Method for cleaning a test card, and use and device

Abstract

The invention relates to a method for cleaning contact pins (4) and an optical functional surface of a test card (1) for testing optoelectronic chips.

Description

[0001] JENOPTI K Optical Systems GmbH

[0002] LO-24-013-P-WO

[0003] 1 / 16

[0004] Method for cleaning a test card, as well as use and apparatus

[0005] Technical field

[0006] The invention relates to the testing and qualification of chips with photonic integrated circuits (PICs) at the wafer level. Unlike conventional, purely electrically integrated circuits (ICs), PICs integrate optical functionalities in addition to electrical circuitry.

[0007] The present invention relates to a method for cleaning contact needles and optical functional surfaces of test cards for testing optoelectronic chips. Such a test card is also known as a probe card.

[0008] Such a test card is known to be used in a wafer prober, which allows the simultaneous testing of the functionality of electrical and optical components or circuits of a chip at the wafer level. Such a method is known by type from US 2011 / 0279812 A1. DE 102018 108283 A1 further discloses an electro-optical circuit board for contacting photonic integrated circuits, in which an optical beam path passes through the circuit board. WO 2019 / 029 765 A1 discloses a position tolerance-insensitive contacting module for contacting optoelectronic chips.

[0009] As a result of the operation of the wafer prober, the test card, in particular the optical and electronic contact module, may become contaminated, which may necessitate cleaning.

[0010] State of the art

[0011] A cleaning device for test cards, which operates with compressed air, is known from KR 0835467 B. A disadvantage is that compressed air can stir up dust.

[0012] A needle cleaning device is known from KR 19970030555 A, in which a brush is used on a ceramic wafer. A disadvantage is that the cleaning device requires complex placement. JENOPTIK Optical Systems GmbH

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[0015] From KR 20050071 117 A, another device for cleaning the contact needles of test cards is known, which is attached laterally to the wafer holding device. The complexity of this system is a disadvantage.

[0016] Other similar systems are known from JP 11 054572 A, CN 103878150 B, US 5968282 A, the latter of which, in addition to the brushes, has a rubber filled with an inorganic filler material as a cleaning element.

[0017] A cleaning system with a rotating wiper arm for contact needles is known from CN 219850969 U.

[0018] A wet cleaning system for contact needles is known from CN 116099819 A.

[0019] From DE 10201621 7098 A1 a handheld joint cleaning device with a swinging brush is known.

[0020] None of the known systems are suitable for cleaning both the contact needles and the optical functional surfaces of a test card. JENOPTI K Optical Systems GmbH

[0021] LO-24-013-P-WO

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[0023] Object of the invention

[0024] The object of the invention is to provide a cost-effective, energy-efficient, universally applicable cleaning method for optoelectronic test cards.

[0025] Solution to the task

[0026] The problem is solved by a cleaning method according to claim 1, a use according to claim 14 and a brush arrangement according to claim 15.

[0027] Advantages of the invention

[0028] The inventive method allows for the gentle and residue-free removal of solid particles (e.g., dust and abrasion) from the contact needles and optical functional surfaces. The advantage of the invention over known cleaning methods for test cards lies in its cost-efficiency and energy efficiency. Furthermore, the invention is sustainable because the cleaning method is gentle, thus enabling a long service life for the test cards. In addition, the brush arrangement according to the invention can be reused multiple times. Cleaning can be carried out without the use of additives such as cleaning agents. The inventive cleaning process can be performed when the test card is installed in a test card holder of a wafer-level test device, which can also be referred to as a wafer prober. This eliminates the need to remove the test card or the optical contact module from the wafer-level test device for cleaning.

[0029] Description

[0030] The following describes the procedure for cleaning contact needles and optical functional surfaces of test cards for testing optoelectronic chips.

[0031] The test card can comprise an electrical contact module and at least one optical contact module. These can advantageously be interconnected. The optical contact module can essentially consist of a glass body containing integrated optical waveguides and monolithically mounted, totally reflecting mirrors. During operation of the module, a coupled light beam, for example, a laser beam, can be guided through the waveguides to the mirror, which reflects the beam and shapes it so that the light beam exits the module at the beam exit surface as collimated as possible. A total of, for example, 4 to 1024 such optical channels can be provided in the optical module. The beam spacing (pitch) can advantageously be between 20 pm and 500 pm, for example, 127 pm. The mirrors can be concave. (In analogous JENOPTI K Optical Systems GmbH)

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[0034] Alternatively, the optical channels can be configured with reversed beam paths as inputs. Instead of light sources, light detectors, such as photodiodes, can then be used. In this case, light emanating from the optoelectronic chip under test can be detected during operation of the test card. The optical contact module can be made of glass, quartz glass, or a transparent plastic. It can incorporate the aforementioned waveguide structures, for example, by inscribing them into the base material using a laser beam. When using the test card for testing optoelectronic chips, the optical contact module can be used to transmit optical signals to and from the chip under test.

[0035] The method comprises providing a test card with contact needles and at least one optical functional surface, providing a brush arrangement with a vibration drive, a cantilever and a plurality of bristles, each mechanically connected at one end to the cantilever and each having an opposite free end.

[0036] The test card can have one or more optical functional surfaces. These surfaces can be configured as mirror surfaces or as optical interfaces, i.e., beam entry or beam exit surfaces. The optical functional surfaces can be located on the optical test module. When using the test card to test optoelectronic chips, the optical interfaces can serve as light entry and / or light exit surfaces of the optical test module. They can be configured as flat surfaces, lenses, and / or diffraction gratings.

[0037] Furthermore, the optical test module can include additional optical functional surfaces, which may, for example, be designed as external mirror surfaces. These can be intended for deflecting light rays within the material of the optical contact module, for example by internal total internal reflection, and may also be intended for reshaping, in particular focusing, the light rays.

[0038] The contact pins can be arranged on the electrical test module. They can be designed as metallic contacts or as spring-loaded tips. When using the test card for testing optoelectronic chips, the contact pins can be used to electrically contact the chip under test. JENOPTI K Optical Systems GmbH

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[0041] The bristles can be made of materials such as carbon fibers, polyimide fibers, or glass fibers. This ensures low bristle wear. The bristles can be electrically conductive with a resistivity of less than 100 pQm, particularly between 1 pQm (microohms per meter) and 30 pQm, or electrostatically dissipative with a resistivity between 100 pQm and 1 MQm (megaohms per meter). This prevents electrostatic charging of the bristles.

[0042] The bristles can be straight when at rest. They can be arranged essentially parallel in one direction z. The term "essentially" can mean that the divergence of the bristles from the clamped ends to the free ends is less than 30°, which can be determined as a full angle at half the number of bristles, advantageously less than 15°, and particularly advantageously less than 8°. Then the cleaning effect can be especially homogeneous across the entire bristle bundle.

[0043] The bristles can have a diameter between 3 pm (micrometers) and 30 pm, particularly between 5 pm and 15 pm, and a length between 5 mm and 30 mm, particularly between 10 mm and 20 mm. The ratio of bristle length to bristle diameter can be between 500 and 10,000, advantageously between 800 and 3,000. The bristles of the brush assembly can be arranged as a bristle bundle with a bundle diameter between 2 mm and 20 mm or as a flat bristle bundle with a cross-section of 0.5 mm x 3 mm to 3 mm x 30 mm. The cross-section can lie in an xy-plane, with the x, y, and z directions forming a Cartesian coordinate system. An aspect ratio of the cross-section, i.e., the ratio of the long extent to the short extent of the bundle cross-section, can be between 2 and 10, particularly between 2 and 5.

[0044] The bristles are set into mechanical vibration by generating mechanical energy from the vibration drive and transferring it to the bristles via the arm. The arm can be rod-shaped. It can advantageously be rigid to transmit the vibration without damping. It can be essentially straight and extend in the z-direction. This means that the center line is angled, for example, at most 30°, advantageously at most 10°, between the two ends of the arm. A slight curvature of the arm, which is negligible for the brushing effect, can, however, improve the visibility of the brushed area at the free ends of the bristles during brushing. The mechanical vibration can be white noise or pink noise in the frequency spectrum. This can have the advantage of preventing resonant vibration of the contact pins.The frequency spectrum of the mechanical vibration JENOPTI K Optical Systems GmbH.

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[0047] However, it can also be harmonic with a fundamental frequency between 100 Hz and 1500 Hz, particularly between 150 Hz and 500 Hz. In this case, the brush assembly can be operated resonantly, resulting in energy savings compared to noise operation. In addition to a fundamental frequency, the mechanical vibration can exhibit frequency multiples in the power spectrum, which, on average, have a peak height of more than -30 dB relative to the peak of the fundamental frequency in the range between 2 kHz and 8 kHz. Higher harmonics in this frequency range can improve cleaning efficiency for small particles. The mechanical vibration can be stationary. Alternatively, it can be modulated with a modulation frequency between 5 Hz and 30 Hz, where the modulation depth can advantageously be between 10% and 100%, or between 20% and 80% amplitude reduction relative to the maximum amplitude.A modulation depth of 100% can mean that the amplitude of the oscillation is zero at the minima of the envelope. A modulation depth of 10% can mean that the amplitude of the oscillation at the minima of the envelope is 90% of the amplitude at the maxima of the envelope. The modulation frequency can, but does not have to, also be swept. Amplitude modulation and / or swept can improve the cleaning effect and also make the sound of the brush device more pleasant. However, these functions may involve additional effort in controlling the oscillation drive.

[0048] The process involves bringing the free ends of the bristles close to the contact needles, and then brushing the contact needles with the vibrating free ends of the bristles for an initial period to clean them. The bristles can be set into mechanical vibration either after being brought close to the contact needles or before reaching them. The latter approach can be advantageous because the vibrating bristles, as described below, can be gentler than when the bristles are fully extended.

[0049] The method comprises bringing the free ends of the bristles close to the at least one optical functional surface, and then brushing the at least one optical functional surface with the vibrating free ends of the bristles for a second period to clean the at least one optical functional surface. The bristles can be set into mechanical vibration either after being brought close to the optical functional surface or before reaching it. The latter approach can be advantageous because the bristles, curved by vibration as described below, can be gentler than bristles in a straight state. Furthermore, the effective length of the bristles can change due to vibration-induced curvature. JENOPTI K Optical Systems GmbH

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[0052] Bringing the free ends of the bristles to the contact needles and to at least one optical functional surface can be done manually. For this purpose, the vibration drive can be located in a handpiece that can be used to manually guide the brush assembly. Alternatively, the process can be automated, for example with a robot, or by moving the test card to the bristles. In the latter case, the brush assembly can be stationary.

[0053] The first and second brushing cycles can last between 0.5 and 10 seconds, and the amplitude of the vibration at the clamped ends of the bristles can range from 0.1 mm to 5 mm, particularly between 0.2 mm and 2 mm. The amplitude of the vibration at the free ends of the bristles can be in the same range. However, it is also possible that the free ends of the bristles will exhibit a larger or smaller amplitude due to their elasticity. Here, as well as in the following, the term amplitude can be understood as the magnitude of the maximum displacement from the rest position.

[0054] The process can also be carried out by simultaneously brushing the contact needles and the optical functional surface. In this case, the bristle bundle must be sufficiently large to brush the contact needles and one or more optical functional surfaces at the same time. The first and second brushing times can then be identical. Similarly, bringing the free ends of the bristles to the contact needles and bringing them to at least one optical functional surface can also be done simultaneously. A potential advantage of this approach is that simultaneous brushing reduces cleaning time.However, it can also be advantageous to choose a smaller cross-section for the bristle bundles and to perform several brushing processes in succession for the contact needles and the one or more optical functional surfaces, because then the accuracy requirements for the brush arrangement allow for greater tolerances.

[0055] Cleaning can be performed dry, without the use of cleaning agents. Dry cleaning is particularly effective for removing solid particles (such as abrasion, dust, and mites) from the contact needles and optical functional surfaces. Optionally, a suction system can be provided to collect the brushed-off particles. However, suction may also be unnecessary. If the vibration energy is sufficiently high, the brushed-off particles can be accelerated to such an extent that they reach safe locations without the need for suction. In principle, liquid or pasty contaminants can also be removed. However, in this case, inJENOPTI K Optical Systems GmbH

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[0058] This means that cleaning or replacement of the bristles will be necessary at correspondingly shorter intervals.

[0059] The bristles can be essentially oriented in the direction of the boom. The mechanical vibration of the bristles can be essentially transverse, for example, in the x and / or y direction. This transverse direction of vibration, in conjunction with the frequency spectrum and geometry of the bristles, can cause the vibrating bristles to exhibit one or more time-dependent curvatures. As a result, the force exerted by the bristles on the objects to be cleaned in the z direction, i.e., in the longitudinal direction, can be considerably smaller than it would be with straight bristles. Particularly in the presence of higher harmonics of the transverse vibration component, a wave-like pattern of bristles can develop, for example, with several turning points in its geometric profile.

[0060] Based on the above considerations regarding the reduced longitudinal force exerted by transversely oscillating bristles, a certain longitudinal oscillation component of the bristles can even be advantageous in order to address a larger cleaning volume in the z direction without placing excessive stress on the objects being cleaned. It can be advantageous if the longitudinal oscillation is performed with a smaller amplitude, for example, 0.5% to 20%, or advantageously 1% to 10%, of the transverse amplitude. The amplitude of the longitudinal oscillation component can, for example, be between 10 pm and 200 pm. The longitudinal oscillation frequency can be higher than that of the transverse oscillation. For example, the fundamental frequency of the longitudinal oscillation can be a multiple, for example, twice, of the fundamental frequency of the transverse oscillation component.Then the bristles, measured at the clamped end, can, for example, execute a vibration pattern in the form of a figure eight lying on its side.

[0061] The optical functional surfaces can be designed as a beam entry surface, beam exit surface and / or mirror surface.

[0062] The present invention also relates to the use of an oscillating brush driven by a vibration drive for cleaning contact needles and optical functional surfaces of test cards for testing optoelectronic chips. JENOPTI K Optical Systems GmbH

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[0065] The bristles can be aligned with the boom at an angular deviation of less than 30°, advantageously less than 10°. This can mean that the bristles are essentially aligned with the boom.

[0066] The figures show the following:

[0067] Fig. 1 shows a first embodiment.

[0068] Fig. 2 shows a sequence of the process.

[0069] Fig. 3 shows a second sequence of the process.

[0070] Fig. 4 shows an example of bristle movement in an xz-view.

[0071] Fig. 5 shows an example of bristle movement in an xy-view.

[0072] Fig. 6 shows a power spectrum of the mechanical vibration.

[0073] Examples of implementation

[0074] The invention is explained below using exemplary embodiments.

[0075] Fig. 1 shows a first embodiment. In this embodiment, a test card 1 with electrical test modules 2 and optical test modules 3 is shown. The test card 1 has several contact pins 4 on the electrical test module 2 and several optical functional surfaces 5 on the optical test module 3. These can be configured as optical interfaces 6, which subsequently serve as light entry surfaces and / or light exit surfaces of the optical test module when the test card 1 is used to test optoelectronic chips. These can be configured as flat surfaces, lenses, or diffraction gratings. Furthermore, the optical test module can include optical functional surfaces 5, which are configured as external mirror surfaces 7.

[0076] In this embodiment, the brush assembly 8 is shown spaced apart from the test card 1. The brush assembly comprises a vibration drive 9, which is housed in a handpiece 14 and connected to a boom 10. The brush assembly 8 has bristles 11, each of which is mechanically connected at one end 12 to the boom 10 and each has an opposite free end 13.

[0077] The handpiece 14 is connected to the brush assembly 8. The handpiece 14 allows the brush assembly 8 to be handled and manually brought into contact with the contact needles 4 and the optical functional surfaces 5. JENOPTI K Optical Systems GmbH

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[0080] In this embodiment, the brush arrangement 8 is positioned such that the bristles 11 are aligned in the direction of the arm 10, here designated as the z-direction. The mechanical vibration of the bristles 11 can be essentially transverse. The bristle ends can therefore vibrate predominantly in the x or y direction, or in both directions.

[0081] In a modification of the embodiment, a suction device 15 can be provided, which is arranged as a suction nozzle with the suction end next to the bristles.

[0082] Fig. 2 shows a sequence of the process.

[0083] • Provide 16 of the test card 1 with the contact needles 4 and the at least one optical functional surface 5,

[0084] • Providing 17 a brush arrangement 8 comprising a vibration drive 9, a boom 10 and a plurality of bristles 11, each mechanically connected at one end 12 to the boom 10 and each having an opposite free end 13,

[0085] • Setting 18 of the bristles 11 into a mechanical vibration by means of the vibration drive 9, wherein mechanical energy is generated by the vibration drive 9 and transferred to the bristles 11 via the boom 10,

[0086] • Bringing 19 of the free ends 13 of the bristles 11 to the contact needles 4,

[0087] • Brushes 20 of the contact needles 4 with the oscillating free ends 13 of the bristles 11 for an initial period to clean the contact needles 4,

[0088] • Bringing 21 the free ends 13 of the bristles 11 towards at least one optical functional surface 5,

[0089] • Brushes 22 of the at least one optical functional surface 5 with the oscillating free ends 13 of the bristles 11 for a second period to clean the at least one optical functional surface 5.

[0090] Fig. 3 shows a second sequence of the process. In this sequence, the bringing 19 of the free ends 13 of the bristles 11 to the contact needles 4 and the bringing 21 of the free ends 13 of the bristles 11 to the at least one optical functional surface 5 take place simultaneously.

[0091] Furthermore, the brushing 20 of the contact needles 4 with the oscillating free ends 13 of the bristles 11 takes place for a first period of time to clean the contact needles 4 and the brushing 22 of the at least one optical functional surface 5 with the oscillating free ends 13 of the JENOPTI K Optical Systems GmbH

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[0094] Bristles 11 are used for a second duration to clean at least one optical functional surface 5 simultaneously. The first and second durations are then necessarily identical.

[0095] For this purpose, a bundle of bristles can be used which is sufficiently large to brush both the contact needles 4 and the optical functional surface 5 or several optical functional surfaces simultaneously.

[0096] Fig. 4 shows an example of bristle movement in an xz-direction. It depicts an example of a trajectory 16 of the clamped ends of the bristles, i.e., at the end of the boom. The amplitude in the z direction is smaller than that in the x direction. This indicates a predominantly transverse brushing action. It can also be seen that the frequency of the deflection in the z direction is a multiple of the frequency of the deflection in the x direction, in this case, twice as much. In a variation of the example not shown in the figure, both frequencies are equal. In another variation not shown in the figure, a zigzag line caused by harmonics can be superimposed on the trajectory of the fundamental frequency. In yet another variation not shown in the figure, the amplitude in the z direction, i.e., the longitudinal oscillation component, can be zero, resulting in a purely transverse oscillation.The figure-eight shape of the track is only meant as an example. A variety of other Lissajous figures can also occur.

[0097] Fig. 5 shows an example of bristle movement in an xy-axis representation. The trajectory 16 of the fundamental frequency is shown here as an oval. However, it can also degenerate into a circle or a line in variations. In the latter case, a linear transverse oscillation may be present.

[0098] Fig. 6 shows a power spectrum of the mechanical vibration. Here, the vibration power in arbitrary units (au) is plotted against the frequency. The curve is normalized to the global peak value at 275 Hz. The mechanical vibration has a fundamental frequency of 275 Hz. In addition to the fundamental frequency, the mechanical vibration exhibits frequency multiples in the power spectrum shown. On average, these have a peak amplitude of more than -30 dB relative to the peak of the fundamental frequency at 275 Hz in the range between 2 kHz and 8 kHz. In the measurement of a brush arrangement according to the invention shown here, the value is -22 dB. The value of -22 dB is indicated in the graph.

[0099] Please note that the figures are not to scale. JENOPTI K Optical Systems GmbH

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[0102] The reference symbols used uniformly in all figures are as follows:

[0103] 1. Test card (probecard)

[0104] 2. Electrical test module

[0105] 3. Optical test module

[0106] 4. Contact needle

[0107] 5. optical functional area

[0108] 6. optical interface

[0109] 7. Mirror surface

[0110] 8. Brush arrangement

[0111] 9. Vibration drive

[0112] 10. Boom

[0113] 11. Bristles

[0114] 12. Clamped end

[0115] 13. Free ending

[0116] 14. Handpiece

[0117] 15. Extraction (optional)

[0118] 16. Movement of the clamped ends

[0119] 17. Providing the test card

[0120] 18. Providing a brush arrangement

[0121] 19. Setting the bristles into a mechanical vibration

[0122] 20. Bringing the free ends of the bristles towards the contact needles

[0123] 21. Brushes of the contact needles,

[0124] 22. Bringing the free ends of the bristles towards at least one optical functional surface.

[0125] 23. Brushing of at least one optical functional surface

Claims

JENOPTI K Optical Systems GmbH LO-24-013-P-WO 13 / 16 Patent claims 1. Method for cleaning contact needles (4) and at least one optical functional surface (5) of a test card (1) for testing optoelectronic chips, comprising: • Providing (17) the test card (1) with the contact needles (4) and the at least one optical functional surface (5), • Providing (18) a brush arrangement (8) comprising a vibration drive (9), a boom (10) and a plurality of bristles (11) which are mechanically connected at one end (12) to the boom (10) and each have an opposite free end (13), • Setting (19) the bristles (11) into a mechanical vibration by means of the vibration drive (9), wherein mechanical energy is generated by the vibration drive (9) and transferred to the bristles (11) via the boom (10), • Bringing (20) the free ends (13) of the bristles (11) to the contact needles (4), • Brushes (21) of the contact needles (4) with the oscillating free ends (13) of the bristles (11) for an initial period to clean the contact needles (4), • Bringing (22) the free ends (13) of the bristles (11) towards at least one optical functional surface (5), • Brushing (23) of the at least one optical functional surface (5) with the oscillating free ends (13) of the bristles (11) for a second period to clean the at least one optical functional surface (5).

2. Method according to claim 1, characterized in that the at least one optical functional surface (5) is designed as a beam entry surface, beam exit surface and / or mirror surface.

3. Method according to one of the preceding claims, characterized in that the bristles (11) are formed as carbon fibers or as polyimide fibers or as glass fibers.

4. Method according to one of the preceding claims, characterized in that the bristles (11) are electrically conductive with a specific electrical resistance of less than 100 pQ*m, in particular between 1 pQ*m and 30 pQ*m, or electrostatically dissipative with a specific resistance between 100 JENOPTI K Optical Systems GmbH LO-24-013-P-WO 14 / 16 pQ*m and 1 MQ*m are trained.

5. Method according to one of the preceding claims, characterized in that the bristles (11) have a diameter between 3pm and 30pm, in particular between 5pm and 15pm, and / or the bristles (11) have a length between 5mm and 30mm, in particular between 10mm and 20mm.

6. Method according to one of the preceding claims, characterized in that the bristles (11) are arranged in a round brush-like manner as a bristle bundle with a diameter between 2 mm and 20 mm, or that the bristles (11) are arranged in a flat brush-like manner as a bristle bundle with a cross-section of 0.5 mm x 3 mm to 3 mm x 30 mm and an aspect ratio between 2 and 10, in particular between 2 and 5.

7. Method according to one of the preceding claims, characterized in that the mechanical vibration is resonant and / or has a fundamental frequency between 100Hz and 1500Hz, in particular between 150Hz and 500Hz.

8. Method according to one of the preceding claims, characterized in that the mechanical vibration, in addition to a fundamental frequency, has frequency multiples in the power spectrum which, on average, in the range between 2kHz and 8kHz, have a peak height of more than -30db relative to the peak of the fundamental frequency.

9. Method according to one of the preceding claims, characterized in that the brushing of the contact needles (4) and the brushing of the at least one optical functional surface (5) are carried out simultaneously.

10. Method according to one of the preceding claims, characterized in that the cleaning of the contact needles (4) and the at least one optical functional surface (5) is carried out dry.

11. Method according to one of the preceding claims, characterized in that the bristles (11) are substantially aligned in the direction of the boom (10) and / or that the mechanical vibration of the bristles (11) is substantially JENOPTI K Optical Systems GmbH LO-24-013-P-WO 15 / 16 is directed transversely.

12. Method according to one of the preceding claims, characterized in that the first and second duration of the brushing is between 0.5 seconds and 10 seconds and / or that the amplitude of the vibration at the clamped ends (12) of the bristles (11) is between 0.1mm and 5mm, in particular between 0.2mm and 2mm.

13. Method according to one of the preceding claims, characterized in that the mechanical vibration is generated in an amplitude-modulated and / or wobbled manner, or that it is generated as white noise or pink noise.

14. Use of an oscillating brush driven by an oscillating drive (9), comprising an oscillating drive (9), a cantilever (10), and a plurality of bristles (11), for cleaning contact needles (4) and optical functional surfaces (5) of test cards (1) for testing optoelectronic chips.

15. Brush arrangement (4) for cleaning contact needles (4) and optical functional surfaces (5) of test cards (1) for testing optoelectronic chips, comprising: • a vibration drive (9), • a boom (10) and • a plurality of bristles (11) which are mechanically connected at one end (12) to the cantilever (10) and each have an opposite free end (13), wherein the bristles (11) are oriented in the direction of the cantilever (10) with an angular deviation of less than 30°.

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