DE102021110102B4 - Wafer-level test methods for opto-electronic chips - Google Patents

Abstract

Method for testing optoelectronic chips (1) arranged on a wafer, with electrical interfaces in the form of contact pads (1.1) and optical elements fixed thereto, which comprise optical interfaces in the form of optical deflection elements (1.2) with a specific coupling angle (α), in which the wafer is received by a positioning table (3) which is adjustable in the x, y and z directions of a Cartesian coordinate system relative to a contacting module (2) and rotatable about the z axis, wherein the contacting module (2) has electrical interfaces in the form of needles (2.1) with needle tips (2.3) assigned to the contact pads (1.1) and optical interfaces (2.2) assigned to the optical deflection elements (1.2), and• in a first alignment step, the wafer is fed to the contacting module (2) in such a way that the needle tips (2.3) are in a first position (x1, y1, z1) each spaced apart in the z direction over a predetermined Position (1.4) of the associated contact pad (1.1) of a first of the chips (1), wherein the contacting module (2) has a first distance (a) in the z direction from the first of the chips (1), wherein the first distance (a) is greater than a maximum distance (I) of the needle tips (2.3) from the contacting module (2),• starting from the first position (x1, y1, z1), a second position (x2, y2, z2) is determined in a second alignment step by moving the positioning table (3) to the contacting module (2) in the z direction to a second distance (b), at which the needle tips (2.1) rest against the contact pads (1.1), so that electrical signals can be transmitted via the interfaces assigned to one another and optical signals can be transmitted via the optical interfaces assigned to one another, wherein in the second alignment step a relative alignment of the optical deflection elements (1.2) to the optical interfaces (2.2) present on the contacting module (2) is carried out in that in a scan field (4, 5) smaller than an xy extension of the contact pads (1.1) the positioning table (3) is deflected in scan positions Ps[i] = (xs[i], ys[i], zs[i]) within a scan area (7) relative to the x and/or y coordinates of the first position in the x and/or y direction and/or fed in the z direction, and at the scan positions (xs[i], ys[i], zs[i]) an optical signal is coupled via at least one of the optical interfaces of the contacting module (2.2) and one of the optical deflection elements (1.2), wherein the second position (x2, y2, z2) is defined by the optical signal being coupled with a maximum degree of coupling, wherein the degree of coupling is determined by means of at least one electrical signal transmitted via at least one electrical interface (1.2, 2.1), and• in a third Alignment step, the first of the chips (1) is brought to a third position (x3, y3, z3) by first setting the x3 and y3 coordinates of the positioning table and then feeding it in the z direction to a third distance c which is smaller than the second distance b, wherein the needle tips (2.3) rest against the contact pads (1.1) with a predetermined pressing force, wherein the third position (x3, y3, z3) was calculated from the second position and the coupling angle (α) of the respective optical deflection element (1.2) for the third distance (c) before the third alignment step.

Description

Technical area

The invention relates to a method with which the functionality of electrical and optical components or circuits of a chip can be tested simultaneously on wafer level in a wafer probe. Such a method is generically known from US 2011 / 0 279 812 A1 known.

The invention is located in the field of testing and qualifying chips with optical-electrical integrated circuits, so-called PICs (Photonic Integrated Circuits), at wafer level. In contrast to conventional, purely electrically integrated circuits, so-called ICs (Integrated Circuits), PICs have optical functionalities integrated in addition to the electrical circuits.

When manufacturing ICs, e.g. using CMOS technology, tests and measurements are carried out in various manufacturing steps in order to monitor the process on the one hand and to carry out quality control on the other. An established test is the electrical wafer level test after the wafer has been completed. Here, functional and non-functional chips are identified, recorded in a wafer map and the yield is determined. Functional chips are also known as known good dies (KGD). When the wafer is separated into individual chips, the non-functional chips are then sorted out. The test equipment required for the wafer level test is available in the form of wafer probes and wafer testers with associated contact modules (also called probe cards). The contact module is used to connect the device-side interfaces of the wafer tester to the individual interfaces of the chips on the wafer fixed to the wafer probe. In principle, the contact module can be designed so that it contacts just one chip or several chips at the same time. It is also not absolutely necessary for the chips to be present in the wafer assembly for contacting. In order to contact several chips on a wafer at the same time or one after the other, the chips only need to have a fixed and defined position relative to one another.

Test equipment for testing purely electronic chips (semiconductor chips with ICs) has been optimized and diversified over decades in order to be able to qualify high volumes of a wide variety of ICs with high throughput in order to optimize costs.

PICs are usually manufactured using the same established semiconductor processes, e.g. CMOS technology. The very low production volumes of PICs compared to IC production mean that only tests for process characterization, but not functional tests of the PICs, are usually carried out in a semiconductor factory. Functional characterization is the responsibility of the end customer and is often carried out on sawn chips. The test equipment used uses independent, separate electrical and optical contact modules and is not optimized for throughput.

State of the art

Testing PICs at the wafer level requires coupling light into and out of the PIC plane, usually using integrated grating couplers as coupling points, as described in the technical literature "Grating Couplers for Coupling between Optical Fibers and Nanophotonic Waveguides" (D. Taillaert et al, Japanese Journal of Applied Phys[i]cs, Vol. 45, No. 8A, 2006, pp. 6071-6077) The grating couplers can be a functional component in the chip or sacrificial structures on the wafer, e.g. in the scribe trench or on neighboring chips.

According to the state of the art, fiber optic based systems are used for the wafer level test, as described in the technical literature: “Test station for flexible semi-automatic wafer-level silicon photonics testing” ( J. De Coster et al., 21th IEEE European Test Symposium, ETS 2016, Amsterdam, Netherlands, May 23-27, 2016. IEEE 2016, ISBN 978-1-4673-9659-2 ). These contain a glass fiber-based optics module that couples light into and out of the chip's coupling points via individual glass fibers. In order to ensure repeatable optical coupling, the glass fibers must be adjusted to the coupling points with submicrometer precision at a distance of up to a few micrometers. This is only possible with the help of high-precision adjustment elements, e.g. in combination with hexapods and piezo elements. In addition, a time-intensive, active adjustment process designed to achieve maximum coupling efficiency must be carried out before each individual optical coupling.

• • sequentielles, zeitintensives Kontaktieren aller optischen Koppelstellen eines Chips nacheinander, d. h. eine parallele Kontaktierung aller optischen Koppelstellen eines Chips ist nicht oder nur stark eingeschränkt möglich, eine parallele Kontaktierung mehrerer Chips gar nicht möglich.
• • geräteseitige Sonderlösungen, so dass herkömmliche Waferprober nur mit aufwendigen und kostenintensiven Modifikationen umrüstbar und danach nicht mehr oder nur bedingt bzw. erst nach zeitaufwendiger Umrüstung für den Wafer Level Test von ICs einsetzbar sind.
• • getrennte, nicht fest miteinander verbundene Elektronik- und Optikmodule, d. h. beide müssen separat gehaltert und justiert werden.

Existing wafer-level test systems are characterized by

• • sequential, time-consuming contacting of all optical coupling points of a chip one after the other, ie parallel contacting of all optical coupling points of a chip is not possible or only possible to a very limited extent, parallel contacting of several chips is not possible at all.
• • Special device solutions, so that conventional wafer probes can only be used with complex and cost-intensive modifications and can then no longer be used for wafer level testing of ICs or can only be used to a limited extent or only after time-consuming conversion.
• • separate electronic and optical modules that are not firmly connected to each other, ie both must be mounted and adjusted separately.

From the above US 2006 / 0 109 015 A1 is an optoelectronic contact module (probe module) for testing chips (object to be tested - DUT 140) with electrical and optical inputs and outputs. The contact module represents an interface between a test apparatus (ATE) and the device under test (DUT) and is designed with electrical contacts (electronic probes), optical contacts (optical probes), optical elements and combinations thereof in order to conduct signals from and to the DUT and to redistribute these signals for an interface to the test apparatus.

Regarding the optical inputs and outputs, it is disclosed that these are created via optical elements that are located on the contact plate and/or the redistribution plate and are matched to various coupling mechanisms, e.g. free radiation, quasi-free radiation or waveguides. Diffractive elements and refractive elements are specified as suitable optical elements for this purpose. It is also stated that a photodetector or a light source can be arranged directly at the interface to the DUT, which then represents the optical input or output on the contact plate.

In addition, the aforementioned US 2006 / 0 109 015 A1 that for designs of optical coupling via free-space or quasi-free-space connections, where the optical signal is guided through a free space between the optical element and the interface to the DUT, the optical signal is focused or collimated in order to achieve a high coupling efficiency of the transmitted signal. The signal coupling here is therefore based on the concept of coupling the signal as completely as possible.

According to an embodiment of the above US 2006 / 0 109 015 A1 the optical and electrical signal lines (optical and electrical distribution network) are implemented on separate redistribution plates. It is proposed to guide the electrical signals from the DUT to the edge regions of the contacting plate, so that the electrical signals are coupled into the first redistribution plate arranged above the edge region. As a result, an opening can be formed in the first redistribution plate, in which only the electrical signals are redistributed, through which the optical signals are guided into a separate second redistribution plate arranged above it.

In summary, the above US 2006 / 0 109 015 A1 A number of ideas have been presented as to how a contact module, which is divided into a contact plate and a redistribution plate for reasons such as wear on the mechanical contacts for electrical signal transmission, could be additionally equipped with optical signal lines. This completely ignores the fact that the possible tolerances for the mechanical contact of the electrical inputs and outputs of the contact module to the DUT cannot be transferred to the optical inputs and outputs.

While the transmission of a constant electrical signal only requires mechanical contact between needles on the contact module and contact pads on the DUT, which can be ensured within a comparatively large position tolerance of a few µm in all three spatial directions, the quality of the optical signal transmission is already affected by a much smaller deviation from a target position in the sub-µm range.

If, as in the above-mentioned US 2006 / 0 109 015 A1 To optimize the coupling efficiency of the optical signal by collimating or focusing the optical beam, the entire contact module must be adjusted with high precision in the sub-µm range. Otherwise, the adjustment-dependent repeatability of the measurement is not sufficient for the applications described. This in turn means that the contact module cannot utilize the adjustment tolerances for electrical contacting in the range of a few micrometers in the X, Y and Z directions that are typical in conventional electrical wafer probes. Complex and expensive special wafer probe solutions are required, including various adjustment elements, such as piezo adjustment elements, and linear axes or hexapods, in order to adjust the contact module to the DUT with high precision.

Another critical point is that for the clean electrical adjustment of the needles, a so-called overdrive of typically a few 10 µm is set in the Z direction, i.e. after the initial contact of the needles with the electrical contact pads, the contacting module is moved by an additional amount in the Z direction in order to ensure reliable electrical contact. Wear and deformation of the needles are usually compensated by adjusting the overdrive during operation. With a simple collimation or focusing of the optical beam, as in the above-mentioned US 2006 / 0 109 015 A1 However, as described above, for a repeatable coupling the working distance in the Z direction may only vary in the range of micrometers. This means that this type of optical coupling is not compatible with established electrical contacting methods.

The US 2011 / 0 279 812 A1 discloses a contacting module for testing chips with electrical and optical inputs and outputs. The chip is mounted on a movable carrier, with which it can be roughly aligned with the contacting module. The rough alignment is sensor-controlled using position monitoring of the chip or the alignment marks of the chip. The fine alignment of the chip takes place in two process steps. In the first process step, it is checked whether the electrical inputs and outputs are in contact with the contacting module. For this purpose, the chip is sucked in with the contacting module so that the electrical inputs and outputs of the chip come into contact with the electrical contacts of the contacting module. A test signal is used to check for successful contacting and, if the contacting is faulty, it is corrected by means of repeated rough alignment. In the second process step, the alignment takes place at the optical inputs and outputs. The optical inputs and outputs of the chip can receive or emit focused or collimated beams with an adjusted aperture and focus position. The optical inputs and outputs of the contacting module have variable optics with which beams focused perpendicular to the surface of the optical inputs and outputs can be generated, whereby the axial and lateral focus position and the aperture of the beams can be adjusted. For the adjustment, the variable optics have at least one optical element with a variable focal length and/or at least one optical element that is movable. The axial adjustment of the focus position is carried out on the basis of distance measurements using additional distance sensors or intensity measurements using an optical test signal. The lateral adjustment of the focus position is carried out on the basis of intensity measurements in which a test beam already focused on the surface of the chip is moved in a scanning relative movement relative to the optical inputs and outputs until the test beam is optimally coupled into the optical inputs and outputs. The time required for this can be shortened by initially carrying out the scanning relative movement with an enlarged focus diameter of the test beam. After fine alignment, the chip is tested using special electrical and optical test signal sequences. When testing several similar chips, parts of the coarse and fine adjustment can be saved and reused.

Out of EN 10 2018 108 283 A1 An electro-optical circuit board for contacting photonic integrated circuits is known, in which an optical beam path is guided through the circuit board. The disadvantage is the low achievable precision of the optical coupling between the circuit board and the chip to be tested.

Out of WO 2019 / 029 765 A1 A contacting module that is insensitive to position tolerances is known for contacting optoelectronic chips. The disadvantage is that the alignment of the contacting module to the chip to be tested can be suboptimal.

For testing electronic chips at wafer level, it is known from practice that the spatial position of the tips of the needles is determined using a first camera and the spatial position of the centers of the contact pads of a chip is determined using a second camera. From the relative position of the tips to the contact pads derived from this, control signals are generated with which a positioning table is controlled and the centers of the contact pads are positioned vertically below the tips before the contact needles are brought into contact with the contact pads.

Object of the invention

It is the object of the invention to find a method for testing optoelectronic chips arranged on a wafer with electrical interfaces in the form of contact pads and optical interfaces fixed thereto in the form of optical deflection elements with a specific coupling angle, which is based on proven methods for testing electronic chips.

Solution to the task

This object is achieved with a method according to claim 1 for testing optoelectronic chips arranged on a wafer with electrical interfaces in the form of contact pads and optical interfaces fixed thereto in the form of optical deflection elements.

Description

The method according to the invention is used for testing optoelectronic chips arranged on a wafer.

The chips have electrical interfaces in the form of contact pads and optical elements fixed to them. Optical elements can be, for example, passive optical elements, optoelectronic actuators and/or optoelectronic sensors and/or electro-optical modulators. The optical elements comprise optical interfaces in the form of optical deflection elements with a specific coupling angle α. The deflection elements can be designed, for example, as grating couplers or mirrors. The specific coupling angle α can represent an angle that an optical signal or its central beam encloses with a plumb line on the wafer. The plumb line can be the z-direction. The specific coupling angle can typically be greater than 0° and less than 25°. A common value for the specific coupling angle α can be, for example, 11.6° and can be advantageous for the glass fibers with corresponding wedge grinding used for coupling in the end application of the chip. The beams deflected by several, particularly advantageously by all, optical deflection elements can advantageously be parallel. If the optical elements are designed for beams, the parallelism can be determined with respect to the central rays of the beams. The coordinate system can then be chosen such that the y-components of the deflected rays disappear, ie that the deflected rays can lie in xz-planes.

In the method according to the invention, the wafer is picked up by a positioning table which is adjustable in the x, y and z directions of a Cartesian coordinate system relative to a contacting module and rotatable about the z axis. The x and y coordinates can advantageously be adjusted using the positioning table. The z coordinate can also advantageously be adjusted using the positioning table. Alternatively or additionally, adjustability in the z direction can be provided by means of a vertical lifting and lowering device of the contacting module. Rotatability about the z axis can advantageously be provided by means of a rotatability of the positioning table. Alternatively or additionally, rotatability about the z axis can be provided by means of a rotating device of the contacting module. The contacting module has electrical interfaces in the form of needles with needle tips assigned to the contact pads. In addition, the contacting module has optical interfaces assigned to the optical deflection elements.

Any deviation of the x-travel path or y-travel path of the positioning table from the x-direction or y-direction of the chip can, for example, be compensated for by means of the above-mentioned rotatability around the z-axis or by means of a rotation matrix in the sense of a rotation as a linear coordinate transformation (so-called alias transformation in the xy plane) when controlling the travel paths of the positioning table.

In a first alignment step, the wafer is fed to the contacting module in such a way that the needle tips are arranged in a first position (x1, y1, z1) each spaced apart in the z direction above a predetermined location of the associated contact pad of a first of the chips, the contacting module having a first distance a from the first of the chips in the z direction. The first distance a is greater than a maximum distance I of the needle tips from the contacting module. The maximum distance of the needle tips from the contacting module can also be considered as an effective needle length in the z direction in the mechanically unloaded (i.e. force-free) state. The predetermined location can be a specific xy position, i.e. a location in an xy plane. The predetermined location can, but does not have to, be the center of the contact pad. The needles can, for example, be designed as vertical needles (i.e. arranged in the z direction), so-called “vertical needles”, which may have only a slight spring effect. The needles can advantageously be designed as spring-loaded vertical or diagonally arranged needles. The needles can particularly advantageously be designed as cantilever needles. Cantilever needles can be needles that are arranged spring-loaded in the sense of a bending beam. Cantilever needles can preferably be arranged in such a way that, with respect to their bending beam section, in the unloaded (force-free) state they enclose an angle of greater than 60° to the z-axis, particularly preferably greater than 70° and most particularly preferably greater than 80°. The first alignment step can be carried out, for example, using one or more cameras, with the aid of which the first position can be found. Alternatively, manual positioning or passive positioning by striking the wafer edge against provided stops can also be carried out.

Starting from the first position (x1, y1, z1), a second position (x2, y2, z2) is determined in a second alignment step by moving the positioning table to the contacting module in the z direction to a second distance b, at which the needle tips rest on the contact pads, so that electrical signals can be transmitted via the interfaces assigned to one another and optical signals can be transmitted via the optical interfaces assigned to one another. According to the invention, in the second alignment step, the optical deflection elements are aligned relative to the optical interfaces present on the contacting module by small ner an xy extension of the contact pads, the positioning table is deflected in scan positions (xs[i], ys[i], zs[i]) within a scan area relative to the x and/or y coordinates of the first position in the x and/or y direction and/or fed in the z direction, and at the scan positions P[i] = (xs[i], ys[i], zs[i]) an optical signal is coupled via at least one of the optical interfaces of the contacting module and one of the optical deflection elements, wherein the second position (x2, y2, z2) is defined in that the optical signal is coupled with a maximum degree of coupling, wherein the degree of coupling is determined by means of at least one electrical signal transmitted via at least one electrical interface. If several optical interfaces are coupled in the second alignment step, the mean value of the degree of coupling from the several optical interfaces can be used for further consideration.

The scan area can be understood as the generally spatially formed area in which the scan takes place. As a special case, the scan area can be flat, as a special case of this special case, linear. The scan area can be defined mathematically as the smallest convex polyhedron that contains all scan positions (xs[i], ys[i], zs[i]). The scan area can advantageously be provided two-dimensionally, in which case the polyhedron can be degenerated into a polygon. The index i can be provided as a running index i=1, 2 ... imax for consecutively numbering the scan positions. This means that the temporal sequence of the scan positions can be predetermined. The scan positions can also be determined adaptively. This can mean that the position P[i+1] is only determined at the runtime of the scan, taking into account the coupling degrees at the scan positions P[1] to P[i]. The scan field can be understood as a z-projection of the scan area onto the xy plane of the wafer. The scan field can generally be two-dimensional (i.e. flat), as a special case one-dimensional (i.e. linear) and as a special case of the special case zero-dimensional (i.e. point-shaped). The xy extension of the contact pads can be understood as its geometric extension in the x and y directions. The fact that the scan field is smaller than the xy extension of the contact pads can mean that the needle tips do not leave the respective assigned contact pad during the scan. In a special version, the scan can be linear, which corresponds to a one-dimensional scan area. In a special case of this special version, the scan can be linear in the z direction. The scan field can then be point-shaped. This can have the advantage that a scan is possible even with very small contact pads.

A coupling degree k[i] can be determined at each of the scan positions P[i]. The second position (x2, y2, z2) can be an interpolated maximum position of the coupling degrees determined at the scan positions. Alternatively, the scan position at which the maximum coupling degree was measured can be used as the second position without interpolation. At the end of the second alignment step, the determined second position (x2, y2, z2) can, but does not have to, be approached.

According to the invention, in a third alignment step, the first of the chips is brought to a third position (x3, y3, z3) by first setting the x3 and y3 coordinates of the positioning table and then feeding it in the z direction to a third distance c, which is smaller than the second distance b, wherein the needle tips rest on the contact pads with a predetermined pressure force, wherein the third position (x3, y3, z3) was calculated from the second position and the coupling angle α of the respective optical deflection element for the third distance c before the third alignment step.

The third position (x3, y3, z3) can be determined by the fact that for the third distance, which is assigned to the coordinate z3, there is a maximum degree of coupling at the point with the coordinates x3, y3. The third distance c can be a predetermined optical working distance at which the needles rest against the contact pads with a predetermined pressure force.

The third position can be calculated, for example, in the following way. The beam deflected by a deflection element at the specific coupling angle α can, without any limitation, lie in an xz plane (i.e. perpendicular to y). Then the third position (x3, y3, z3) can be determined as x3 = x2+(z3-z2) tan α and y3 = y2, where the coordinate z3 can be fixed by the fact that the specified working distance c is present there. As described below, the required working distance c (and thus the coordinate z3) can change over time.

In the third position, electrical or optical signals can then be transmitted via the respective interfaces for testing the optoelectronic chip.

Advantageously, a position difference resulting from the first position and the second position of the first of the chips is stored as an offset and taken into account for the adjustment of further chips after they have been positioned in a first position relative to the contacting module.

In order to take into account the wear of the needle tips in particular, it is advantageous if the optical working distance c, at which the needles rest against the contact pads with a predetermined contact force, is monitored and the third position is corrected if the contact force changes.

By determining the change in the optical working distance c over the long-term period of use of the contacting module, it is advantageously possible to replace the needles of the contacting module with new needles for the process implementation if the optical working distance c falls below a predetermined minimum distance.

As the optical signals advantageously outshine the respective interface during coupling, position tolerances of the optical interfaces of the chips are compensated for.

It is even more advantageous if the optical signals transmitted via the respective interfaces exhibit a tophat distribution of their radiation intensity when coupled in.

Advantageously, the third alignment step can follow immediately after the second alignment step. This can minimize the processing time. Also advantageously, a fourth alignment step can be provided between the second and third alignment steps, in which the positioning table is placed in the -z direction (i.e. in the negative z direction) at a fourth distance d, the fourth distance d being greater than the maximum distance I of the needle tips to the contact module. This can reduce scratching of the pads.

Advantageously, in the second alignment step, the scan positions (xs[i], ys[i], zs[i]) can lie in an xy plane (x, y, z2). The scan positions Ps[i] can then be (xs[i], ys[i], z2).

Also advantageously, in the second alignment step, the scan positions (xs[i], ys[i], zs[i]) can lie in a yz plane (x2, y, z). The scan positions Ps[i] can then be (x2, ys[i], zs[i]). The scan positions can advantageously be chosen so that z increases monotonically, i.e. z[i+1] ≥ z[i] ∀ i ∈ {1, 2, ..., imax-1}.

It is also advantageous if the scan positions (xs[i], ys[i], zs[i]) lie on a line (x2, y2, z) in the second alignment step. The scan positions Ps[i] can then be (x2, y2, zs[i]). In this case, alignment with respect to y can be omitted.

Advantageously, at least one of the optical elements of a chip can comprise an electro-optical sensor and/or an electro-optical actuator and/or an electro-optical modulator. An electro-optical element can be electrically contacted via two electrical interfaces. It can also be possible for one of the electrical interfaces to be replaced by a substrate contact (ground contact). When using such a ground contact, one electrical interface of the electro-optical element can be sufficient for the method. For example, the anode of an electro-optical element can be connected to ground and only the cathode can be guided via one of the electrical interfaces, or vice versa.

The electro-optical sensor can advantageously be designed as a photodiode or a phototransistor. Then, in the second alignment step, the degree of coupling can be determined by coupling light into the deflection element assigned to the optical sensor via the optical interface of the contact module and measuring a photocurrent via the corresponding electrical interfaces. The degree of coupling can be determined from the ratio of the photocurrent to the light output by the contact module at the interface.

The electro-optical actuator can advantageously be designed as a light emitting diode (LED) or a laser diode. A light emitting diode can be designed as a conventional light emitting diode or as a superluminescent diode. In an electro-optical actuator, the degree of coupling can be determined in the second alignment step by supplying an operating current to the electro-optical actuator via electrical interfaces, whereby the actuator generates light. The generated light is supplied to the optical interface of the contact module via the associated deflection element and the incoming amount of light is measured. The degree of coupling can be determined from the ratio of the photocurrent to the light output by the optical element at the interface. The operating current of the optical element can be used as a measure of the optical light output.

The electro-optical modulator can advantageously be designed as a Mach-Zehnder interferometer. In this case, the degree of coupling can be measured via two optical interfaces. The interferometer can expediently be switched to a conducting state via a voltage applied to the corresponding electrical interfaces.

Advantageously, in the second alignment step, at least one optical actuator can be used as a sensor For example, an LED or laser diode can be operated as a photodiode. This can have the advantage that no operating current needs to be passed through the electrical contacts, but only a comparatively low photocurrent to determine the degree of coupling.

Examples of implementation

• 1a einen Chip eines ersten Ausführungsbeispiels nach dem ersten Ausrichtungsschritt in einer ersten Position zum Kontaktierungsmodul angeordnet,
• 1b das erste Ausführungsbeispiel beim zweiten Ausrichtungsschritt
• 1c das erste Ausführungsbeispiel nach dem dritten Ausrichtungsschritt.
• 2a einen Chip eines zweiten Ausführungsbeispiels nach dem ersten Ausrichtungsschritt in einer ersten Position zum Kontaktierungsmodul angeordnet,
• 2b das zweite Ausführungsbeispiel beim zweiten Ausrichtungsschritt
• 2c das zweite Ausführungsbeispiel nach dem dritten Ausrichtungsschritt.
• 3 den vierten Schritt einer Abwandlung des zweiten Ausführungsbeispiels
• 4 eine Scanvorschrift beim zweiten Ausrichtungsschritt.
• 5 zeigt das dazugehörige Scanfeld

The invention will be explained in more detail below using exemplary embodiments with the aid of drawings.

• 1a a chip of a first embodiment arranged in a first position relative to the contacting module after the first alignment step,
• 1b the first embodiment in the second alignment step
• 1c the first embodiment after the third alignment step.
• 2a a chip of a second embodiment arranged in a first position relative to the contacting module after the first alignment step,
• 2 B the second embodiment in the second alignment step
• 2c the second embodiment after the third alignment step.
• 3 the fourth step of a modification of the second embodiment
• 4 a scanning instruction for the second alignment step.
• 5 shows the corresponding scan field

Using a method according to the invention, optoelectronic chips 1 arranged on a wafer with electrical interfaces in the form of contact pads 1.1 and optical interfaces arranged fixedly thereto in the form of optical deflection elements 1.2, e.g. grating couplers or mirrors, are tested with a specific coupling angle α. The specific coupling angle α represents an angle that an optical signal or its central beam encloses with a perpendicular to the chip 1. It is typically greater than 0° and less than 25°. A common value for the specific coupling angle α in air is e.g. 11.6° and refers to the glass fibers with the corresponding wedge cut used for coupling in the final application of the chip.

In this case, similar to a prior art method, a wafer is picked up by a positioning table 3 that is adjustable in the x, y and z directions of a Cartesian coordinate system relative to a contacting module 2 and rotatable about the z axis. In addition to electrical interfaces 2.1 that can be assigned to the chips 1, the contacting module 2 also has optical interfaces 2.2 that can be assigned to them. The electrical interfaces and the optical interfaces of the chips 1 are manufactured in the wafer assembly in different process steps, so that although they each have only small positional tolerances among themselves, the arrangements formed by the electrical interfaces can have tolerance deviations from the arrangements formed by the optical interfaces, in particular from wafer to wafer. The same applies to the tolerances of the contacting module. In addition, the coupling angle α can be subject to tolerances. In particular, the coupling angle can differ from wafer to wafer due to tolerances.

In a first alignment step, the wafer is delivered to the contacting module 2 in such a way that the electrical interfaces in the form of needles 1.1 present on the contacting module 2 are arranged in a first position (nominal position), perpendicularly above predetermined locations 1.4, which correspond to the centers 1.3 of the contact pads 1.1, of a first of the chips 1. During the alignment, the contacting module has an alignment distance a in the z direction, greater than the free length l of the needles 2.1 to the chip 1, so that no contact can occur between the needle tips of the needles 2.1 and the contact pads 1.1. This corresponds to a first position (x1, y1, z1) with a z coordinate z1. See 1a , whereby for the sake of simplicity, as in the other figures, the representation of the adjustment in the y-direction has been omitted. Accordingly, adjustment paths during individual adjustment steps are only shown as adjustment paths in the x-direction. In a modification of this embodiment not shown in the figure, the predetermined locations are arranged at a distance from the centers of the contact pads.

The process of this first adjustment step is advantageously carried out according to a fixed routine known from practice. One camera measures the needles 2 by focusing on the needle tips, and a second camera measures the contact pads 1.1 of the chip 1. Both cameras have previously been referenced to each other via a standard. This then enables the precise calculation of the optimal position (nominal position) of the needles 2 in relation to the contact pads 1.1 and thus the positioning of the contact module 2 to the chip 1. This is usually done via regression and extrapolation of the measured values. In addition, a contact pad 1.1 is expected as a counterpart for each needle 2 found. The user cannot usually intervene in this routine. Also, alternative structures such as jus animal tags etc. can be used for positioning. The cameras not only correct the xy position and a rotation around the z-axis, but also determine the z-position. The reference points for the needles 2.1 are their needle tips 2.3.

It is clear to the person skilled in the art that due to positional tolerances of the needle tips 2.3 relative to one another and the centers of the contact pads 1.1 relative to one another, not all needle tips can actually be arranged exactly above the centers of the contact pads 1.1 at the same time and that ultimately a position is adjusted in which the average deviation is the smallest. The positional tolerance of the electrical interfaces relative to one another is, however, negligible compared to the positional tolerance of the arrangement of the electrical interfaces relative to the arrangement of the optical interfaces of a chip 1, which is caused in particular by the fact that the electrical interfaces and the optical interfaces are manufactured one after the other using different process steps. The same applies to the tolerances of the contact module. In addition, the coupling angle α can be subject to tolerances. In particular, the coupling angle can differ from wafer to wafer due to tolerances.

Since the production of the electrical interfaces or the production of the optical interfaces for all chips of a wafer takes place in one process sequence, the positional deviation between the arrangements of the electrical interfaces and the arrangements of the optical interfaces of individual chips of a wafer is at least approximately the same.

After the arrangement of the electrical contacts of the chip 1 (needles 2.1) is aligned with the electrical contacts of the contacting module 2 (contact pads 1), the actual position of the optical interfaces of the chip to the optical interfaces on the contacting module 2.2 deviates from a target position due to various reasons, see 1a .

Firstly, the position of the optical interfaces on the chip 1 deviates from their target position in the x, y, z and z directions in a statistically variable manner for each wafer, while the deviations in the x and y directions can be assumed to be unchangeable due to the alignment of the positioning table 3 after the position of the chip 1 and thus of the wafer has been fixed (systematic deviations). In addition, the coupling angle can deviate from a target angle.

Secondly, there is a deviation in the position of the optical interfaces on the contact module 2.2 from a target position in relation to the needles 2.1, depending on the assembly accuracy of the optical module, which is an integral part of the contact module. This affects not only positional deviations in the x, y and z directions, but also tilting in the z, x and y directions. All 6 parameters are assembly-related, systematic deviations.

And thirdly, there are variable deviations over the service life of the contact module 2. The reason for this is the mechanical wear and tear and thus the change in shape of the needle tips as well as possible bending and the resulting change in the fit result.

Using the camera measurement, the positioning table 3 de facto corrects the position of the entire contacting module 2 to the chip/wafer accordingly - and thus also changes the position of the arrangement of the optical interfaces of the contacting module 2.2 to the arrangement of the optical interfaces on the chip 1 due to the specific coupling angle of the respective optical deflection element, in particular a grating coupler.

Starting from the first position (x1, y1, z1), a second alignment step (see 1b) a second position (x2, y2, z2) is determined. To do this, the positioning table (3) is first moved to the contacting module (2) in the z direction to a second distance (b) at which the needle tips (2.1) rest against the contact pads (1.1). This means that electrical signals can be transmitted via the interfaces assigned to one another and optical signals can be transmitted via the optical interfaces assigned to one another. In the second alignment step, the optical deflection elements (1.2) are aligned relative to the optical interfaces (2.2) present on the contacting module (2). In a scan field (4, 5) smaller than an xy extension of the contact pads (1.1), the positioning table (3) is deflected in scan positions Ps[i] = (xs[i], ys[i], z2) within a scan area (4, 5) relative to the x and/or y coordinates of the first position in the x direction (4) and y direction (5). The scan is therefore carried out in an xy plane. In other modifications of the example not shown in the figure, the scan can also include a feed in the z direction, for example the scan can be carried out in a yz plane.

At the scanning positions (xs[i], ys[i], zs[i]) an optical signal is coupled via at least one -here two- of the optical interfaces of the contacting module (2.2) and one of the optical deflection elements (1.2).

The second position (x2, y2, z2) is defined by coupling the optical signal with a maximum coupling degree, where the The coupling degree is determined by means of at least one electrical signal transmitted via at least one electrical interface (1.2, 2.1). Two of the three electrical interfaces shown are used for a photodiode. Alternatively, one of the two interfaces used can be replaced by a substrate contact of the wafer. If several optical interfaces are coupled simultaneously, the average value of the coupling degree from several interfaces can be used for further analysis.

In the illustration, the second position is at the x-coordinate x2, which is different from x1. See 1b The second position does not need to be approached; it is sufficient if it is determined, for example, by means of regression from the coupling degrees at the scan positions.

In a third alignment step, the first of the chips (1) is brought to a third position (x3, y3, z3) by first setting the x3 and y3 coordinates of the positioning table and then moving it in the z direction to a third distance c, which is smaller than the second distance b. The needle tips (2.3) are in contact with the contact pads (1.1) with a predetermined pressure force, as shown in 1c This is achieved by selecting an optical working distance smaller than the free length of the needles I. The difference is referred to as overtravel or overdrive. The needles can, for example, bend or compress elastically. Elastic bending is exaggerated here to illustrate the effect. In the second alignment step, a smaller overtravel is selected than in the third alignment step. The second distance b is larger than the third distance c. This means that the contact pads can be protected during scanning.

The overtravel ensures a secure electrical contact between the needles and the contact pads (low contact resistance). After the first light contact between the needles and the contact pads, the wafer is moved upwards by a few 10µm in the z-direction in the third alignment step. This achieves two things. On the one hand, this causes any oxide surfaces that may be present to break through, so that a reproducible, low-resistance contact is achieved. On the other hand, the overtravel creates a constant contact pressure for the needles, as the needles spring in accordingly due to the additional travel and exert a pressure on the contact pad. This pressure force varies depending on the type of needle used, but one can assume a magnitude of around 0.03 N per needle. The overtravel is typically specified in multiples of MIL (American 1mil = 1/1000 inch = 0.0254mm)

In a contacting module in which the arrangement of the needle has a fixed position in relation to the optical interfaces, the value of the overtravel must be taken into account in the final adjustment of the chip so that, in the contacted state, it is ensured that there is an optical working distance between the optical interfaces of the contacting module and the chip at which maximum coupling of the optical signals is possible. The third position (x3, y3, z3) is calculated from the second position and the coupling angle (α) of the relevant optical deflection element (1.2) for the third distance (c) before the third alignment step. This makes it possible to first set the x and y coordinates of the third position and then adjust the z coordinate to the desired working distance c. This prevents the needles from scratching the pads.

To set the optical working distance c, the chip is brought into the third position. This is the final adjustment state in which both the electrical and optical interfaces of the chip and contact module are aligned with each other as best as possible, i.e. the optical signal flow can be measured as best as possible (maximum position of the optical coupling), whereby the electrical signal flow is also present. To test the chip, electrical or optical signals are then passed through the interfaces assigned to each other.

The adjustment path from the first position (nominal position) to the third position (maximum position of the optical coupling) represents an offset (in 1c , shown as coordinate comparison of the coordinates x3 to x1 and x2 and as coordinate z3) which is advantageously stored and taken into account when adjusting all other chips on this wafer. This means that the nominal position of the positioning table is adjusted by the corresponding offset as the difference x3-x1 in the x-direction (see 1c ) and difference y3-y1 in the y-direction (not shown). However, this requires a corresponding displacement of the needles over the contact pads, which is still tolerable.

This procedure only needs to be performed once per wafer or at longer intervals.

In addition, this offset value, i.e. the difference x3-x1 as well as the difference y3-y1 and its changes over time can be observed and thus allows statements to be made about the wear and associated changes in the needles.

To increase the accuracy of the offset determination, the raster scan can also be performed on several chips of the wafer before starting the inspection of all chips of the wafer and the results averaged.

The required overtravel, i.e. the difference c-I, can change over the lifetime of the contacting module due to “running in” or wear of the needles, which reduces the working distance c.

The direct monitoring of the optical working distance is important to ensure the defined optical coupling properties between the contacting module and the chip (measurement capability) and to prevent collisions of the optical interfaces of the contacting module and chips (a few 10-100 µm distance during operation).

A distance sensor that is permanently integrated into the contact module can be used for this purpose, e.g. a capacitive distance sensor. This enables the real optical working distance to be monitored. In combination with an active control, the working distance can be actively adjusted by moving the positioning table in the z direction and, if necessary, a hard stop can be programmed in if a minimum working distance is not reached, e.g. to avoid a collision due to incorrect operation by the operator.

Typically, a change in the needle tip positions is to be expected when the contact module is first put into operation (run-in behavior). This can be anticipated by multiple contact simulations before adjusting a first chip (pre-aging), which can reduce the value of the offset to be set.

Typically, all optical interfaces of the chips, embodied by grating couplers, have coupling angles with the same angular magnitude and the same orientation, so that a change in the optical working distance Δb results in an equal relative change ΔX of the optimal coupling position for all optical interfaces.

Typically, the change in the optical working distance Δb is in the range <50µm. If the optical working distance changes by, for example, 10µm and the coupling angle α is 11.6° in the x-z plane, this would result in a change in the optimal coupling position Δx of 2µm in the x-direction. If the optical working distance Δb changes by 20µm, this would result in a change in the optimal coupling position Δx of 4µm in the x-direction. This can be compensated for by the correction described above, i.e. the positioning table is corrected in the x-direction using a control command, assuming that the needles still hit the contact pads with sufficient reliability.

The requirements for the accuracy of the adjustment steps can be reduced if the optical signals transmitted for testing the chip via the respective associated interfaces outshine the coupling interface.

2a shows a chip of a second embodiment after the first alignment step arranged in a first position relative to the contacting module. In contrast to the first embodiment, cantilever needles 2.1 are used here. In a first alignment step, the wafer is fed to the contacting module (2) such that the needle tips (2.3) are arranged in a first position (x1, y1, z1) each spaced apart in the z direction above a predetermined point (1.4) of the associated contact pad (1.1) of a first of the chips (1), wherein the contacting module (2) has a first distance (a) in the z direction from the first of the chips (1), wherein the first distance (a) is greater than a maximum distance (I) of the needle tips (2.3) from the contacting module (2). The predetermined points 1.4 are provided here outside the centers 1.3 of the contact pads 1.1.

2 B shows the second embodiment in the second alignment step. In contrast to the first embodiment, a yz scan is provided here over a scan area in the y direction 5 and z direction 6 with a fixed coordinate x2=x1. Here, the scan can advantageously be aborted after the maximum degree of coupling at z2 is exceeded. In a modification of the second embodiment, the scan is carried out with a fixed y coordinate y2=y1, which is a line scan. In a further modification, the scan is carried out with a fixed z coordinate in an xy plane.

2c shows the second embodiment after the third alignment step. With the cantilever needles, the pressure force can be applied by means of elastic bending. The third alignment step takes place immediately after the second.

3 shows a fourth alignment step of a further modification of the second embodiment. Here, the positioning table is set in the -z direction at a fourth distance d, whereby the fourth distance d is greater than the maximum distance I of the needle tips (2.3) to the contacting module (2). The fourth alignment step occurs between the second and third alignment steps.

4 shows a scanning specification for a yz scan in the second alignment step. The z coordinate of the scan trajectory minus the z2 coordinate of the second position is plotted on the abscissa. The y coordinate of the scan trajectory minus the y2 coordinate of the second position is plotted on the ordinate. The trajectory (yz path) is shown as a thick line in the diagram, the direction of travel indicated by arrows. The scan positions are selected one after the other at the corners of the drawn trajectory. The scan begins in the middle right of the illustration at the second position (x2, y2, z2). 55 scan positions are used. The z coordinate increases monotonically in 54 steps, alternating between 1 µm and 0 µm. The y coordinate moves in step sizes of -1 µm, 0 µm and 1 µm in the manner shown between y2-3 µm and y2+3 µm. The scanning range is 6µm in the y-direction and 27µm in the z-direction. The path of 28µm in the z-direction is equivalent to a beam offset in the x-direction of 5.5µm at a specific coupling angle α=11.6°. The 5 The scan area 7 shown is the polygon surrounding the scan positions. It is formed flat in a yz plane. The scan field (not shown in the figure) as a vertical z-projection of the scan area would be one-dimensional (linear) running in the y-direction.

In a non-figuratively represented variation of the 4 and 5 In the scanning rule shown, the y-coordinate remains fixed and the scan is performed as a line scan in the z-direction. The scan field would then be point-shaped.

List of reference symbols

1

chip

1.1

Contact pad

1.2

optical deflection element

1.3

Center of the contact pad

1.4

predetermined location

2

Contacting module

2.1

needle

2.2

optical interface on the contact module

2.3

Needle tip

3

Positioning table

4

x- Scan range

5

y- scan range

6

z- scan range

7

Scan area

α

Coupling angle

a

first distance

b

second distance

c

third distance

l

free length of the needle

Claims (11)

Method for testing optoelectronic chips (1) arranged on a wafer, with electrical interfaces in the form of contact pads (1.1) and optical elements fixed thereto, which comprise optical interfaces in the form of optical deflection elements (1.2) with a specific coupling angle (α), in which the wafer is received by a positioning table (3) which is adjustable in the x, y and z directions of a Cartesian coordinate system relative to a contacting module (2) and rotatable about the z axis, wherein the contacting module (2) has electrical interfaces in the form of needles (2.1) with needle tips (2.3) assigned to the contact pads (1.1) and optical interfaces (2.2) assigned to the optical deflection elements (1.2), and • in a first alignment step, the wafer is fed to the contacting module (2) in such a way that the needle tips (2.3) are in a first position (x1, y1, z1) each spaced apart in the z direction over a predetermined Position (1.4) of the associated contact pad (1.1) of a first of the chips (1), wherein the contacting module (2) has a first distance (a) in the z direction from the first of the chips (1), wherein the first distance (a) is greater than a maximum distance (I) of the needle tips (2.3) from the contacting module (2), • starting from the first position (x1, y1, z1), a second position (x2, y2, z2) is determined in a second alignment step by moving the positioning table (3) to the contacting module (2) in the z direction to a second distance (b), at which the needle tips (2.1) rest against the contact pads (1.1), so that electrical signals can be transmitted via the interfaces assigned to one another and optical signals can be transmitted via the optical interfaces assigned to one another, wherein in the second alignment step a relative alignment of the optical deflection elements (1.2) to the optical interfaces (2.2) present on the contacting module (2) is carried out by deflecting the positioning table (3) in scan positions Ps[i] = (xs[i], ys[i], zs[i]) within a scan area (7) in relation to the x and/or y coordinates of the first position in the x and/or y direction and/or feeding it in the z direction in a scan field (4, 5) smaller than an xy extension of the contact pads (1.1), and at the scan positions (xs [i], ys[i], zs[i]) via at least one of the optical interfaces of the contacting module (2.2) and one of the optical deflection elements (1.2), wherein the second position (x2, y2, z2) is defined by the optical signal being coupled with a maximum degree of coupling, wherein the degree of coupling is determined by means of at least one electrical signal transmitted via at least one electrical interface (1.2, 2.1), and • in a third alignment step, the first of the chips (1) is brought to a third position (x3, y3, z3) by first setting the x3 and y3 coordinates of the positioning table and then feeding it in the z direction to a third distance c, which is smaller than the second distance b, wherein the needle tips (2.3) rest on the contact pads (1.1) with a predetermined pressure force, wherein the third position (x3, y3, z3) is determined from the second position and the Coupling angle (α) of the respective optical deflection element (1.2) for the third distance (c) was calculated before the third alignment step. Method for testing optoelectronic chips (1) arranged on a wafer according to Claim 1 , characterized in that the third alignment step immediately follows the second alignment step or that a fourth alignment step is provided between the second and third alignment steps, in which the positioning table is placed at a fourth distance d in the -z direction, wherein the fourth distance d is greater than the maximum distance I of the needle tips (2.3) to the contacting module (2). Method for testing optoelectronic chips (1) arranged on a wafer according to one of the preceding claims, characterized in that in the second alignment step the scanning positions (xs[i], ys[i], zs[i]) lie in an xy plane (x, y, z2) and/or on a line (x2, y2, z) and/or in a yz plane (x2, y, z). Method for testing optoelectronic chips (1) arranged on a wafer according to one of the preceding claims, characterized in that a position difference resulting from the first position and the second or the third position of the first of the chips (1) is stored as an offset and is taken into account for the alignment of further chips (1) after they have been positioned in a first alignment step with respect to the contacting module (2). Method for testing optoelectronic chips (1) arranged on a wafer according to one of the preceding claims, characterized in that the third distance c, at which the needles (2.1) rest against the contact pads (1.1) with a predetermined contact force, is monitored and the third distance is corrected in the event of changes in the contact force. Method for testing optoelectronic chips (1) arranged on a wafer according to one of the preceding claims, characterized in that the change in the third distance c is determined over the long term over the service life of the contacting module (2) and, for carrying out the method, the needles (2.1) of the contacting module (2) are replaced by new needles (2.1) when the third distance c falls below a predetermined minimum distance. Method for testing optoelectronic chips arranged on a wafer according to one of the preceding claims, characterized in that the optical signals conducted via the interfaces assigned to one another outshine one of the interfaces when coupled into said interfaces. Method for testing optoelectronic chips (1) arranged on a wafer according to one of the preceding claims, characterized in that the optical signals conducted via the respective interfaces assigned to one another have a tophat distribution of their radiation intensity when coupled into one of the interfaces. Method for testing optoelectronic chips (1) arranged on a wafer according to one of the preceding claims, characterized in that at least one of the optical elements of a chip comprises an electro-optical sensor and/or an electro-optical actuator and/or an electro-optical modulator. Method for testing optoelectronic chips (1) arranged on a wafer according to Claim 9 , characterized in that the electro-optical sensor is designed as a photodiode or a phototransistor and/or that the electro-optical actuator is designed as a light emitting diode or a laser diode or that the electro-optical modulator is designed as a Mach-Zehnder interferometer. Method for testing optoelectronic chips (1) arranged on a wafer according to one of the Claims 9 until 10 , characterized in that in the second alignment step at least one optical actuator is operated as a sensor.

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