A method and apparatus for evaluating the quality of semiconductor wire bonding joints using laser periodic heating.

Abstract

A method and apparatus for evaluating the quality of wire bonding joints using a laser cyclic heating method is provided, replacing the non-contact, non-destructive instantaneous inspection method for metal joints based on the relationship between phase difference and bonding area. This method is based on the relationship between phase difference and thermal resistance of the bonding area. In this invention, when heating the junction from above by periodic heating laser irradiation and continuously checking the quality of the bonding state of each tiny spherical junction of the semiconductor element supported by the lead frame based on the phase difference between the intensity of the periodic heating laser (which varies with a sinusoidal waveform) and the intensity of the sinusoidal infrared radiation emitted from the heated portion, the thermal resistance of the wire bonding portion is used to evaluate the phase difference.

Description

Technical Field

[0001] This invention relates to a method and apparatus for evaluating the quality of bonding at semiconductor wire bonding sites using a laser periodic heating method. More specifically, it is a method and apparatus for evaluating the quality of bonding at semiconductor wire bonding sites using a laser periodic heating method: after wire bonding using fine-diameter leads in a semiconductor manufacturing process, it is used to check the quality of bonding at micro-spherical bonding (mainly on the semiconductor element side) or wedge bonding (mainly on the external connection terminal, i.e., the lead frame side) in a non-contact and non-destructive manner. Prior Technology

[0002] Semiconductor wire bonding is a technique that connects the connection electrodes on semiconductor devices such as LSIs or transistors to external connection terminals, i.e., lead frames, using gold, copper, or aluminum wires. It is crucial that the two ends of the wires are bonded in a state of low resistance and high mechanical strength. Therefore, after wire bonding, the quality of the wire bonding is inspected. Various non-contact, non-destructive inspection methods have been proposed and implemented for rapid testing.

[0003] For example, Japanese Patent No. 6620499 discloses a non-contact, non-destructive instantaneous inspection method for metal joints. In this method, when one of the tiny metals being joined is periodically heated with varying intensity (sine wave), the heated portion also undergoes a temperature change (temperature response) synchronized with the heating cycle. Furthermore, the phase difference between the heating cycle and the temperature change cycle is related to the area of ​​the joint. Based on this innovation, periodic heating is performed using a laser, and the temperature change is measured using infrared radiation measurement.

[0004] However, the method of this invention is based on the premise that the phase difference between a heating laser with a sinusoidal waveform and an infrared radiation with a sinusoidal waveform is related to the area of ​​the joint. That is, the method calculates the joint area based on the premise that if the phase difference is similar, the joint area will also be similar. So far, the idea that the phase difference is related to the area of ​​the joint has been accepted and is being tested in practice based on this idea. [Previous Technical Documents] [Patent Literature]

[0005] [Patent Document 1] Japanese Patent No. 6620499 Summary of the Invention

[0006] (The problem the invention aims to solve) As described above, the method has been accepted and tested to date, namely, calculating the area of ​​the joint, i.e., the joint area, based on the phase difference between a heating laser exhibiting a sinusoidal waveform and an infrared radiation exhibiting a sinusoidal waveform. However, in order to verify whether the method that has been accepted to date is correct, the inventors have repeatedly conducted various experimental studies, and the results show that the aforementioned phase difference is not necessarily closely related to the joint area, but is closely related to the thermal resistance of the joint, thus completing the present invention.

[0007] That is, the objective of this invention is to provide a method and apparatus for evaluating the quality of the bonding state of semiconductor wire bonding parts using a laser periodic heating method, which replaces the non-contact, non-destructive instantaneous inspection method of metal bonding parts that has become mainstream in the prior art based on the idea that phase difference is related to bonding area, and is based on the idea that phase difference is related to the thermal resistance of the bonding parts. (Technical means to solve the problem)

[0008] One aspect of the present invention, intended to address the aforementioned problems, is a method for evaluating the quality of semiconductor wire bonding joints using a periodic laser heating method. This method utilizes a periodic heating laser to heat the bonding joint from above, and continuously evaluates the bonding quality of the bonding joints of the semiconductor elements supported by the leadframe using the phase difference between the periodic heating laser (which exhibits a sinusoidal waveform) and the sinusoidal infrared radiation emitted from the heated portion. The quality of the bonding state of the bonding area is judged by evaluating the phase difference as the thermal resistance of the semiconductor wire bonding area.

[0009] Another aspect of the present invention for solving the above-mentioned problems is a device for evaluating the quality of wire bonding joints using a laser periodic heating method. This device utilizes a periodic heating laser to heat the bonding joint from above, and continuously evaluates the bonding status of the bonding joints of the micro-spherical or wedge-shaped bonding joints of the semiconductor element supported by the lead frame based on the phase difference between the periodic heating laser (which exhibits a sinusoidal waveform) and the sinusoidal infrared radiation emitted from the heated portion. The device includes: A laser irradiation method that emits the periodically heated laser with an intensity that varies sinusoidally within the heated portion; A laser intensity detection method that detects the intensity of the periodically heated laser on the heated part; An infrared radiation intensity detection method that detects the intensity of the infrared radiation emitted from the heated portion, which exhibits a sinusoidal waveform. A phase difference detection method acquires detection signals from the aforementioned laser intensity detection method and detection signals from the aforementioned infrared radiation intensity detection method, and detects the phase difference between the intensity of the aforementioned periodically heated laser, which exhibits a sinusoidal waveform change, and the intensity of the aforementioned infrared radiation, which also exhibits a sinusoidal waveform change; and The evaluation method uses the phase difference detected by the aforementioned phase difference detection method as the thermal resistance in the semiconductor wire bonding area to evaluate the quality of the bonding state of the bonding area. (Compared to the effectiveness of previous technologies)

[0010] As described above, the method and apparatus for evaluating the bonding status of wire bonding parts using the laser periodic heating method of the present invention are based on the idea that phase difference is related to the thermal resistance of semiconductor wire bonding parts. The measured value of phase difference is regarded as the thermal resistance of the aforementioned semiconductor wire bonding parts for evaluation. Therefore, compared with the prior art method, it has the effect of performing more accurate and reliable inspections that meet the original purpose. Simple Explanation of the Diagram

[0011] Figure 1 is a schematic diagram of the device for evaluating the quality of the bonding state of wire bonding parts using the laser periodic heating method of the present invention. Figure 2 is a diagram illustrating the selection method of the inspection object in the verification test related to the evaluation method of the bonding state of the wire bonding part performed by the laser periodic heating method of the present invention. Figure 3 is a schematic image of the bonding object used in the evaluation method of the present invention for evaluating the quality of bonding of wire bonding parts using laser periodic heating method. Figure 4 is a diagram showing the method for measuring the thickness of the bonding area in a verification test related to the evaluation method for the bonding state of wire bonding using the laser periodic heating method of the present invention. Figure 5 is a graph showing the correlation between the bonding area and the phase value obtained from the verification test results related to the evaluation method of the bonding state of the wire bonding part performed by the laser periodic heating method of the present invention. Figure 6 is a graph showing the correlation between thermal resistance and phase value obtained from the verification test results related to the evaluation method of the bonding state of wire bonding parts using the laser periodic heating method of the present invention. Implementation

[0012] Referring to the accompanying drawings, the configuration for implementing the present invention will be described. The method for evaluating the quality of the bonding state of wire bonding portions using a laser periodic heating method according to the present invention is characterized in that, when heating the bonding portion from above using a periodic heating laser, and continuously checking the quality of the bonding state of each tiny spherical bonding portion of the semiconductor element supported by the lead frame based on the phase difference between the intensity of the periodic heating laser (which exhibits a sinusoidal waveform) and the intensity of the sinusoidal infrared radiation emitted from the heated portion, the phase difference is not considered as the bonding area, but rather as the thermal resistance of the semiconductor wire bonding portion for evaluation.

[0013] The present invention discloses an evaluation device for assessing the bonding condition of wire bonding joints using a laser periodic heating method. This device is used to implement the aforementioned method and includes: a laser irradiation means 1 that emits a heating laser with an intensity that varies sinusoidally at the heated point (measuring point); a laser intensity detection means 2 that detects the intensity of the heating laser in the heated portion; an infrared intensity detection means 3 that detects the intensity of sinusoidally varying infrared radiation emitted from the heated portion; a phase difference detection means (lock-in amplifier) ​​4 that acquires detection signals from the laser intensity detection means 2 and the infrared intensity detection means 3, and detects the phase difference between the intensity of the sinusoidally varying heating laser and the intensity of the sinusoidally varying infrared radiation; and a quality evaluation means 5 that evaluates the phase difference detected by the phase difference detection means 4 as the thermal resistance of the semiconductor wire bonding joint (Figure 1).

[0014] Since the methods of irradiating the micro spherical or wedge-shaped joint with a periodic heating laser exhibiting a sinusoidal waveform by means of laser irradiation 1, the methods of detecting the intensity of the heating laser on the heated part by means of laser intensity detection 2, the methods of measuring the intensity of the sinusoidal infrared radiation emitted from the heated part by means of infrared radiation intensity detection 3, and the methods of measuring the phase difference between the intensity of the periodic heating laser exhibiting a sinusoidal waveform and the intensity of the sinusoidal infrared radiation emitted from the heated part by means of phase difference detection 4 can all be performed according to prior art methods, such as those described in the aforementioned Patent Document 1, detailed descriptions of these methods are omitted.

[0015] Prior art believed that the aforementioned phase difference was closely related to the bonding area, but the inventors proposed that the aforementioned phase difference might be more closely related to the thermal resistance system than to the bonding area. In order to verify this idea, the following verification experiment was conducted.

[0016] The verification test was conducted according to the following key points. 1) Selection of the joining object Using a wire bonding device, the bond strength standard for wire bonding is adjusted to three strength standards: strong, medium, and weak. The 4 wires that have obtained the grade (bond numbers 1, 2, 6, and 10) and the 3 wires that have not obtained the medium and weak grades (bond numbers 5, 8, and 9) out of the 10 wires that have undergone optimal thermal diffusion length analysis are selected as the objects (see Figure 2).

[0017] The so-called optimal thermal diffusion length analysis is based on the thermal diffusivity of the heating element material. The frequency is obtained by measuring the thermal diffusion length from the shortest distance from the laser heating point to the wire bonding joint to the longest distance + α. The phase value is then determined using an appropriate heating power and an optimal thermal diffusion length pitch (e.g., 10 μm pitch). This method can identify the modulation frequency (optimal thermal diffusion length) with the best grading capability. The optimal thermal diffusion length varies depending on the lead diameter or the aluminum electrode construction.

[0018] 2) Evaluation Object The bonding area and alloy layer thickness (bonding interface thickness) of the aluminum electrode at the wire bonding part of the object are measured after etching. The relationship between the effective cross-sectional area and alloy layer thickness (bonding interface thickness) values ​​(thermal resistance) and the measured value of thermal diffusion length (e.g., 100 μm) obtained by optimal thermal diffusion length analysis is evaluated.

[0019] In other words, in order to evaluate the suitability of the measured phase value, the aluminum electrode of the wire bonding part of the object is etched, the effective bonding area of ​​the alloy layer on the back of the bonding is measured by digital microscopy, and the average thickness of the alloy layer (usually about 1 to 3 μm) is measured by 3D photography. The thermal resistance is calculated based on this value, and the suitability of the correlation (coefficient of determination R2) and the grading ability (each standard) is evaluated based on the distribution diagram of the measured phase value and thermal resistance.

[0020] The verification test on the correlation between phase difference and bonding area was conducted under the following measurement conditions. <Measurement Conditions> • Thermal diffusion length: 100 μm (referring to the modulation frequency of periodic heating) Laser power: minimum 1W, maximum 3W (minimum and maximum for sine wave). Measurement time: 5ms (approximately 5-10 cycles)

[0021] Figure 5 shows the correlation curves between the phase values ​​and the bonding area obtained from these results, with a correlation coefficient (coefficient of determination) of -0.88, which is confirmed as an appropriate negative correlation. Furthermore, it was also confirmed that the distribution of each standard is roughly concentrated in the vicinity. In the table, weak bonding systems are represented by triangles, bonding systems within the standards are represented by squares, and strong bonding systems within the standards are represented by circles.

[0022] The standard weak joints, numbered 5, 8, and 9, were presumed to have similar bond areas to the standard joints because their phase values ​​were comparable. However, the measured areas were smaller than 700 μm², compared to the standard joints (approximately 940–1150 μm²), and were comparable to other standard weak joints (see the thick dashed box). Similarly, the standard strong joint, numbered 9, was presumed to have a similar bond area because its phase value was close to the standard joints. However, the measured areas were larger by approximately 500 μm², and were comparable to other standard strong joints. Based on these results, it is presumed that the phase value and bond area are not closely related, but rather related to other factors.

[0023] Next, the thickness of the alloy layer was measured using the method described below, and a verification experiment was conducted using thermal resistance calculation formula 1 to verify the correlation between the phase value and thermal resistance. The measurement conditions were the same as those for the bonding area described above. R=d / Aλ…Equation 1 R: Thermal resistance (K / W) d: Thickness of the alloy layer (m) A: The bonding area of ​​the alloy layers (m²) λ: Thermal conductivity (W / (mK))

[0024] The alloy layer thickness at the bonding area was measured as follows (see Figure 4): After etching the aluminum electrode at the wire bonding area, 3D photography was performed on the back side of the bonding area using a digital microscope. The height was measured at four points (points A, B, C, and D) that were presumed to be the edge of the alloy layer, and at point O near the center. The alloy layer thickness of each bonding area was calculated by averaging the measured values ​​from A to O, B to O, C to O, and D to O.

[0025] Figure 6 shows the correlation curves between the phase value and the thermal resistance obtained from these results, with a correlation coefficient (coefficient of determination) of 0.92, confirming a close positive correlation between the two. As shown in the curves of Figure 6, the standard weak joints 5, 8, and 9, which have small joint areas but high phase values, have thermal resistances and phase values ​​comparable to those of the standard joints. On the other hand, the standard strong joint 9, which has large joint areas but high phase values, results in thermal resistances and phase values ​​close to those of the standard joints.

[0026] The thermal resistance value is calculated using Equation 1 above based on the bonding area and thickness of the alloy layer as measured in the manner described above. The bonding area, alloy layer thickness, and thermal resistance measured for strong, medium, and weak conditions are shown in Tables 1-3.

[0027] As shown in Tables 1-3, when the bonding area is large and the alloy layer is thin, the thermal conductivity is good and the thermal resistance is low; when the alloy layer is thick, the thermal conductivity is poor and the thermal resistance is high. That is, since the thermal resistance (and resistance) is low when the bonding area is large and the alloy layer is thin, the electrical conductivity path intended for this purpose is in a good bonding state (thermal conductivity and electrical conductivity are closely related).

[0028] Based on the curves shown in Figure 6 and Tables 1-3 above, the following conclusions can be drawn (supporting the inferences of this invention). 1) Since a good bonding state is presumed to be characterized by a large bonding area and a thin alloy layer when the thermal resistance is low, the quality of the bonding state can be judged by evaluating the phase difference, which is closely related to the thermal resistance. 2) The condition for a small phase difference is when the bonding area is relatively large and the alloy layer thickness is relatively thin. Ideally, the effective bonding area is larger than the ideal area and the alloy layer thickness is thinner. 3) It does not have a small phase difference (good product) and a relatively small area (even if the alloy layer is thin).

[0029] In the method and apparatus of the present invention, the phase difference detected by the phase difference detection means 4 is regarded as the thermal resistance of the semiconductor wire bonding part in the quality evaluation means 5. However, the quality judgment criteria (the range of phase difference) required for the evaluation is determined by the semiconductor manufacturer and others, and the quality judgment is made according to the prescribed value in the quality evaluation means 5.

[0030] Of course, the purpose of wire bonding is to electrically connect semiconductor components and leadframes, and to achieve a bonding strength resistant to the difference in the coefficients of thermal expansion of the surrounding environment (semiconductor components, leadframes, molding materials, etc.) relative to temperature changes. Therefore, wire bonding is required to ensure excellent electrical conductivity and bonding strength. As an excellent electrical conductivity path, low resistance, low impedance, and sufficient current tolerance are required. However, since all terminals (leadframes) are usually connected together immediately after the wire bonding step, it is impossible to perform resistance (impedance) measurement. But this becomes possible with the method of the present invention.

[0031] In recent years, due to the increasing demand for high reliability (in industrial applications such as automobiles, aerospace, and industrial machinery), it has become necessary to measure the quality of all bonding components. Furthermore, in power semiconductors, there is a requirement to improve the performance of high-efficiency or high-speed switching (not only semiconductor elements or leads, but also the optimization of low resistance or low impedance of bonding components). The wire bonding evaluation method of the present invention, which uses laser periodic heating, can meet the above requirements. (Industrial applicability)

[0032] As described above, the present invention provides a method and apparatus for evaluating the quality of wire bonding joints using laser periodic heating. Based on the idea that phase difference is related to the thermal resistance of the bonding joint, the measured value of phase difference is regarded as the thermal resistance of the wire bonding joint for evaluation. Compared with the prior art, it has the effect of performing more accurate and reliable inspection, and its industrial applicability is extremely high.

[0033] 1: Laser irradiation method 2: Laser intensity testing methods 3: Infrared intensity detection methods 4: Phase difference detection method (lock-in amplifier) 5: Evaluation methods for advantages and disadvantages

Claims

1. A method for evaluating the quality of semiconductor wire bonding joints using a periodic laser heating method, comprising: heating the joint from above using a periodic heating laser; and continuously evaluating the quality of the bonding state of each micro-spherical or wedge-shaped bonding of a semiconductor element supported by a lead frame based on the phase difference between the periodic heating laser (which exhibits a sinusoidal waveform) and the sinusoidal infrared radiation emitted from the heated part; characterized in that the quality of the bonding state of the joint is determined by evaluating the phase difference as the thermal resistance of the semiconductor wire bonding joint.

2. A device for evaluating the quality of semiconductor wire bonding joints using a periodic laser heating method, comprising: a device that uses a periodic heating laser to heat the bonding joint from above; and continuously evaluates the bonding state of each micro-spherical or wedge-shaped bonding joint of a semiconductor element supported by a lead frame based on the phase difference between the periodic heating laser (which exhibits a sinusoidal waveform) and the sinusoidal infrared radiation emitted from the heated portion; characterized in that it comprises: A laser irradiation method that emits a periodically heated laser with an intensity that varies sinusoidally within the heated portion; a laser intensity detection method that detects the intensity of the periodically heated laser in the heated portion; an infrared radiation intensity detection method that detects the intensity of the sinusoidally varying infrared radiation emitted from the heated portion; a phase difference detection method that acquires detection signals from the laser intensity detection method and the infrared radiation intensity detection method, and detects the phase difference between the intensity of the sinusoidally varying periodically heated laser and the intensity of the sinusoidally varying infrared radiation; and a quality evaluation method that evaluates the phase difference detected by the phase difference detection method as thermal resistance in the semiconductor wire bonding joint, thereby determining the quality of the bonding state of the joint.

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