fabricating a support including a recess having a generally cylindrical wall opening onto a first face of the support, and an orifice centered on the cylindrical wall and passing through a bottom of the recess to a second face of the support; attaching the support to a lens such that a circular section of the lens is centered with respect to the cylindrical wall of the recess and a flat face of the lens rests against the bottom of the recess; using an optical placement machine, centering a chip comprising terahertz components fabricated using semiconductor technology in the orifice; and fixing the chip to the flat face of the lens. The chip fixing step may include depositing an adhesive of the aforementioned type between a face of the chip and the flat face of the lens.

[0010] The step of joining the support to the lens may include the following steps: placing the lens in an opening in a plate with its flat face facing upwards; and placing the support on the flat face of the lens with an optical placement machine, whereby pressure exerted by the placement machine on the lens causes the flat face of the lens to align with the bottom of the recess.

[0011] The adhesive having the aforementioned characteristics can in fact be used to assemble any silicon elements used to create an optical device for terahertz radiation. Brief description of the drawings

[0012] Some embodiments will be described below, which is not exhaustive, in relation to the attached figures, among which: There figure 1 represents a cross-sectional view of an embodiment of a terahertz imager optical module; The figure 2 is a perspective view of the optical module; and La figure 3 illustrates a step in the assembly of optical modules. Detailed description

[0013] To automate the assembly of chips on any substrate, the chips are generally handled by "pick and place" optical placement machines which are capable of precisely placing the chips relative to a visual reference point on the substrate.

[0014] In the case of chips to be mounted in modules including a silicon lens, the flat surface of the lens intended to receive the chip offers very little contrast, even when patterns are etched onto it, so the optical registration of the placement machine is often failed or distorted. As a result, the chip is too often misaligned in an automated manufacturing line.

[0015] Another challenge lies in the optical quality of the interface between the chip and the lens. Adhesives available for bonding silicon chips are generally designed for substrates that do not require optical transmission (ceramic, metal, or resin). It has been observed that these adhesives do not have good terahertz wave transmission properties, so droplets of adhesive are deposited at the periphery of the chip rather than within the interface. However, depending on the fluidity of the deposited adhesive, it can partially penetrate the interface by capillary action, resulting in an uneven contact between the chip and the lens surface and a deterioration in the quality of terahertz wave transmission at the periphery of the sensor array.

[0016] There figure 1 represents a cross-sectional view of an embodiment of a terahertz imager optical module that overcomes the difficulties of centering the chip relative to the lens.

[0017] The module includes a support 10 which can be manufactured using multilayer printed circuit board techniques. The support has one face designed to center a silicon lens 12. The lens 12 is shown as an example of a hyperhemispherical shape, which is the most common case, but it could have other shapes, generally having a circular cross-section.

[0018] More specifically, the face of the support intended to receive the lens, the upper face in the figure, includes a recess having a generally cylindrical wall 14. The bottom of the recess is designed to support the flat face of the lens. The wall 14 has a diameter corresponding to the largest diameter of the lens 12. When the lens is hyperhemispherical, as shown, the largest diameter (the equator) is located away from the flat face; in this case, the wall 14 can flare out, as shown, to locally conform to the shape of the lens.

[0019] The bottom of the recess is pierced by an orifice 16 centered with respect to the cylindrical wall 14 and open on the second face of the support, the lower face in the figure. A chip 18 containing the terahertz components is fixed to the face of the lens 12, inside the orifice 16. By centering the chip 18 with respect to the orifice 16 during its placement and fixing, precise centering of the chip with respect to the lens 12 is obtained. This precision is guaranteed in particular by the dimension chain between the orifice 16 and the cylindrical wall 14, which involves a single part, in combination with the fact that optical registration of the edges of the orifice 16 by the placement machine is particularly easy.

[0020] If, by chance, the manufacturing process of the substrate is not very precise with regard to the shaping of the edges of the orifice 16, the underside of the substrate 10 may include conductive tracks serving as optical markers, in particular peripheral contacts of the orifice 16, shown later, used to connect the chip to circuits in the substrate. Such markers can also be precisely positioned relative to the cylindrical wall 14, and offer high contrast, making optical alignment easy.

[0021] As shown, the underside of the support 10 forms a cup 20 into the bottom of which the orifice 16 opens. This cup can be filled with a product to protect the chip 18, once the chip is fixed and electrically connected to the support.

[0022] There figure 2 is a perspective view from below of the optical module of the figure 1This view reveals, in particular, the bottom of the tray 20 and an example of the electrical connection structure of the chip 18. The visible face of the chip 18 can be its active face, that is, the face on which the components and electrical contacts are located. Terahertz radiation is then received by the components, for example, a terahertz receiver array, through the rear face of the chip. This active face has rows of peripheral contacts 22 which are connected by conductive wires (not shown) to respective contacts 24 of the substrate, arranged in rows at the edges of the hole 16. These contacts 24 can also serve as precise markers for optical recognition by the placement machine.

[0023] The contacts 24 at the edge of the orifice are in turn connected to contacts 26 arranged at the periphery of the support by conductive tracks, not visible, made for example in internal layers of the support in the case of a multilayer printed circuit.

[0024] The bowl 20 can be filled with a protective product that drowns the chip and the conductive wires.

[0025] There figure 3This illustrates a step in the assembly of a batch of modules. The lenses 12 are arranged with their flat faces facing upwards in circular holes in a plate 30. The spherical parts of the lenses thus form ball joints, allowing for self-alignment. A glue dispensing machine, for example, deposits four evenly spaced drops of glue around the periphery of the flat face of each lens, for example, off the four straight edges of the holes 16 of the supports to be fitted. A placement machine then places the supports 10 onto the lenses.

[0026] The dosing and placement machines can be optically oriented by the contour of the lenses. Even if the flat surfaces of the lenses are not necessarily horizontal, since the lenses are hyperhemispherical, the contour perceived from above by the machines remains circular.

[0027] According to an alternative method for aligning machines with lenses, these can be set with the coordinates of the plate's holes, allowing the plate and its holes to be machined with precision.

[0028] During the support placement phase 10, upon contact between a support and the flat face of the corresponding lens, as illustrated for one of the lenses, the lens tilts within its orifice so that its flat face straightens and aligns with the bottom of the recess 14, and the lens centers itself between the cylindrical walls. In the preceding glue droplet deposition phase, the lenses may also tilt due to contact with the nozzle of the dosing machine, but the resulting misalignment is also corrected during the support placement phase.

[0029] Next, with the supports 10 in place and centered with respect to the lenses 12, the holes 16 serve as optical guides for fixing the chips to the exposed portion of the flat surfaces of the lenses. The dosing machine deposits adhesive at the chip locations, and then the placement machine deposits the chips.

[0030] The modules are then transferred to an oven to cure the adhesive, and subsequently to a soldering station where conductive wires are soldered between the contacts 22, 24 of the chips and the sockets, and the wells 20 are filled with a protective agent. For these operations, it is preferable that the lenses remain stable. Thus, for example, the modules are first transferred onto a board with holes larger in diameter than the lenses, so that the sockets 10 lie flat on the board.

[0031] The chip 18 was attached to the flat surface of the lens 12 by direct bonding, namely by depositing a drop of adhesive at the interface between the chip and the lens. Although it is well known that this poses problems with terahertz wave transmission, the inventors conducted various tests. These tests showed that most commercially available adhesives intended for attaching chips to a traditional, usually metallic, substrate did indeed significantly degrade terahertz wave transmission between the lens and the chip. However, a specific adhesive was found that offers satisfactory transmission characteristics. This is the adhesive commercially known as Ablebond 84-3 or Loctite™ Ablestik 84-3.

[0032] This adhesive has the following characteristics which give it good properties for creating interfaces between silicon elements generally intended to perform optical functions in the terahertz range: Hot-cure epoxy resin. Brookfield CP51 viscosity, 25°C, speed 5 rpm: 50,000 mPa·s. No fillers are used to make the adhesive electrically or thermally conductive. The aforementioned adhesive is electrically insulating. Thermal conductivity of 0.8 W / mK at 121°C is achievable without metallic fillers.

[0033] The most important characteristic is the absence of charges that interfere with the transmission of terahertz waves, particularly metallic particles.

[0034] Viscosity is also important. The specified value is intended for liquid-phase interfaces 25 to 50 µm thick with a chamfer of 25 to 50% of the chip height. To achieve good terahertz wave transmission characteristics, an interface thickness of less than 25 µm is preferred, achieved by increasing the pressure applied to the chip during placement. The specified viscosity value is high enough in this case to prevent excessive creep under the chip, which would leave voids at the interface, but low enough to avoid applying excessive pressure to create interfaces less than 25 µm thick. The optimal viscosity value can vary within a certain range around the value specified above, for example, ±20%.

[0035] The adhesive base, in this case a hot-curing epoxy resin, proves to be suitable for silicon-on-silicon bonding and exhibits satisfactory terahertz wave transmission characteristics.

[0036] Thermal conduction is less important, as the chips under consideration dissipate little power.

Claims

1. A method for assembling an optical device compatible with terahertz radiation, comprising the following steps: providing a silicon lens (12) and a silicon chip including terahertz components (18), each having a planar face; depositing on the planar face of the lens or of the chip an adhesive having the following characteristics: epoxy resin curable by heat, Brookfield viscosity CP51, 25°C, speed 5 rpm: 50000 mPa•s ± 20%, and absence of fillers to make the adhesive electrically or thermally conductive; and applying the planar face of the chip to the planar face of the lens.

2. Method according to claim 1, wherein the adhesive is designated commercially as Ablebond 84-3 or Loctite™ Ablestik 84-3.