The successful sales of Samsung’s Galaxy Z foldable phone series in 2021 amazed the mobile phone market. This trend drives the continuing investment in the foldable phone market. Though the price is certainly one consideration, consumers are even more concerned with the durability and functionality of foldable phones.
To better understand the hinge design of foldable phones, MA-tek has dismantled these devices and conducted further analysis and testing. Now let us guide you through an in-depth discussion of the key components and their advanced process trends.
Unlike standard, single-screen mobile phones, the most notable features of the foldable phone are the hinge and the flexible panels in the folding area. |
Firstly, perform a scan with the nondestructive 3D X-Ray to see how the internal components are assembled and the state of the joint during operation. Then disassemble the foldable phone and take out the key components for further analysis. For instance, you can use a Scanning Electron Microscope (SEM) to observe the finer structures of the foldable panel as well as the thickness of each layer inside. You can then use Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) to obtain the elemental composition of the OLED and TFT layers.
In addition, the mobile phone lenses, internal CMOS image sensors and latest NAND flash memory used in flagship models are also worth a closer look. The CMOS structure of the camera lens can be observed using SEM, which can determine the positional relationship between the pixel layer and the logic IC (integrated Circuits) layer. Then there is the cross-sectional structure of the NAND flash memory. Its sample preparation can be performed using a Focused Ion Beam Microscope (FIB). Combining that with the Transmission Electron Microscope (TEM), with its atomic-level resolution, and Energy Dispersive X-ray Spectroscopy (EDS) enables the clear identification of nano-scale NAND string multi-layer films. Thus, it is possible to complete the necessary structure and composition distribution analysis (EDS Mapping).
Hinge Analysis |
Outwardly, the most obvious difference between a folding-screen phone and a single-screen phone is the screen’s folding function. |
The key component here is the hinge used to support the connection between the mobile phone and the screen during the opening and closing process. The internal portion of the hinge is composed of multiple precision components with a complicated assembly. This is the reason that the cost of hinges remains high. By 2021, several mobile phone manufacturers were already in the process of applying for patents on installing the lens module to the hinge structure, to enlarge the external screen display. Clearly, the hinge design is one important area of research and development for foldable mobile phones.
We know that X-rays can penetrate most substances, so X-ray inspection is a nondestructive way to explore the internal structure of objects. MA-tek’s 3D X-ray technology is based mainly on obtaining two-dimensional X-ray images of a sample from multiple angles by rotating said sample. These two-dimensional images are then combined according to computer calculations to form three-dimensional X-ray Tomography images. Therefore, the 3D X-ray is undoubtedly the most suitable tool for studying the joint structure inside the hinge of the folding mobile phone.
Figure 1 is a 3D X-ray image of the hinge of a foldable mobile phone. The three-dimensional structure of the metal hinge is clearly visible in the image. Note that the three-dimensional image can be rotated in any direction during the 3D X-ray scanning process so that it can be observed from various perspectives. In addition, you can use the software’s cross-section observation function to view hinge cross-sections at different positions. In other words, you can randomly select a position on the three-dimensional image (in the example in Figure 1, three planes, A, B, and C, were chosen), and the software can display cross-sectional images of the selected positions. This makes it easy to observe the relative positions between the joint of the hinge and the flexible panel and conduct failure analysis without destroying the sample.
Figure 1. 3D X-ray of the Hinge of a Foldable Phone |
Analysis of the Foldable Panel |
Another key component of the foldable phone is the foldable panel, in which the flexible material undoubtedly plays a key role. |
The folding-screen mobile phone adopts a soft, flexible AMOLED (active-matrix organic light-emitting diode) screen. As opposed to an LCD screen, which requires a backlight, AMOLED is thin and light and has low energy-consumption, vivid colors and a faster screen response. The structure of AMOLED (Figure 2) mainly consists of an OLED layer, a TFT layer and a flexible substrate layer. The OLED layer can be further divided into a cathode layer, an organic light-emitting layer and an anode layer. |
Figure 2. AMOLED Structure Diagram |
Every layer of the foldable panel must be flexible. At present, the mainstream flexible substrate layer in the market uses the flexible, transparent Polyimide (PI) film, which has high light transmittance. However, folding can cause the organic film layer to separate or become brittle and crack. Therefore, it is essential to prevent moisture and oxygen from invading the interior of the panel and causing the deterioration of organic materials, leading to display failures. As such, multi-layer thin film encapsulation technology is another focus of development for foldable phones.
Figure 3 is an SEM image of a cross-section of the folding panel taken after sample preparation. It can be clearly seen in Figure 3(a) that the OLED layer is located above the TFT array and has a thickness of less than 50um. In addition, the TFT array is comprised of multiple repeated electrode units. These are used to control the glow of the pixels. The length of each electrode unit is measured at 60um, which is also the size of each pixel. Figure 3(b) is a magnified SEM image of one of these electrode units.
Figure 3. (a) AMOLED Cross-Section SEM Image and (b) SEM Image of the TFT Array |
Figure 4. TOF-SIMS Ion Image of the TFT Layer |
As for the plane arrangement of the TFT array layer, it can be observed via the Ti+, In+ secondary ion images of TOF-SIMS (Figure 4). TOF-SIMS ion imaging of different layer structures and their characteristics can be applied to the research and analysis of AMOLED and organic and inorganic materials as well as contaminants and process defects. |
Chip Analysis - CMOS |
The pursuit of high-resolution mobile CIS (CMOS Image Sensors) and the current trend in small pixels have made backlit, stacked structures the mainstream in the market. |
The Galaxy Z Flip3 has three main lenses which the two rear cameras and a front camera. The main camera, the wide-angle camera and the front camera use the SONY IMX 563, the IMX 258, and the IMX 374 respectively. Figure 5 is the SEM image of the cross-section of the image sensor after the Micro Lens is removed. It can be seen that the pixel layer and the logic IC layer are bonded together top to bottom. The pixel layer contains photodiodes (PD) and wiring. From the Back Side Illuminated (BSI) CIS and stacked CIS structures shown in the SEM images, it can be seen that the photodiodes and pixel transistors are on the same layer. |
Figure 5. CIS Cross-Section SEM Image |
Process improvements on the pixel and logic IC layers continue to advance the development of CIS technology. Recently, the industry has developed a stacked CMOS image sensor with twice the photosensitivity of the traditional sensor. It is a two-layer transistor pixel structure. Its design places the photodiodes and pixel transistors on separate layers. This design is a breakthrough for the hardware of this type of lens.
Chip Analysis - Flash |
In addition to efficient processing, foldable mobile phones also need large capacity data storage chips to meet the needs of next generation applications. The huge amount of data storage demanded by emerging applications such as 5G, AR/VR and AI requires continuous innovations in 3D NAND flash memory process architecture. |
Figure 6. Samsung-KLUDG4UHDC-B0E1-128GB Flash Memory Chip (Blue Frame) |
The chip in Figure 7(a) is the Samsung-KLUDG4UHDC-B0E1-128GB. From the X-Ray image, it can be seen that there are four dies inside. After the chip is delayered, the appearance of these dies can be observed directly (Figure 7b). The Focused Ion Beam Microscope (FIB) utilizes gallium (Ga) or xenon (Xe) ions, which are much heavier than electrons, to perform selective milling via bombardment or sputtering at specific locations on the test piece to achieve fixed point processing and test piece cutting. This allows for precise positioning and cutting at the nanometer level as well as the preparing of TEM sample slices. The TEM projects high-energy electron beams onto ultra-thin samples to perform three-dimensional scatter imaging. This is ideal for observing fine structures in samples. |
Figure 7. (a) Optical Image and X-ray Image of the Chip’s Front and Back (b) After the chip is delayered, 4 dies can be seen (c) FIB Optical Image and TEM Analysis at the Lower Cutting Position |
Figure 8(a) is a TEM image of a cross-section of the NAND chip taken after FIB processing. The calculations determined that there are 136 gate layers. Increasing the number of stacked layers can increase the storage capacity of 3D NAND flash memory. However, another challenge is how to maintain the stability of the structure throughout related processes such as alternative layer stacking (Module Layers), WL ladder optimization, vertical channel hole HAR etching and ALD layer processes, ONO cutting, sacrificial layer removal, replacement gate filling, CSL groove profiling, and chip warpage, etc. Energy Dispersive X-ray Spectroscopy (EDS) has always played an important role in the field of semiconductor structure and failure analysis because of its ease of use. The EDS element composition analysis function is often integral to determining structural composition and defect sources. The EDS detector recently acquired by MA-tek is installed on a TEM. Its windowless design significantly improves its performance in the detection of light elements, and the large surface area enhances its detection efficiency. Its ability to determine the concentration levels of elements in particular is excellent (Figure 8d). |
Figure 8. (a) TEM Image of NAND (b)(c) TEM Images of the NAND string at Different Magnifications (d) NAND string EDS Element Image; A clear visualization of the alternately deposited oxide and nitride film structure of the NAND string channel polysilicon ring. |
As related technologies continue to mature, many mobile phone manufacturers are investing in the foldable phone market, proposing various types of folding-screen phones. In response to the development of increasingly sophisticated processes for various electronic products, MA-tek strives constantly to develop and update our analysis machines and equipment hardware based on more than 20 years of analytical technology and experience, thus ensuring our ability to solve all kinds of problems in the industry. It is our honor to provide excellent, high-quality analytical services to customers in the industry, academia, and government.