Silica gel packaging power module of novel chip bonding point reinforcing process scheme

Abstract

The invention relates to the technical field of semiconductors, in particular to a silica gel packaging power module of a novel chip bonding point reinforcing process scheme. Comprising a chip; the back surface of the chip is arranged in a conductive copper layer power circuit etching region of the insulating ceramic substrate, and the front surface of the chip is connected to the conductive copper layer power circuit etching region of the insulating ceramic substrate through a bonding wire to form a power loop; and the reinforcing layer is arranged on the bonding points on the front surface of the chip. According to the invention, the bonding point is coated with the reinforcing layer, and the bonding point is cured and reinforced, so that in a power cycle reliability tolerance experiment with higher chip junction temperature and larger temperature change, the bonding point is less deformed under the constraint of the reinforcing layer and is subjected to relatively small stress, and the bonding and binding with the chip are less prone to failure. Therefore, the application of higher junction temperature resistance of the module can be improved, the power cycle or temperature cycle resistance of the bonding and binding process is higher, and the reliability of the module is improved.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor technology, and in particular to a silicone gel encapsulated power module with a novel chip bonding point reinforcement process solution. Background Art

[0002] At present, power modules are being used more and more widely in the fields of automobiles, wind power, solar power and storage, and other new energy vehicles. These modules have higher requirements for operating junction temperature and high reliability. This requires the power modules to be connected close to the chip end, such as the front bonding process of the silicon gel encapsulated chip to withstand higher temperature applications and requirements such as stronger resistance to power cycling or temperature cycling. In the existing technology, in the power cycling reliability tolerance test with high chip junction temperature and greater temperature changes, the bonding point is prone to deformation and receives greater stress, and the bonding with the chip is prone to failure. Summary of the Invention

[0003] The purpose of the present invention is to provide a silicone gel encapsulated power module with a novel chip bonding point reinforcement process solution to solve the above technical problems;

[0004] The technical problem solved by the present invention can be achieved by adopting the following technical solutions:

[0005] A new silicon gel encapsulated power module with a new chip bonding point reinforcement process solution, including:

[0006] chip;

[0007] An insulating ceramic substrate, wherein the back side of the chip is arranged in a conductive copper layer power circuit etching area of ​​the insulating ceramic substrate, and the front side of the chip is connected to the conductive copper layer power circuit etching area of ​​the insulating ceramic substrate via bonding wires to form a power loop;

[0008] The reinforcement layer is arranged on the bonding point on the front side of the chip.

[0009] Preferably, it also includes,

[0010] a substrate, the insulating ceramic substrate being disposed on the substrate;

[0011] A housing with metal terminals is provided on the outer sides of the insulating ceramic substrate and the substrate, and the housing is filled with transparent silicone gel.

[0012] Preferably, the back side of the chip is connected to the conductive copper layer power circuit etching area of ​​the insulating ceramic substrate by a silver sintering process or a soft soldering process, the silver sintering layer used in the silver sintering process is a silver paste or a silver film, the silver sintering temperature is 200° C. to 350° C., and the silver sintering pressure is 5 MPa to 50 MPa;

[0013] The solder used in the soft soldering process is tin flakes or tin paste, and the melting temperature of the solder is 240° C. to 350° C.

[0014] Preferably, the chip includes an IGBT tube, a diode and a MOS tube, and the back of the chip is provided with a first metal layer for silver sintering or soft soldering; the front of the chip is provided with a second metal layer for bonding.

[0015] Preferably, the first metal layer includes a nickel-palladium-gold alloy layer, an aluminum-titanium-nickel-vanadium-silver alloy layer and an aluminum-nickel-vanadium-silver alloy layer, and the second metal layer includes an aluminum-copper alloy layer, an aluminum-silicon-copper alloy layer and a copper-containing alloy layer.

[0016] Preferably, the bonding wire is connected from the front surface of the chip to the conductive copper layer power circuit etching area of ​​the insulating ceramic substrate by ultrasonic bonding.

[0017] Preferably, the bonding wire is an aluminum wire or a copper wire, and the wire diameter of the bonding wire is 12 mil to 20 mil.

[0018] Preferably, the reinforcement layer is a solid material formed by drying a mixed solution containing polyamide-imide applied to the bonding points in a mist or dot coating manner.

[0019] Preferably, the insulating ceramic substrate is an active metal brazing ceramic substrate or a bonded copper ceramic substrate.

[0020] Preferably, the thickness of the reinforcement layer is 10um to 50um.

[0021] The present invention has the following beneficial effects: By employing the above technical solution, the present invention solidifies and reinforces the bonding points by applying a reinforcement layer to the bonding points. In power cycling reliability tests at higher chip junction temperatures and greater temperature variations, the bonding points deform less under the restraint of the reinforcement layer, experiencing relatively less stress and making the bonding to the chip less likely to fail. This improves the module's ability to withstand higher junction temperatures, and the bonding process's ability to withstand power and temperature cycling, enhancing module reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a schematic diagram of the overall structure of a silicone gel encapsulated power module with a novel chip bonding point reinforcement process solution in an embodiment of the present invention.

[0023] Figure 2 Schematic diagram of the bonding point reinforcement structure of the silicone gel encapsulated power module according to the novel chip bonding point reinforcement process solution in an embodiment of the present invention.

[0024] Figure 3 It is a partially enlarged schematic diagram of the bonding point reinforcement structure of the present invention.

[0025] In the accompanying drawings: 1. Insulating ceramic substrate; 2. Chip; 3. Bonding wire; 4. Bonding point; 5. Reinforcement layer; 6. Housing; 7. Silicone gel; 8. Substrate. DETAILED DESCRIPTION

[0026] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0027] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0028] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but they are not intended to limit the present invention.

[0029] A new chip bonding point reinforcement process solution for silicone gel encapsulated power modules, such as Figures 1 to 3 Shown, including,

[0030] Chip 2;

[0031] Insulating ceramic substrate 1, the back of chip 2 is set in the conductive copper layer power circuit etching area of ​​insulating ceramic substrate 1, and the front of chip 2 is connected to the conductive copper layer power circuit etching area of ​​insulating ceramic substrate 1 through bonding wire 3 to form a power loop;

[0032] The reinforcement layer 5 is provided on the bonding points 4 on the front surface of the chip 2 .

[0033] In a preferred embodiment, it also includes:

[0034] A substrate 8, on which the insulating ceramic substrate 1 is disposed;

[0035] A housing 6 having metal terminals is provided outside the insulating ceramic substrate 1 and the substrate 8 , and the interior of the housing 6 is filled with a transparent silicone gel 7 .

[0036] The reinforcement layer 5 with polyamideimide as the main component coated on the bonding point 4 is formed into a solid coating (thickness 10um to 50um) after being coated with a solution mixture (mist spray or point-to-point solution coating) and dried, which has a significant reinforcement effect on the bonding point 4.

[0037] During the operation of the power module, especially when facing power cycles with higher chip 2 junction temperatures and greater temperature changes, the reinforcement layer 5 can effectively restrain the deformation of the bonding point 4, reduce stress concentration caused by factors such as temperature, thereby reducing the risk of failure of the bonding between the bonding point 4 and the chip 2, greatly improving the reliability and stability of the bonding point 4, and ensuring the continuous and stable operation of the power circuit.

[0038] Specifically, the present invention mainly provides a silicone gel encapsulated power module with a novel chip bonding point reinforcement process solution, which mainly includes a chip 2 with a back surface suitable for welding process and a front surface suitable for bonding process, an insulating ceramic substrate 1 (AMB or DBC), bonding wires 3, a reinforcement layer 5 (represented by a rectangular long transparent component) coated on the bonding point 4, an external encapsulation substrate 8, a housing of the encapsulation module, and a transparent silicone gel 7 forming an insulating package;

[0039] The substrate 8 is specifically a copper substrate;

[0040] The back side of the chip 2 is connected to the conductive copper layer power circuit etching area of ​​the insulating ceramic substrate 1 by silver sintering or welding process;

[0041] The front side of the chip 2 is connected to the conductive copper layer power circuit etching area of ​​the insulating ceramic substrate 1 using a bonding wire 3 bonding process to form a local circuit connection of the power loop;

[0042] The bonding points 4 on the surface of the chip 2 are coated and reinforced with a reinforcement layer 5; the internal components formed by the insulating ceramic substrate 1 are welded on the substrate 8, and the whole is encapsulated and wrapped by a shell 6 with metal terminals and transparent silicone gel 7 to form a complete power module.

[0043] More specifically, the present invention can strengthen the bonding connection of the bonding point 4 by applying a reinforcement layer on the bonding point 4, such as a silicone gel encapsulation module. Compared with the flexible packaging material of the silicone gel 7, the reinforcement layer solidifies and strengthens the bonding point 4 to a certain extent. In the power cycle reliability tolerance test with higher chip 2 junction temperature and greater temperature variation;

[0044] The chip 2 is connected to the insulating ceramic substrate 1 through back welding and front bonding processes, and then the power circuit is constructed and the bonding point 4 is reinforced with the help of bonding wires 3 and reinforcement layer 5. Finally, it is externally packaged by substrate 8, shell and silicone gel 7.

[0045] Thanks to the reinforcement layer 5, bond points 4 are less deformed and subject to relatively less stress, making the bond to chip 2 less likely to fail. This improves the module's ability to withstand higher junction temperatures and makes the bonding process more resistant to power and temperature cycling, thereby enhancing module reliability.

[0046] In a preferred embodiment, the back side of the chip 2 is connected to the conductive copper layer power circuit etching area of ​​the insulating ceramic substrate 1 by a silver sintering process or a soft soldering process. The silver sintering layer used in the silver sintering process is a silver paste or a silver film. The silver sintering temperature is 200°C to 350°C, and the silver sintering pressure is 5Mpa to 50Mpa.

[0047] The solder used in the soft soldering process is tin flakes or tin paste, and the melting temperature of the solder is 240℃~350℃.

[0048] Specifically, the back side of the chip 2 is connected to the conductive copper layer power circuit etching area of ​​the insulating ceramic substrate 1 using a silver sintering process or a soft soldering process;

[0049] The silver sintering process uses silver paste or silver film as the silver sintering material. The maximum silver sintering temperature is controlled between 200℃-350℃, and the sintering pressure is controlled between 5Mpa-50Mpa.

[0050] Soft soldering uses a solder with a melting temperature of 240℃-350℃, which can be tin flake or tin paste.

[0051] Specifically, the silver sintering process uses silver paste or silver film, and under specific temperature (200° C.-350° C.) and pressure (5 MPa-50 MPa) conditions, can achieve a high-strength and high-conductivity connection between the chip 2 and the insulating ceramic substrate 1 .

[0052] Silver's good conductivity ensures efficient power transmission, while the strong bonding interface formed by sintering can withstand large mechanical stress and remain stable in temperature changes and vibration environments.

[0053] Soft soldering (solder melting temperature 240℃-350℃, such as tin sheet or solder paste) has a relatively low soldering temperature that has little thermal impact on the chip 2 and substrate 8. It is suitable for some temperature-sensitive chip 2 or substrate 8 materials. At the same time, it can meet certain electrical connection requirements and ensure the reliable operation of the power module in different application scenarios.

[0054] In a preferred embodiment, the chip 2 includes an IGBT tube, a diode and a MOS tube, and the back of the chip 2 is provided with a first metal layer for silver sintering or soft soldering; the front of the chip 2 is provided with a second metal layer for bonding.

[0055] In a preferred embodiment, the first metal layer includes a nickel-palladium-gold alloy layer, an aluminum-titanium-nickel-vanadium-silver alloy layer and an aluminum-nickel-vanadium-silver alloy layer, and the second metal layer includes an aluminum-copper alloy layer, an aluminum-silicon-copper alloy layer and a copper-containing alloy layer.

[0056] Specifically, the chip 2 includes but is not limited to an IGBT tube, a diode (DIODE) and a SiC MOSFET tube. The back of the chip 2 is covered with a first metal layer that can be used for silver sintering process and soft soldering. The first metal layer includes but is not limited to Ni / Pd / Au, Al / Ti / NiV / Ag or Al / NiV / Ag.

[0057] The front surface of the chip 2 may be covered with a second metal layer suitable for a bonding process. The second metal layer includes but is not limited to Al / Cu, Al / Si / Cu or a Cu-containing alloy.

[0058] In a preferred embodiment, the bonding wire 3 is connected from the front surface of the chip 2 to the conductive copper layer power circuit etching area of ​​the insulating ceramic substrate 1 by ultrasonic bonding.

[0059] In a preferred embodiment, the bonding wire 3 is an aluminum wire or a copper wire, and the wire diameter of the bonding wire 3 is 12 mil to 20 mil.

[0060] Specifically, the bonding wire 3 on the front of the chip 2 is connected by an ultrasonic bonding process. The bonding wire 3 can be made of pure aluminum or pure copper, and its wire diameter includes but is not limited to 12 mil, 16 mil and 20 mil.

[0061] In practical applications, optimization is required based on factors such as the power requirements of the chip 2, the operating frequency, and the heat dissipation conditions of the module. For example, in high-power applications, a bonding wire 3 with a thicker wire diameter (such as 20 mil) can be selected to improve the current carrying capacity.

[0062] The bonding wire 3 of the present invention can be made of pure aluminum or pure copper;

[0063] The pure aluminum bonding wire 3 has the advantages of good electrical conductivity, light weight, relatively low cost, and high electromagnetic compatibility.

[0064] The pure copper bonding wire 3 has higher conductivity and can carry a larger current under the same cross-sectional area, and is suitable for power modules with higher power transmission requirements.

[0065] In a preferred embodiment, the reinforcement layer 5 is a solid formed by drying a mixed solution containing polyamide-imide that is applied on the bonding points 4 in a mist or dot coating manner.

[0066] Specifically, the reinforcement layer 5 with polyamideimide as the main component coated on the bonding point 4 is coated with a solution mixture (mist spray or point-to-point solution coating) and dried to form a solid coating (thickness 10um to 50um), which has a significant reinforcement effect on the bonding point 4.

[0067] Specifically, the main component of reinforcement layer 5 is polyamide-imide (PAI). PAI is a high-performance organic polymer material with many excellent properties, making it suitable for reinforcing chip 2 bond pads 4. It has excellent high-temperature resistance, maintaining a stable chemical structure and physical properties in high-temperature environments. It will not soften, deform, or decompose due to prolonged exposure to high temperatures, thereby effectively protecting the integrity of bond pads 4 at high operating junction temperatures.

[0068] Polyamide-imide also offers high mechanical strength, including excellent tensile strength, flexural strength, and impact resistance. During power module operation, when the bond points 4 are subjected to various mechanical stresses (such as those caused by thermal expansion and contraction, and vibration), the reinforcement layer 5 provides sufficient support and restraint, preventing failures such as displacement, cracking, or breakage of the bond points 4.

[0069] In addition, PAI also has good chemical stability and strong tolerance to common chemical substances (such as acids and alkalis, etc.), and is not easily susceptible to chemical corrosion. This helps to maintain the performance of the reinforcement layer 5 in a complex working environment, thereby ensuring the long-term reliability of the bonding point 4.

[0070] More specifically, the coating material is in the form of a solution mixture, which is convenient for handling and coating.

[0071] The specific coating process includes converting the coating material solution into a mist spray through special equipment or using point-to-point solution coating.

[0072] The mist spray coating can evenly distribute the reinforcement material on the surface of the bonding point 4, ensuring that each bonding point 4 can be coated more uniformly. It is particularly suitable for fast and efficient coating of multiple bonding points 4 in large-scale production.

[0073] Point-to-point solution dispensing can more accurately control the coating amount and position, and is suitable for situations where high coating accuracy is required or the distribution of bonding points 4 is relatively special.

[0074] The drying process causes the liquid coating material to undergo physical and chemical changes, turning it into a solid state. During the drying process, the solvent gradually evaporates, and cross-linking reactions occur between the polyamide-imide molecules to form a three-dimensional network structure, thereby giving the reinforcement layer 5 stable mechanical and chemical properties.

[0075] The drying temperature is, for example, 150°C-250°C, and the drying time is determined by factors such as the coating amount and the efficiency of the drying equipment. It usually takes several minutes to several tens of minutes. It is necessary to ensure that the coating material is completely cured to achieve the best reinforcement effect.

[0076] During the operation of the power module, especially when facing power cycles with higher chip 2 junction temperatures and greater temperature changes, the reinforcement layer 5 can effectively restrain the deformation of the bonding point 4, reduce stress concentration caused by factors such as temperature, thereby reducing the risk of failure of the bonding between the bonding point 4 and the chip 2, greatly improving the reliability and stability of the bonding point 4, and ensuring the continuous and stable operation of the power circuit.

[0077] More specifically, the present invention can strengthen the bonding connection of the bonding point 4 by applying a reinforcement layer on the bonding point 4, such as a silicone gel encapsulation module. Compared with the flexible packaging material of the silicone gel 7, the reinforcement layer solidifies and strengthens the bonding point 4 to a certain extent. In the power cycle reliability tolerance test with higher chip 2 junction temperature and greater temperature variation;

[0078] The chip 2 is connected to the insulating ceramic substrate 1 through back welding and front bonding processes, and then the power circuit is constructed and the bonding point 4 is reinforced with the help of bonding wires 3 and reinforcement layer 5. Finally, it is externally packaged by substrate 8, shell and silicone gel 7.

[0079] Thanks to the reinforcement layer 5, bond points 4 are less deformed and subject to relatively less stress, making the bond to chip 2 less likely to fail. This improves the module's ability to withstand higher junction temperatures and makes the bonding process more resistant to power and temperature cycling, thereby enhancing module reliability.

[0080] In a preferred embodiment, the insulating ceramic substrate 1 is an active metal brazed ceramic substrate 8 or a bonded copper ceramic substrate 8 .

[0081] Specifically, both the active metal brazed ceramic substrate (AMB) and the bonded copper ceramic substrate (DBC) have excellent electrical insulation properties, which can effectively prevent electrical short circuits inside the power module and ensure the safety and stability of the circuit.

[0082] In addition, the active metal brazed ceramic substrate (AMB) and the bonded copper ceramic substrate (DBC) have a certain thermal conductivity, which can quickly conduct the heat generated by the chip 2 to the external heat dissipation component (the substrate 8 in the present invention), which helps to maintain the normal operating temperature of the chip 2 and improve the heat dissipation efficiency of the power module, thereby improving the power density and reliability.

[0083] In a preferred embodiment, the thickness of the reinforcement layer 5 is 10 um to 50 um.

[0084] Specifically, the reinforcement layer 5 coated on the bonding point 4 is a coating material including but not limited to polyamide-imide as a main component, and the coating material is a solution mixture;

[0085] The specific coating process includes but is not limited to converting the coating material solution into a mist spray through special equipment or applying the solution point-to-point, which needs to be evenly covered on the surface of the bonding point 4 as much as possible. After coating, a drying process is required to convert the liquid coating material into a solid state to solidify the connection of the bonding point 4;

[0086] The thickness of the reinforcement layer 5 applied after baking can be between 10 μm and 50 μm.

[0087] In the present invention, under conditions of higher chip 2 junction temperature (such as in high-temperature working environments such as automotive electronics and industrial power supplies) and greater temperature changes (such as during power cycling), the reinforcement layer 5 (mainly composed of polyamide-imide and with a thickness of 10um to 50um) plays a role in curing and reinforcing the bonding point 4.

[0088] It is used to constrain the deformation of the bonding point 4 so that the stress it is subjected to is relatively small when the temperature changes, thereby reducing the possibility of failure of the bonding between the bonding point 4 and the chip 2 and significantly improving the reliability of the power module under harsh working conditions.

[0089] The presence of the reinforcement layer 5 at the bonding point 4 allows for better connection stability at high temperatures, thereby enhancing the module's ability to withstand higher junction temperatures. This allows the power module to operate normally in higher temperature environments or extend its service life at the same temperature, broadening its application range to applications with high temperature requirements, such as high-temperature industrial environments and electric vehicle power systems.

[0090] In summary, the reinforcement layer 5 applied to the bonding point 4 of the present invention can strengthen the bonding connection of the bonding point 4. For example, in a silicone gel encapsulated module, compared to the flexible packaging material of silicone gel 7, the reinforcement layer 5 solidifies and reinforces the bonding point 4 to a certain extent. In power cycling reliability tests with higher chip 2 junction temperatures and greater temperature variations, due to the reinforcement of the reinforcement layer 5, the bonding point 4 deforms less under the constraints of the coating material and is subjected to relatively less stress, making the bonding to the chip 2 less likely to fail. This can improve the module's ability to withstand higher junction temperatures, and the bonding process has stronger resistance to power cycling or temperature cycling, thereby improving module reliability.

[0091] The above description is only a preferred embodiment of the present invention and does not limit the implementation mode and protection scope of the present invention. For those skilled in the art, it should be aware that all solutions obtained by equivalent substitutions and obvious changes made using the description and illustrations of the present invention should be included in the protection scope of the present invention.

Claims

1. A new silicon gel encapsulated power module with a new chip bonding point reinforcement process, characterized in that: include, Chip (2); An insulating ceramic substrate (1), the back side of the chip (2) being arranged in a conductive copper layer power circuit etching area of ​​the insulating ceramic substrate (1), and the front side of the chip (2) being connected to the conductive copper layer power circuit etching area of ​​the insulating ceramic substrate (1) via a bonding wire (3), thereby forming a power circuit; A reinforcement layer (5) is provided on the bonding point (4) on the front side of the chip (2).

2. The silicone gel encapsulated power module of the novel chip bonding point reinforcement process according to claim 1 is characterized in that: Also includes, A substrate (8), the insulating ceramic substrate (1) being arranged on the substrate (8); A shell (6) having metal terminals is arranged outside the insulating ceramic substrate (1) and the substrate (8), and the shell (6) is filled with transparent silicone gel (7).

3. The silicone gel encapsulated power module of the novel chip bonding point reinforcement process according to claim 1 is characterized in that: The back side of the chip (2) is connected to the conductive copper layer power circuit etching area of ​​the insulating ceramic substrate (1) by a silver sintering process or a soft soldering welding process, the silver sintering layer used in the silver sintering process is a silver paste or a silver film, the silver sintering temperature is 200° C. to 350° C., and the silver sintering pressure is 5 MPa to 50 MPa; The solder used in the soft soldering process is tin sheet or tin paste, and the melting temperature of the solder is 240°C to 350°C.

4. The silicone gel encapsulated power module of the novel chip bonding point reinforcement process solution according to claim 1 is characterized in that: The chip (2) comprises an IGBT tube, a diode and a MOS tube; the back of the chip (2) is provided with a first metal layer for silver sintering or soft soldering; the front of the chip (2) is provided with a second metal layer for bonding.

5. The silicone gel encapsulated power module of the novel chip bonding point reinforcement process according to claim 4 is characterized in that: The first metal layer includes a nickel-palladium-gold alloy layer, an aluminum-titanium-nickel-vanadium-silver alloy layer and an aluminum-nickel-vanadium-silver alloy layer, and the second metal layer includes an aluminum-copper alloy layer, an aluminum-silicon-copper alloy layer and a copper-containing alloy layer.

6. The silicone gel encapsulated power module of the novel chip bonding point reinforcement process according to claim 1 is characterized in that: The bonding wire (3) is connected from the front side of the chip (2) to the conductive copper layer power circuit etching area of ​​the insulating ceramic substrate (1) by ultrasonic bonding.

7. The silicone gel encapsulated power module of the novel chip bonding point reinforcement process solution according to claim 6 is characterized in that: The bonding wire (3) is an aluminum wire or a copper wire, and the wire diameter of the bonding wire (3) is 12 mil to 20 mil.

8. The silicone gel encapsulated power module of the novel chip bonding point reinforcement process according to claim 1 is characterized in that: The reinforcement layer (5) is a solid material formed by drying a mixed solution containing polyamide-imide which is applied on the bonding point (4) in a mist or dot coating manner.

9. The silicone gel encapsulated power module of the novel chip bonding point reinforcement process solution according to claim 1 is characterized in that: The insulating ceramic substrate (1) is an active metal brazed ceramic substrate or a bonded copper ceramic substrate.

10. The silicone gel encapsulated power module of the novel chip bonding point reinforcement process solution according to claim 1, characterized in that: The thickness of the reinforcement layer (5) is 10 um to 50 um.

Images