JP7468402B2 - How to make a sample - Google Patents

Description

This disclosure relates to a method for creating samples prior to analysis of minute observation points on a resin package.

In recent years, the use of resin packages in semiconductor products has rapidly increased due to demands for lower costs. However, resin packages can cause problems such as interface peeling due to corrosion from impurities, stress during formation, and heat generation during operation. When device degradation occurs, unless it can be determined whether the degradation is due to the interface or the semiconductor itself, the reliability of the entire device, including the resin material, cannot be improved. Interface-related degradation is limited to a maximum of a few microns at the interface between the resin material and the semiconductor. Such minute areas of degradation must be analyzed using SEM, TEM, STEM, etc.

The typical method for analyzing resin-packaged semiconductor devices involves removing the resin by dissolving it in a chemical solution or ashing it with a laser, and then opening the package. However, the interface between the resin material to be analyzed and the semiconductor chip is affected by opening the package, and in the worst case scenario, disappears.

There is also a method in which processing is performed using only a FIB (Focused Ion Beam) processing device, and degradation analysis is performed using an SEM or similar device. However, because minute observation points such as deteriorated areas exist deep within the resin material, FIB processing must be performed over a wide area and deep into the device over a long period of time. Furthermore, when performing TEM or STEM analysis, a thin sample must be prepared from a deep position in the device, making analysis virtually impossible. To solve these problems, for example, there is a method in which mechanical polishing is performed from the back of the semiconductor chip, the chip is thinned, and then a cross section of the observation point is produced using FIB processing, ion milling, or the like, and observed using an SEM or similar device (see, for example, Patent Document 1).

JP 2009-288029 A

However, the stress caused by mechanical polishing affects the interface between the resin material and the semiconductor chip. Therefore, there is a risk that the semiconductor will detach from the resin material. In addition, since the resin material is opaque to prevent semiconductor deterioration due to external light, the chip thickness cannot be measured, and there is a risk of over-polishing. Furthermore, when the resin interface at the observation point in the device is finally analyzed using TEM, STEM, etc., the observation point must be cut into a slice with a thickness of tens to hundreds of nm using an FIB processing device. If the length and width of the slice is too large, damage will occur due to bending, so the limit is cutting to 20 μm or less in length and width. In order to cut out a cross section including an interface part with a thickness of several μm in length and width to 20 μm or less, the thickness of the chip substrate part must be thinned to about 10 μm, but it is very difficult to process with an accuracy of about 10 μm using mechanical polishing.

There is also a method in which the observation location is identified by optical observation or X-ray transmission observation, and a cross section of the observation location is cut out by irradiating Ar ions while the observation location is half covered with a shielding plate. This method is suitable for interface observation because the stress on the semiconductor chip is very small. However, it is difficult to identify and process the location of micro-leaks of about a few micrometers that cannot be observed by optical observation or X-ray transmission observation. There is also a method in which the observation location is searched for by digging through a wide area using ion milling, but there is a risk that the observation location will disappear during the search process.

As described above, conventional sample preparation methods were not suitable for analyzing minute observation points measuring up to a few microns at the interface between the resin material and the semiconductor chip.

This disclosure has been made to solve the problems described above, and its purpose is to provide a sample creation method that can be applied to the analysis of minute observation points at the interface between a resin material and a semiconductor chip.

The sample preparation method according to the present disclosure is a method for preparing a sample for analyzing an observation location at the interface between the resin material of a resin package and the upper surface of a semiconductor chip, and is characterized by comprising the steps of: attaching a shielding plate to the lower surface of the resin package, which is in close contact with the lower surface of the semiconductor chip exposed from the resin material and covers the observation location in a plan view but does not cover the periphery of the observation location; irradiating an ion beam to the lower surface of the resin package to which the shielding plate is attached, thereby exposing the side surface of the semiconductor chip from the resin material; and removing the shielding plate, focusing an optical length measuring device on the exposed side surface of the semiconductor chip to measure the thickness of the semiconductor chip while irradiating the ion beam to thin the semiconductor chip.

In the present disclosure, the observation area is covered with a shielding plate and the side of the semiconductor chip is exposed by ion milling, and the semiconductor chip is thinned to the desired thickness while the thickness is measured with an optical length measuring device. This allows the semiconductor chip to be thinned with good controllability from the observation area being the sample surface. In addition, because the semiconductor chip is thinned by ion milling, which has a small processing stress, the effect of stress on the observation area at the interface between the resin material of the resin package and the top surface of the semiconductor chip can be suppressed. Therefore, the sample creation method according to the present disclosure can be applied to the analysis of a minute observation area at the interface between the resin material and the semiconductor chip.

1 is a cross-sectional view showing a resin package that is the subject of degradation analysis; 1 is a flowchart of a sample preparation method according to an embodiment. 1A to 1C are cross-sectional views showing a sample preparation method according to an embodiment. 1A to 1C are cross-sectional views showing a sample preparation method according to an embodiment. 1A to 1C are cross-sectional views showing a sample preparation method according to an embodiment. 1A to 1C are cross-sectional views showing a sample preparation method according to an embodiment. 1A to 1C are cross-sectional views showing a sample preparation method according to an embodiment. FIG. 1 is a perspective view showing a resin package in which a semiconductor chip is thinned. 1 is a side view showing a resin package in which a semiconductor chip is thinned; 1 is a bottom view showing a resin package in which a semiconductor chip is thinned. FIG. 13 is a diagram showing the results of observing the prepared sample with an STEM.

Figure 1 is a cross-sectional view of a resin package that is the subject of degradation analysis. A semiconductor chip 1 is covered with a resin material 2 to form a resin package 3. The semiconductor chip 1 is, for example, a GaN-HEMT (High Electron Mobility Transistor) chip. A metal plate 4 is provided on the underside of the semiconductor chip 1. The metal plate 4 is a metal frame or a backside metal, etc. At the underside of the resin package 3, the metal plate 4 is exposed from the resin material 2.

The semiconductor chip 1 has wiring and electrodes 5 and an insulating film 6 on its top surface. At the interface between the resin material 2 of the resin package 3 and the top surface of the semiconductor chip 1, there are minute observation points 7, such as deteriorated areas. The observation points 7 exist in a range of up to several μm.

In this embodiment, the resin package is processed to create a sample for analyzing the observation point 7 at the interface between the resin material 2 of the resin package 3 and the top surface of the semiconductor chip 1 using SEM, TEM, or STEM. Figure 2 is a flowchart of the sample creation method according to the embodiment. Figures 3 to 7 are cross-sectional views showing the sample creation method according to the embodiment.

First, as shown in FIG. 3, the metal plate 4 is removed by mechanical polishing or the like to expose the underside of the semiconductor chip 1. Note that a resin package in which the metal plate 4 is not present and the underside of the semiconductor chip 1 is exposed may be used, in which case the step of removing the metal plate 4 can be omitted.

Next, as shown in FIG. 4, the shielding plate 8 is attached to the bottom surface of the resin package 3 and is brought into close contact with the bottom surface of the semiconductor chip 1 exposed from the resin material 2 (step S1). Here, the shielding plate 8 is arranged so as to cover the observation point 7 but not to cover the periphery of the observation point 7 in a plan view seen from a direction perpendicular to the bottom surface of the semiconductor chip 1. The end of the shielding plate 8 is arranged at one boundary between the resin material 2 and the side surface of the semiconductor chip 1, but may be arranged inside the semiconductor chip 1 from the boundary. For example, when the shielding plate 8 is attached so that at least one side surface of the rectangular semiconductor chip 1 is exposed, it is preferable that the shielding plate 8 is rectangular. If it is necessary to identify the observation point 7, a known identification method such as optical observation, OBIRCH (Optical Beam Induced Resistance CHange), light emission analysis, or heat generation analysis is used.

Next, the resin package 3 is placed in an ion milling device. The ion milling device has an optical length measuring device 9 for measuring the thickness of the semiconductor chip 1, and a transparent body 10 for protecting the optical length measuring device 9 from debris caused by ion milling. An ion beam 11 is irradiated onto the underside of the resin package 3 to which the shielding plate 8 is attached, ion milling the resin material 2, and exposing the side of the semiconductor chip 1 from the resin material 2 (step S2). This makes it possible to measure the chip thickness from the side of the semiconductor chip 1. As shown in FIG. 5, the side of the semiconductor chip 1 is observed (step S3), and if the chip thickness can be measured from the chip side, this process is completed (step S4). To observe the chip side, the chip is removed from the ion milling device and optical observation or SEM observation, or the optical length measuring device 9 is used.

Note that it is not necessary for the entire side of the semiconductor chip 1 to be exposed, as long as the chip thickness can be measured from the side of the chip. Furthermore, it is not necessary for the area not shielded by the shielding plate 8 to be completely removed by the ion beam 11. The acceleration voltage and processing time of the ion beam 11 are adjusted appropriately according to the material to be processed. The observation point 7 is not ion milled because it is covered by the shielding plate 8.

Next, as shown in FIG. 6, the shielding plate 8 is removed, and the underside of the resin package 3 is irradiated with an ion beam 11 to thin the semiconductor chip 1 (step S5). At this time, the optical length measuring device 9 is focused on the exposed side of the semiconductor chip 1 to measure the thickness of the semiconductor chip 1 while performing ion milling. The acceleration voltage of the ion beam 11 and the center position of the irradiated beam are adjusted appropriately according to the shape and material of the semiconductor chip 1 to be observed.

As shown in FIG. 7, the thickness of the semiconductor chip 1 is measured (step S6), and the ion beam 11 is stopped when the desired thickness is reached (step S7). Note that when performing TEM or STEM analysis, it is necessary to prepare a thin sample, so the semiconductor chip 1 needs to be thinned to a thickness of approximately 10 μm.

The optical length measuring device 9 is a device that uses an electronic imaging element such as an objective lens, an eyepiece lens, or a CCD sensor. In this case, the resolution of the optical length measuring device 9 depends not only on the lens magnification but also on the number of pixels and pixel size of the imaging element. Since it is necessary to determine whether the desired thickness has been reached in the process of thinning the chip, the optical length measuring device 9 needs to have sufficient resolution when focusing on the side of the semiconductor chip 1. When observing the processed sample with a TEM, it is necessary to create a thin slice sample with a length and width of 10 μm. If the final chip thickness is too thick, there are problems such as the thin slice sample used for TEM observation bending under its own weight and being damaged during handling. Therefore, it is necessary to recognize a step of several μm when observing the side of the chip. For this reason, the optical length measuring device 9 needs to have a resolution of several μm.

Figure 8 is a perspective view showing a resin package in which a semiconductor chip has been thinned. Figure 9 is a side view showing a resin package in which a semiconductor chip has been thinned. Figure 10 is a bottom view showing a resin package in which a semiconductor chip has been thinned. FIB processing or the like is performed from the bottom surface of the thinned semiconductor chip 1 to cross-process the observation area 7 along I-II in Figure 8. Further thin-section processing can be performed to create a sample for analyzing the observation area.

Figure 11 shows the results of observing the prepared sample with an STEM. The observation point 7 was identified by OBIRCH. It can be seen that the interface between the resin material 2 and the semiconductor chip 1 can be observed by both the SEM and STEM methods.

Here, the method of preparing a sample for analyzing a minute observation point of up to several μm in size at the interface between the resin material 2 and the semiconductor chip 1 must satisfy the following three requirements.
(1) The effects of stress on the interface between the resin material and the semiconductor chip must be minimized.
(2) It is possible to perform controlled processing in the thickness direction so that minute observation points such as deteriorated areas are located at a maximum of about 10 μm from the sample surface.
(3) It is possible to prepare thin specimens for cross-sectional analysis and TEM and STEM analysis with a spatial resolution of about several micrometers.

In contrast, in this embodiment, the observation point 7 is covered with a shielding plate 8, ion milling is performed to expose the side of the semiconductor chip 1, and the semiconductor chip 1 is thinned to the desired thickness while measuring the thickness with an optical length measuring device 9. This allows the semiconductor chip 1 to be thinned with good control so that the observation point 7 is located at a maximum of about 10 μm from the sample surface. In addition, since the semiconductor chip 1 is thinned by ion milling, which has a small processing stress, the effect of stress on the observation point 7 at the interface between the resin material 2 of the resin package 3 and the upper surface of the semiconductor chip 1 can be suppressed. Then, FIB processing or the like is performed from the lower surface of the thinned semiconductor chip 1 to cross-process the observation point 7. This makes it possible to create a thin slice sample for cross-sectional analysis and TEM and STEM analysis with a spatial resolution of about several μm.

Therefore, the sample creation method according to this embodiment can be applied to the analysis of minute observation points at the interface between the resin material and the semiconductor chip.

1 Semiconductor chip, 2 Resin material, 3 Resin package, 7 Observation point, 8 Shielding plate, 9 Optical length measuring device, 11 Ion beam

Claims (2)

1. A method for preparing a sample for analyzing an observation location at an interface between a resin material of a resin package and an upper surface of a semiconductor chip, comprising:
a step of attaching a shielding plate to a lower surface of the resin package, the shielding plate being in close contact with a lower surface of the semiconductor chip exposed from the resin material, and covering the observation area in a plan view but not covering a periphery of the observation area;
a step of irradiating an ion beam onto a lower surface of the resin package to which the shielding plate is attached, thereby exposing a side surface of the semiconductor chip from the resin material;
and removing the shielding plate, and measuring the thickness of the exposed side of the semiconductor chip by focusing an optical length measuring device on the side of the semiconductor chip and irradiating the ion beam while thinning the semiconductor chip. The sample creation method according to claim 1, further comprising a step of cross-machining the observation portion of the thinned semiconductor chip.

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