Embedded radio frequency probe of surface acoustic wave force sensor and preparation method of embedded radio frequency probe

Abstract

The invention belongs to the technical field of semiconductor testing, and provides a surface acoustic wave force sensor embedded radio frequency probe and a preparation method thereof.The radio frequency probe comprises a probe base body and a surface acoustic wave force sensing unit, and the preparation method comprises the steps that the probe base body is formed through a UV-LIGA technology; and the insulating layer, the piezoelectric film and the interdigital transducer are sequentially prepared on the probe substrate by adopting a micro-nano processing technology. The radio frequency probe obtained by the preparation method realizes miniaturization and integration. According to the invention, real-time monitoring of micro-Newton level contact force is realized by detecting the frequency change of the surface acoustic wave signal, and meanwhile, the high-frequency signal transmission capability is maintained. The probe supports various surface acoustic wave unit configurations such as a single port and double ports, and a differential measurement system can be constructed. The probe solves the problems of parameter misalignment and chip test damage caused by improper contact force in wafer test, realizes synchronous acquisition of contact force and electrical performance parameters, and remarkably improves test precision and yield.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor testing technology, specifically relating to an embedded radio frequency probe for a surface acoustic wave force sensor and its preparation method. Background Technology

[0002] In semiconductor manufacturing, wafer electrical testing is a critical step. During testing, the metal probe tips of the probe card need to physically contact the pads on the wafer to form an electrical connection. At this point, the contact force applied to the probes is crucial. Insufficient force will result in excessively high or unstable contact resistance, leading to inaccurate test results and misclassifying good products as defective ones. Excessive force can pierce the oxide or aluminum layer on the pad surface, or even create excessively deep indentations on the pads, causing physical damage to the chip, affecting the quality of subsequent packaging and bonding, and even rendering the chip unusable.

[0003] Currently, monitoring of probe contact force mainly relies on offline calibration before probe card installation or indirect inference from electrical parameters, making it impossible to achieve real-time monitoring of the precise force of each probe. Existing technology lacks a solution that can be integrated into a confined space, without affecting electrical testing, and can directly measure micro-Newton level contact forces with high precision. Furthermore, in high-end chip testing (such as RF chips), minute fluctuations in probe contact force directly affect contact resistance and inductance, introducing testing errors. Currently, there is a lack of means to simultaneously and in-situ measure contact force and electrical parameters, making it impossible to establish a correlation model between the two, thus hindering accurate compensation and root cause analysis of test results.

[0004] Traditional force sensors are too bulky to be integrated into the probe body. Therefore, force sensors (such as piezoresistive or capacitive sensors) are usually placed at the bottom base of the probe, indirectly calculating the contact force at the probe tip by sensing the overall stress transmitted through the base. This approach suffers from problems such as long force transmission paths, severe signal attenuation, difficulty in distinguishing the force on a single needle in a multi-needle array, and susceptibility to interference from installation torque, resulting in limited force measurement accuracy and spatial resolution. On the other hand, there is a lack of dedicated equipment for sensitivity testing of the core unit of the force-sensitive probe, and general-purpose testing devices suffer from low alignment accuracy and insufficient frequency resolution, leading to inaccurate chip performance calibration.

[0005] To address the aforementioned issues, existing technologies propose using MEMS processes to fabricate MEMS probes. The probe assembly comprises a probe body and a substrate, which are fixedly connected via anchor points. The probe body is the core movable and sensing unit, specifically including a T-shaped probe head, a force-sensitive mass block, an elastic beam, and a scale structure. Based on the mechanical principle that "the deformation of an elastic beam under stress is proportional to the applied external force," the magnitude of the external force on the sample can be calculated. This method improves the miniaturization manufacturing technology chain of force-sensitive probes from the perspectives of basic structural design and MEMS process compatibility. However, it requires indirect derivation of the force value through displacement measurement, and cannot directly obtain contact force data, thus limiting measurement efficiency and accuracy. Another existing technology proposes a method where three force-sensitive resistor probes are arranged in the X (left / right) and Y (front / back) directions of a U-shaped base, with the probe tips directly contacting the car guide shoes or rails for measurement. When an imbalance occurs during car operation (such as uneven load leading to increased force on one side of the guide shoe), the contact force acts on the probe tip, causing the resistance of the force-sensitive resistor built into the probe to change with the pressure—the greater the force, the more significant the resistance change, thus converting the "mechanical signal" into a "detectable electrical signal." This method senses the contact force through changes in the resistance of the force-sensitive resistor, but it does not consider the interference of coaxial stiffness on minute force signals, cannot distinguish weak force changes, and the multi-probe layout design is difficult to adapt to micro-device testing scenarios in confined spaces, resulting in insufficient compatibility. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides an embedded radio frequency probe for a surface acoustic wave force sensor and its fabrication method, thereby resolving the issues in the prior art. The technical solution adopted by this invention is as follows: An embedded radio frequency probe for a surface acoustic wave force sensor includes a probe base and a radio frequency probe; The probe base includes a signal transmission structure and a magnetic base; the signal transmission structure includes a coaxial cable and an SMA interface; the coaxial cable connects the SMA interface to the RF probe by soldering. The radio frequency probe includes a probe base, which includes a central signal pin and an outer grounding pin. An insulating layer, a piezoelectric film, and an interdigital transducer are disposed on the central signal needle; the interdigital transducer is disposed on the surface of the piezoelectric film, and the piezoelectric film is disposed on the surface of the central signal needle; the piezoelectric film and the interdigital transducer together constitute a surface acoustic wave force-sensitive unit for sensing the strain of the central signal needle caused by contact force; the insulating layer is disposed between the central signal needle and the surface acoustic wave force-sensitive unit. The interdigital transducer is a single-port resonator structure or a two-port resonator structure.

[0007] Furthermore, the central signal needle includes a probe tip and a cantilever beam; the surface acoustic wave force-sensitive unit is disposed on the cantilever beam.

[0008] Furthermore, the piezoelectric film is aluminum nitride or zinc oxide, providing stable piezoelectric properties.

[0009] Furthermore, the insulating layer is made of silicon nitride or silicon dioxide to prevent electrical short circuits and external interference.

[0010] Furthermore, two SMA interfaces are provided; one SMA interface is used to connect the surface acoustic wave force-sensitive unit and the first test port of the vector network analyzer; the other SMA interface is used to connect the probe substrate and the second test port of the vector network analyzer.

[0011] Furthermore, both the central signal pin and the outer grounding pin are provided with surface acoustic wave (SAW) force-sensitive units; there are at least two SAW force-sensitive units, which are used to generate response signals. By comparing the changes in multiple response signals, differential calculations are performed to eliminate common-mode interference and output the final measurement result.

[0012] A method for fabricating an embedded radio frequency probe for a surface acoustic wave force sensor includes the following steps: A conductive substrate is provided, which serves as a temporary support layer for the probe substrate and is eventually removed by chemical etching. A probe substrate is formed on a conductive substrate using a UV-LIGA process, which includes spin coating, exposure, development, electroforming, and demolding. An insulating layer and a piezoelectric thin film are fabricated on the probe substrate using a surface processing technology, namely sputtering. The patterned structure of the interdigital transducer is formed on the piezoelectric thin film using micro-nano fabrication processes, including spin coating, exposure, development, sputtering, resist removal, and chemical etching for stripping. The center signal needle is obtained.

[0013] The present invention has the following beneficial effects: (1) This invention develops a surface acoustic wave (SAW) radio frequency probe capable of real-time monitoring of probe contact force and achieves miniaturization of the radio frequency probe. The high sensitivity of the SAW force-sensitive unit to strain makes accurate measurement of micro-Newton level contact force possible. The introduction of the insulating layer achieves electrical isolation between the force sensing path and the electrical testing path, ensuring the normal operation of both functions and signal integrity.

[0014] (2) Function of the radio frequency probe of the present invention: When the probe is working, it can perform the traditional electrical signal transmission function and simultaneously sense the contact force, realizing the integration and multiplexing of functions. By optimizing the surface acoustic wave resonator design, it has higher force sensitivity at the working frequency of 78.9MHz, while maintaining excellent high-frequency signal transmission characteristics.

[0015] (3) The probe of this invention has good electroacoustic conversion efficiency and stable signal transmission. Through the design of the insulating layer and the protective layer, signal crosstalk and external interference are effectively suppressed. The combination of aluminum nitride piezoelectric film and nickel-based probe body has both good mechanical properties and electrical characteristics.

[0016] (4) The device of this invention, when connected to a vector network analyzer, can simultaneously acquire force signals and electrical performance signals (such as S-parameters), providing direct data support for analyzing the influence of contact force on test results and establishing an error compensation model. This RF probe can significantly improve the accuracy and reliability of wafer testing. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the overall radio frequency probe holder of the present invention.

[0018] Figure 2 This is a magnified view of the surface acoustic wave force-sensitive unit of the present invention on a probe. Among them, (a) is a schematic diagram of a single-port resonator structure, and (b) is a schematic diagram of a two-port resonator structure.

[0019] Figure 3 This is a schematic diagram of the process flow for the probe preparation method of the present invention.

[0020] Figure 4 This is a schematic diagram of the process flow for fabricating the surface acoustic wave force-sensitive unit of the present invention.

[0021] Figure 5 This is a schematic diagram of the operation of the radio frequency probe of the present invention in a force-electric synchronous detection system.

[0022] Figure 6 This is a schematic diagram of the radio frequency probe based on differential measurement according to the present invention. Detailed Implementation

[0023] The following will be based on embodiments of the present invention. Figures 1-6 The technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art.

[0024] like Figure 1 , Figure 2 An embedded radio frequency probe for a surface acoustic wave force sensor includes a probe base 1 and a radio frequency probe 2. The probe base 1 includes a signal transmission structure and a magnetic base 13; the signal transmission structure includes a coaxial cable 11 and an SMA interface 12; the coaxial cable 11 connects the SMA interface 12 to the radio frequency probe 2 by brazing. The radio frequency probe 2 includes a probe substrate, which includes a central signal pin 21 and an outer grounding pin 22. The probe substrate is made of nickel and is formed by UV-LIGA process, which has excellent elasticity and conductivity.

[0025] An insulating layer 23, a piezoelectric film 24, and an interdigital transducer 25 are disposed on the central signal needle 21; the interdigital transducer 25 is disposed on the surface of the piezoelectric film 24, and the piezoelectric film 24 is disposed on the surface of the central signal needle 21; the piezoelectric film 24 and the interdigital transducer 25 together constitute a surface acoustic wave force-sensitive unit for sensing the strain of the central signal needle 21 caused by contact force; the insulating layer 23 is disposed between the central signal needle 21 and the surface acoustic wave force-sensitive unit. The interdigital transducer 25 is a single-port or dual-port resonator structure, fabricated on the surface of the piezoelectric thin film 24 using micro-nano fabrication technology, combining miniaturization and high sensitivity. The single-port resonator structure consists of an interdigital transducer (IDT) and symmetrical reflective gratings at both ends. The IDTs share a set of electrodes and have no independent input or output terminals, resulting in a simple structure that is easy to integrate. It uses resonant frequency shift to feedback the strain caused by contact force. The dual-port resonator structure contains independent input and output IDTs, arranged along the surface acoustic wave propagation direction. Signal excitation and reception are separated, resulting in sensitivity to corresponding changes. This manifests as a linear change in signal phase, facilitating complex signal processing and adapting to high-precision applications such as differential measurements.

[0026] Furthermore, the central signal needle 21 includes a probe tip and a cantilever beam; the surface acoustic wave force-sensitive unit is disposed on the cantilever beam.

[0027] Furthermore, the piezoelectric film 24 can be a piezoelectric material such as aluminum nitride, with a thickness of 300-800 nm, thereby stabilizing the performance of the piezoelectric film.

[0028] Furthermore, the insulating layer 23 can be an insulating material such as silicon nitride, deposited by a sputtering process to prevent electrical short circuits and external interference.

[0029] like Figure 1 , Figure 5 Two SMA interfaces 12 are provided; one SMA interface 12 is used to connect the surface acoustic wave force-sensitive unit and the first test port of the vector network analyzer 3; the other SMA interface 12 is used to connect the probe substrate and the second test port of the vector network analyzer 3.

[0030] During operation, the prepared RF probe 2 is installed at the designated position on the probe card, making physical contact between the contact tip and the wafer pad. When the probe is compressed, it generates micro-strain, which is transmitted to the surface acoustic wave force-sensitive unit, causing a change in the surface acoustic wave propagation characteristics, manifested as a shift in the resonant frequency. The vector network analyzer can quickly switch between the measurement modes of Port1 (force signal) and Port2 (electrical signal), or achieve synchronous measurement through a switch matrix. By monitoring this frequency shift through the vector network analyzer 3, the magnitude of the contact force can be calculated in real time, and high-frequency testing of the S-parameters of the chip under test can be completed simultaneously.

[0031] The surface acoustic wave (SAW) force-sensitive unit can adopt a two-port SAW resonator configuration. Its input interdigital transducer receives the radio frequency excitation signal and generates a SAW wave. After propagating on the surface of the piezoelectric film 24, the SAW wave is received by the output interdigital transducer 25 and converted into an electrical signal output. This structure is more sensitive to strain caused by contact force, especially exhibiting a linear change in signal phase.

[0032] like Figure 1 , Figure 6 The center signal pin 21 and the outer grounding pin 22 are both provided with surface acoustic wave force-sensitive units; there are at least two surface acoustic wave force-sensitive units, which are used to generate response signals, perform differential calculations by comparing the changes of multiple response signals, eliminate common-mode interference, and output the final measurement result.

[0033] Figure 6 This is a schematic diagram of the radio frequency probe based on differential measurement of the present invention, showing a structure in which surface acoustic wave force-sensitive units are integrated on both the central signal pin 21 and the outer grounding pin 22. Specifically, single-port resonators with identical performance can be integrated on the central signal pin 21 and the outer grounding pin 22 respectively. During operation, the resonant frequencies of the two resonators are alternately scanned by a vector network analyzer 3 through a subsequent signal processing device, and the difference in their frequency offset (e.g., Δf) is calculated. diff =Δf signal -Δf ground The difference value Δf diff It exhibits a clear linear relationship with the total contact force on the probe and is insensitive to environmental interference such as temperature. It can effectively deduct common drift caused by environmental factors, eliminate common-mode interference, and achieve high-precision differential force measurement.

[0034] The preparation method of this invention is mainly achieved through the following two core process stages. The first stage is the micromachining of the probe substrate, i.e., using the UV-LIGA process (see...). Figure 3 The first stage involves electroforming a nickel probe substrate onto a silicon substrate; the second stage involves integrating the on-needle surface acoustic wave sensor, i.e., through micro / nano fabrication processes (see...). Figure 4An insulating layer 23 and a piezoelectric film 24 are sequentially deposited in the elastic beam region of the probe substrate, and an interdigital transducer 25 is patterned to form a surface acoustic wave force-sensitive unit.

[0035] A method for fabricating an embedded radio frequency probe for a surface acoustic wave force sensor includes the following steps: Step 1: Provide a conductive substrate, which serves as a temporary support layer for the probe substrate and is eventually removed by chemical etching; Step 2: Form a probe substrate on a conductive substrate using a UV-LIGA process, which includes spin coating, exposure, development, electroforming, and demolding; to obtain a probe substrate without surface acoustic wave force-sensitive units.

[0036] Step 3: An insulating layer 23 and a piezoelectric thin film 24 are fabricated on the probe substrate by a surface processing technology, wherein the surface processing technology is a sputtering process. Step 4: The patterned structure of the interdigital transducer 25 is formed on the piezoelectric thin film 24 by micro-nano processing technology, which includes spin coating, exposure, development, sputtering, resist removal, and chemical etching process for peeling. Step 5, obtain the center signal needle 21.

[0037] like Figure 3 Specifically, step 2 of the UV-LIGA process includes: (1) Provide a substrate as a base material; (2) A conductive thin film is deposited on a substrate by sputtering; (3) Photoresist is spin-coated onto the conductive film using a spin coater; (4) Photoresist is exposed by photolithography; (5) A photoresist mold is formed by immersing the mold in a developing solution; (6) Deposit metallic nickel in a photoresist mold using an electroforming process; (7) Remove the photoresist by soaking in an acetone solution; (8) Release the probe’s three-dimensional structure through chemical etching process.

[0038] like Figure 4 Specifically, the micro-nano fabrication process in step 4 includes: (1) Pretreatment of the probe substrate; (2) An insulating layer is deposited on the probe substrate by sputtering; (3) A piezoelectric thin film is deposited on the insulating layer by sputtering; (4) Photoresist is spin-coated onto the piezoelectric thin film using a spin coater; (5) Photoresist is exposed by photolithography; (6) A photoresist mold is formed by immersing the mold in a developing solution; (7) Depositing a thin film of metallic aluminum by sputtering; (8) The photoresist is removed by soaking in acetone solution to obtain the radio frequency probe.

[0039] This invention integrates a surface acoustic wave (SAW) force-sensitive unit with a probe body using micro-nano fabrication technology. Both the SAW device output signal and the RF probe test signal are frequency response signals, maintaining the electrical signal transmission function of the RF probe while achieving real-time force sensing. Experiments show that the force measurement accuracy of this RF probe reaches the micro-Newton level, effectively distinguishing between test errors caused by poor contact and chip malfunctions. Compared to traditional solutions, this invention provides a complete set of technical solutions ranging from basic to advanced. Specifically, from the basic configuration of integrating a single-port or dual-port sensor into a single probe to constructing a differential measurement system on the center signal pin and ground pin, this invention covers the needs of various application scenarios, providing diverse solutions for precise force monitoring. This invention significantly improves the accuracy and yield of chip testing, offering advantages such as small size, high integration, good compatibility, high reliability, and high sensitivity. It is particularly suitable for high-end chip testing and wafer testing fields sensitive to contact conditions.

[0040] The above embodiments are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Any modifications, alterations, alterations, or substitutions made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. An embedded radio frequency probe for a surface acoustic wave force sensor, characterized in that, Includes a probe base (1) and an RF probe (2); The probe base (1) includes a signal transmission structure and a magnetic base (13); the signal transmission structure includes a coaxial cable (11) and an SMA interface (12); the coaxial cable (11) connects the SMA interface (12) to the radio frequency probe (2) by soldering. The radio frequency probe (2) includes a probe base, which includes a central signal pin (21) and an outer grounding pin (22). An insulating layer (23), a piezoelectric film (24), and an interdigital transducer (25) are disposed on the central signal needle (21); the interdigital transducer (25) is disposed on the surface of the piezoelectric film (24), and the piezoelectric film (24) is disposed on the surface of the central signal needle (21); the piezoelectric film (24) and the interdigital transducer (25) together constitute a surface acoustic wave force-sensitive unit for sensing the strain of the central signal needle (21) caused by contact force; the insulating layer (23) is disposed between the central signal needle (21) and the surface acoustic wave force-sensitive unit. The interdigital transducer (25) is a single-port resonator structure or a two-port resonator structure.

2. The embedded radio frequency probe for a surface acoustic wave force sensor according to claim 1, characterized in that, The central signal needle (21) includes a probe tip and a cantilever beam; the surface acoustic wave force-sensitive unit is disposed on the cantilever beam.

3. The embedded radio frequency probe for a surface acoustic wave force sensor according to claim 1, characterized in that, The piezoelectric film (24) is aluminum nitride or zinc oxide.

4. The embedded radio frequency probe for a surface acoustic wave force sensor according to claim 1, characterized in that, The insulating layer (23) is silicon nitride or silicon dioxide.

5. An embedded radio frequency probe for a surface acoustic wave force sensor according to claim 1, characterized in that, Two SMA interfaces (12) are provided; one SMA interface (12) is used to connect the surface acoustic wave force-sensitive unit and the first test port of the vector network analyzer (3); the other SMA interface (12) is used to connect the probe substrate and the second test port of the vector network analyzer (3).

6. The embedded radio frequency probe for a surface acoustic wave force sensor according to claim 1, characterized in that, The center signal pin (21) and the outer grounding pin (22) are both provided with surface acoustic wave force-sensitive units; there are at least two surface acoustic wave force-sensitive units, which are used to generate response signals, perform differential calculations by comparing the changes of multiple response signals, eliminate common-mode interference, and output the final measurement result.

7. A method for fabricating an embedded radio frequency probe for a surface acoustic wave (SAW) force sensor, used to fabricate the embedded radio frequency probe for a SAW force sensor as described in any one of claims 1-6, characterized in that, Includes the following steps: A conductive substrate is provided, which serves as a temporary support layer for the probe substrate and is eventually removed by chemical etching. A probe substrate is formed on a conductive substrate using a UV-LIGA process, which includes spin coating, exposure, development, electroforming, and demolding. An insulating layer (23) and a piezoelectric thin film (24) are fabricated on the probe substrate by a surface processing technology, wherein the surface processing technology is a sputtering process; The patterned structure of the interdigital transducer (25) is formed on the piezoelectric thin film (24) by micro-nano processing technology, the micro-nano processing technology including spin coating, exposure, development, sputtering, resist removal, and chemical etching process stripping. The center signal needle (21) is obtained.

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