An alloy ribbon magnetic core based toroidal spiral thin film transformer and a preparation method thereof
By using amorphous/nanocrystalline alloy strips and femtosecond laser cold processing to fabricate a toroidal magnetic core, a toroidal solenoid thin-film transformer was developed, which solved the problem of low magnetic coupling efficiency of micro transformers at high frequencies. This resulted in high inductance and high Q values, making it compatible with the planarization process of MEMS devices and improving the miniaturization and integration performance of the devices.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- SOUTHWEAT UNIV OF SCI & TECH
- Filing Date
- 2026-04-09
- Publication Date
- 2026-06-05
Smart Images
Figure CN122158305A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of microelectronic device technology, specifically to a toroidal solenoid thin-film transformer based on an alloy strip magnetic core and its fabrication method. Background Technology
[0002] Planar microtransformers, as core magnetic components of high-frequency power electronic systems, are widely used in 5G base stations, new energy vehicle electronic control systems, and portable smart devices. Their miniaturization, high integration, high frequency, and low loss performance requirements have become key demands driving the upgrading of related industries. However, current mainstream fabrication methods still have many problems and cannot meet the stringent requirements of these application scenarios.
[0003] Traditional wound micro-transformers, manufactured using manual or mechanical winding processes, are not only bulky and loosely structured, but also suffer from poor compatibility with semiconductor processes, making integrated integration with power chips impossible and severely hindering the miniaturization of power modules. While mainstream PCB and thick-film technologies have achieved planar structures, limitations in processing precision make it difficult to precisely control winding linewidth and spacing at the micrometer level. This leads to significantly aggravated skin and proximity effects at high frequencies, resulting in a surge in copper losses. Furthermore, these technologies often employ external magnetic core designs, resulting in poor contact between the core and winding interface, prominent magnetic leakage, and low magnetic coupling efficiency. Although silicon-based CMOS integration technology possesses on-chip integration potential, the parasitic capacitance and resistance effects of the silicon substrate are significant, causing severe energy losses at MHz-level high frequencies.
[0004] The fabrication of planar transformers based on MEMS holds promise for achieving sub-micron precision, superior high-frequency characteristics, and monolithic integration with CMOS circuits, making it a core technology for the future. While MEMS technology enables the miniaturization and integration of transformers, technical bottlenecks remain regarding device inductance, coupling coefficient, power density, and overall device miniaturization. Current mainstream MEMS planar micro-transformer technologies typically employ planar helical coils and solenoid coils. These structures suffer from several problems: planar helical structures inherently exhibit significant edge flux leakage; the non-closed magnetic circuit leads to low coupling coefficients and energy transfer efficiency; and as the number of winding turns increases, the coil diameter grows, hindering improvements in overall device power and miniaturization. For example, publication number CN115719674A utilizes a planar structure, and solenoid coils, due to their closed magnetic core circuit, offer significantly improved performance compared to planar helical structures. However, existing solutions typically employ rectangular core structures, making it difficult to wind the coil uniformly and completely around the entire core. For example, in CN1260749C, the uneven distribution of the coil on the magnetic core and the presence of right-angle corners result in greater magnetic resistance, leading to higher hysteresis loss, eddy current loss, and more significant leakage flux, resulting in low energy utilization.
[0005] Due to limitations in MEMS device fabrication, no mature magnetic core material currently exists that can fully adapt to MEMS devices in terms of size and performance. Traditional electroplated / sputtered soft magnetic thin film cores suffer from inherent defects such as low saturation magnetization and high high-frequency losses, prioritizing improvements in inductance and efficiency, often only reaching the nH level. Furthermore, significant high-frequency eddy current losses severely limit the device's power capacity and DC bias resistance. The deposition process of magnetron sputtered magnetic core films is complex, easily generating internal stress, lattice defects, and inhomogeneous microstructures. These defects pin the movement of magnetic domain walls, directly leading to increased coercivity and decreased permeability. In contrast, amorphous / nanocrystalline ribbon materials possess excellent comprehensive magnetic properties (high permeability, low loss, and relatively high Bs), a golden combination achieved through decades of optimization, and mature processes are available. Mass production at low cost is achievable, but their application still requires further development. Amorphous / nanocrystalline alloy strips have high saturation magnetic induction, high permeability and extremely low high-frequency loss, but their thickness limit is usually higher than 20 µm and they are sensitive to mechanical stress. They are prone to cracking or curling during processing such as cutting and punching, which makes it difficult to use this material for the research and development of MEMS thin film devices. Summary of the Invention
[0006] This invention addresses the problems existing in the prior art by providing a toroidal solenoid thin-film transformer based on an alloy strip magnetic core and its preparation method.
[0007] The technical solution adopted in this invention is: a toroidal solenoid thin-film transformer based on an alloy strip magnetic core, comprising a substrate, a lower coil layer, a magnetic core layer, an upper coil layer and an electrode layer stacked sequentially from bottom to top; The lower coil layer, magnetic core layer, upper coil layer, and electrode layer all use insulating material as the substrate; the magnetic core layer is provided with a ring-shaped magnetic core made of amorphous / nanocrystalline alloy strip; the lower coil layer is provided with a lower solenoid coil array with a ring magnetic core at the corresponding position of the magnetic core; the upper coil layer is provided with an upper solenoid coil array with a ring magnetic core at the corresponding position of the magnetic core; the lower solenoid coil array and the upper solenoid coil array are connected by copper pillars at both ends of the coil array; the upper solenoid coil array is connected to the electrode sheet, and the electrode sheet is connected to the external electrode provided on the electrode layer.
[0008] Furthermore, the magnetic core is made of Fe-based or Co-based amorphous / nanocrystalline alloy strip.
[0009] Furthermore, the thickness of the magnetic core is 16-30 µm.
[0010] Furthermore, the height of the copper pillar is 60-100 µm.
[0011] Furthermore, the substrates of the lower coil layer, magnetic core layer, upper coil layer, and electrode layer are all made of polyimide.
[0012] A method for fabricating a toroidal solenoid thin-film transformer based on an alloy strip magnetic core includes the following steps: Step 1: Deposit a first copper film on the substrate surface by magnetron sputtering as a seed layer for the lower solenoid coil array; Step 2: Coat with photoresist, expose and develop to reveal the pattern of the lower solenoid coil array, obtain a second copper film by electroplating, and remove the photoresist to obtain the lower solenoid coil array; Step 3: Coat with photoresist, expose and develop copper pillars at both ends of the lower solenoid coil array; obtain a third copper film by electroplating, remove the photoresist to obtain the copper pillars; remove the seed layer by dry etching. Step 4: Coat the insulating resin as the substrate for the lower coil layer and cure it; coat the semi-cured insulating resin, place the magnetic core in the center area of the lower solenoid coil array; then coat the insulating resin and cure it to obtain the magnetic core layer; Step 5: The surface of the magnetic core layer is treated to expose the copper pillars; a fourth copper film is deposited by magnetron sputtering as the seed layer for the upper solenoid coil array; Step 6: Coat with photoresist, expose and develop to reveal the upper solenoid coil array pattern and electrode pattern; prepare the fifth copper film by electroplating, remove the photoresist to obtain the upper solenoid coil array and electrode; Step 7: Coat with photoresist, expose and develop to reveal the external electrode pattern, and prepare the external electrode by electroplating; remove the photoresist and remove the seed layer by dry etching. Step 8: Apply insulating resin to achieve encapsulation, and polish to expose the external electrodes.
[0013] Furthermore, the magnetic core is prepared into a ring structure by femtosecond laser cold processing of amorphous / nanocrystalline alloy strip and then vacuum annealed.
[0014] Furthermore, the thickness of the first copper film is 100–300 nm, and the thickness of the second copper film is 8–20 µm.
[0015] Furthermore, the thickness of the third copper film is 60-100 µm.
[0016] Furthermore, the thickness of the fourth copper film is 100–300 nm, and the thickness of the fifth copper film is 8–20 µm.
[0017] The beneficial effects of this invention are: This invention uses amorphous / nanocrystalline alloy strip as the core layer of a thin-film transformer, which can significantly optimize the magnetic flux and inductance performance of the device. Furthermore, it employs femtosecond laser cold cutting of the alloy thin-film strip and stress-reducing post-mold treatment to obtain a high-performance, flattened, ultra-thin alloy strip core, suitable for the planarization process of MEMS devices. Using MEMS, the flattened alloy strip is placed between the upper and lower copper coil arrays as the core, with polyimide used for insulation between the layers. Since the thickness of the strip core is 16–30 µm, this results in a corresponding increase in the thickness of the insulation layer. Layer interconnection is achieved through ultra-high copper pillars (60–100 µm), allowing direct connection between the upper and lower copper coils to form a three-dimensional solenoid structure. Attached Figure Description
[0018] Figure 1 This is a schematic diagram of the thin-film transformer of the present invention.
[0019] Figure 2 This is a schematic diagram of the upper solenoid coil array structure in the thin-film transformer of the present invention.
[0020] Figure 3 The figures show the inductance and Q-value test results of the thin-film transformer of this invention and the comparative example, where a is the inductance and b is the Q-value result.
[0021] In the diagram: 1-substrate, 2-lower coil layer, 3-magnetic core layer, 4-upper coil layer, 401-upper solenoid coil array, 402-electrode sheet, 5-electrode layer, 6-copper pillar, 7-external electrode. Detailed Implementation
[0022] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.
[0023] like Figure 1 and Figure 2 As shown, a toroidal solenoid thin-film transformer based on an alloy strip magnetic core includes a substrate 1, a lower coil layer 2, a magnetic core layer 3, an upper coil layer 4, and an electrode layer 5 stacked sequentially from bottom to top. The lower coil layer 2, the magnetic core layer 3, the upper coil layer 4, and the electrode layer 5 all use insulating materials as substrates; the magnetic core layer 3 is provided with a ring-shaped magnetic core made of amorphous / nanocrystalline alloy strip; the lower coil layer 2 is provided with a lower solenoid coil array with a ring magnetic core at the corresponding position of the magnetic core; the upper coil layer 4 is provided with an upper solenoid coil array 401 with a ring magnetic core at the corresponding position of the magnetic core; the lower solenoid coil array and the upper solenoid coil array are connected by copper pillars 6 provided at both ends of the coil array; the upper solenoid coil array 401 is connected to the electrode sheet 402, and the electrode sheet 402 is connected to the external electrode 7 provided on the electrode layer 5.
[0024] The magnetic core is made of Fe-based or Co-based amorphous / nanocrystalline alloy strip; the thickness of the magnetic core is 16-30 µm; the height of the copper pillar 6 is 60-100 µm. The substrates of the lower coil layer 2, magnetic core layer 3, upper coil layer 4 and electrode layer 5 are all made of polyimide.
[0025] A method for fabricating a toroidal solenoid thin-film transformer based on an alloy strip magnetic core includes the following steps: Step 1: A first copper film is deposited on the surface of substrate 1 by magnetron sputtering as a seed layer for the lower solenoid coil array; Step 2: Coat with photoresist, expose and develop to reveal the pattern of the lower solenoid coil array, obtain the second copper film by electroplating, and remove the photoresist to obtain the lower solenoid coil array; the thickness of the first copper film is 100-300 nm, and the thickness of the second copper film is 8-20 µm.
[0026] Step 3: Coat with photoresist, expose and develop copper pillars at both ends of the lower solenoid coil array; obtain the third copper film by electroplating, and remove the photoresist to obtain copper pillar 6; remove the seed layer by dry etching. Step 4: Coat the lower coil layer 2 with insulating resin and cure it; coat the semi-cured insulating resin and place the magnetic core in the center region of the lower solenoid coil array; then coat the insulating resin and cure it to obtain the magnetic core layer 3; the magnetic core is prepared into a ring structure by femtosecond laser cold processing of amorphous / nanocrystalline alloy strip and then vacuum annealed. The thickness of the third copper film is 60-100 µm.
[0027] Step 5: The surface of the magnetic core layer 3 is processed to expose the copper pillar 6; a fourth copper film is deposited by magnetron sputtering as the seed layer of the upper solenoid coil array 401; Step 6: Coat with photoresist, expose and develop to reveal the upper solenoid coil array pattern and electrode sheet 402 pattern; prepare the fifth copper film by electroplating, remove the photoresist to obtain the upper solenoid coil array 401 and electrode sheet 402; Step 7: Coat with photoresist, expose and develop to reveal the pattern of external electrode 7, and prepare external electrode 7 by electroplating; remove photoresist and remove the seed layer by dry etching. Step 8: Apply insulating resin to achieve encapsulation, and polish to expose the external electrode 7.
[0028] Example 1 A toroidal solenoid thin-film transformer based on an alloy strip magnetic core includes a substrate 1, a lower coil layer 2, a magnetic core layer 3, an upper coil layer 4, and an electrode layer 5 stacked sequentially from bottom to top. The lower coil layer 2, the magnetic core layer 3, the upper coil layer 4, and the electrode layer 5 all use insulating materials as substrates; the magnetic core layer 3 is provided with a ring-shaped magnetic core made of Fe alloy strip; the lower coil layer 2 is provided with a lower solenoid coil array with a ring magnetic core at the corresponding position of the magnetic core; the upper coil layer 4 is provided with an upper solenoid coil array 401 with a ring magnetic core at the corresponding position of the magnetic core; the lower solenoid coil array and the upper solenoid coil array are connected by copper pillars 6 provided at both ends of the coil array; the upper solenoid coil array 401 is connected to the electrode plate 402, and the electrode plate 402 is connected to the external electrode 7 provided on the electrode layer 5.
[0029] The toroidal solenoid thin-film transformer based on an alloy strip magnetic core is prepared according to the following steps: Before fabricating the device, a mask layout was designed and fabricated based on the transformer results, including masks for the lower and upper solenoid coil arrays. The linewidth, number of turns, and electrode spacing were determined according to the structural dimensions. The upper and lower coils were interconnected using copper pillars. Masks for the magnetic core and electrodes were then fabricated. The amorphous alloy thin strip was cut into rings using femtosecond laser cold processing technology and then subjected to vacuum annealing (the annealing temperature was selected according to the type of alloy strip and the actual situation) to relieve stress, resulting in a flat, warp-free, ultra-high permeability magnetic core, which served as the core for the device described below.
[0030] Step 1: Use a 2-inch silicon wafer as substrate 1. The surface of the silicon wafer has an oxide layer of 200-300 nm. Use magnetron sputtering to deposit a first copper film of about 200 nm on the silicon wafer as the seed layer for the lower solenoid coil array.
[0031] Step 2: Coat with 15 µm photoresist, expose and develop to reveal the pattern of the lower solenoid coil array. Prepare a second copper film of 15 µm on substrate 1 by electroplating, and remove the photoresist with acetone solution to obtain the lower solenoid coil array.
[0032] Step 3: Continue coating with an ultra-thick photoresist (thickness matching the height of the copper pillars). After exposure and development, copper pillar points are exposed at both ends of the lower solenoid coil array. An 80 µm third copper film is prepared using electroplating. The photoresist is removed with acetone solution to obtain copper pillar 6. The seed layer is removed using dry etching, exposing copper pillar 6. The overall height of copper pillar 6 is much higher than the height of the magnetic core layer 3, ensuring that the lower and upper solenoid coil arrays can be interconnected. The lower and upper solenoid coil arrays have identical structures.
[0033] Step 4: Coat and cure a layer of polyimide resin as an insulating layer on the substrate obtained in Step 3, then coat a 5 µm layer of polyimide resin and allow it to cure semi-cured. Place the heat-treated magnetic core in the center region of the lower solenoid coil array, ensuring the lower solenoid array surrounds the magnetic core. The heat-treated magnetic core, flat and without warping, is placed between the lower and upper solenoid coil arrays, without exceeding or covering the copper pillar positions. Repeat the coating and curing of polyimide resin until the insulating layer completely covers the magnetic core, resulting in magnetic core layer 3 after curing.
[0034] Step 5: The surface of the magnetic core layer 3 is polished and planarized until the copper pillars 6 are exposed, while the magnetic core itself is not exposed. These copper pillars are used to connect the upper and lower solenoid coil arrays to achieve interlayer interconnection. The thickness of the magnetic core layer 3 is set as needed, based on the requirement that the upper and lower solenoid coil arrays can be interconnected. A 100 nm fourth copper film is deposited by magnetron sputtering as the seed layer for the upper solenoid coil array 401.
[0035] Step 6: Coat with 8 µm thick photoresist, expose and develop to reveal the upper solenoid coil array 401 and electrode plate 402 patterns. Prepare an 8 µm thick fifth copper film using electroplating, and remove the photoresist with acetone solution to obtain the upper solenoid coil array 401 and electrode plate 402.
[0036] Step 7: Coat with photoresist, expose and develop to reveal the pattern of external electrode 7, and prepare external electrode 7 with a thickness of 10 µm by electroplating; remove the photoresist with acetone solution to obtain the lead of external electrode 7, and remove the seed layer by dry etching.
[0037] Step 8: Apply insulating resin to achieve encapsulation, and polish to expose the external electrode 7.
[0038] The external electrode 7 and the electrode sheet 402 in the upper coil layer have the same shape and are interconnected.
[0039] Comparative Example To illustrate the effects of the present invention, a comparative example was set up. All other steps in the comparative example were the same as those in Example 1, except that the magnetic core was made of NiFe alloy and was prepared by magnetron sputtering.
[0040] The inductance and Q-value results obtained from the comparative example and Example 1 are as follows: Figure 3 As shown in the figure, the transformer inductance obtained in Example 1 can reach a maximum of 60 µH, while the thin-film transformer obtained in the comparative example has an inductance of only about 200 nH.
[0041] The transformer obtained by this invention uses amorphous / nanocrystalline alloy strip as the core layer of a thin-film transformer, which can significantly optimize the magnetic flux and inductance performance of the device. Furthermore, a high-performance, flattened, ultra-thin alloy strip core is obtained by cold-working the alloy thin-film strip with femtosecond laser and stress-reducing the alloy thin-film strip after mold fixing, making it suitable for the planarization process of MEMS devices. Using MEMS, the flattened alloy strip is placed between the upper and lower copper coil arrays as the core, with polyimide used for insulation between the layers. Since the thickness of the strip core is 16–30 µm, this results in a corresponding increase in the thickness of the insulation layer. Layer interconnection is achieved through ultra-high copper pillars (60–100 µm), allowing the upper and lower copper coils to be directly connected to form a three-dimensional solenoid structure.
[0042] This invention utilizes a transformer with a "strip magnetic core + toroidal solenoid" structure, aiming for higher power, higher efficiency, higher performance, and better integration. It solves the problems of miniaturization, integration, low inductance, low power, and low efficiency in existing planar transformers. While reducing size, it increases effective magnetic flux. Using amorphous / nanocrystalline alloy strips with high saturation magnetic flux density and low loss, processed by femtosecond laser, as the core magnetic material, the resulting transformer achieves high inductance and a high Q value.
Claims
1. A toroidal solenoid thin-film transformer based on an alloy strip magnetic core, characterized in that, It includes a substrate (1), a lower coil layer (2), a magnetic core layer (3), an upper coil layer (4), and an electrode layer (5) stacked from bottom to top. The lower coil layer (2), the magnetic core layer (3), the upper coil layer (4) and the electrode layer (5) are all made of insulating material as substrate; the magnetic core layer (3) is provided with a ring structure magnetic core made of amorphous / nanocrystalline alloy strip; the lower coil layer (2) is provided with a lower solenoid coil array of ring magnetic core at the corresponding position of the magnetic core; the upper coil layer (4) is provided with an upper solenoid coil array (401) of ring magnetic core at the corresponding position of the magnetic core; the lower solenoid coil array and the upper solenoid coil array are connected by copper pillars (6) provided at both ends of the coil array; the upper solenoid coil array (401) is connected to the electrode sheet (402), and the electrode sheet (402) is connected to the external electrode (7) provided on the electrode layer (5).
2. The toroidal solenoid thin-film transformer based on an alloy strip magnetic core according to claim 1, characterized in that, The magnetic core is made of Fe-based or Co-based amorphous / nanocrystalline alloy strip.
3. A toroidal solenoid thin-film transformer based on an alloy strip magnetic core according to claim 1, characterized in that, The thickness of the magnetic core is 16-30 µm.
4. A toroidal solenoid thin-film transformer based on an alloy strip magnetic core according to claim 3, characterized in that, The height of the copper column (6) is 60-100 µm.
5. A toroidal solenoid thin-film transformer based on an alloy strip magnetic core according to claim 1, characterized in that, The substrates of the lower coil layer (2), magnetic core layer (3), upper coil layer (4) and electrode layer (5) are all made of polyimide.
6. The method for fabricating a toroidal solenoid thin-film transformer based on an alloy strip magnetic core as described in any one of claims 1 to 5, characterized in that, Includes the following steps: Step 1: A first copper film is deposited on the surface of the substrate (1) by magnetron sputtering as the seed layer for the lower solenoid coil array; Step 2: Coat with photoresist, expose and develop to reveal the pattern of the lower solenoid coil array, obtain a second copper film by electroplating, and remove the photoresist to obtain the lower solenoid coil array; Step 3: Coat with photoresist, expose and develop copper pillars at both ends of the lower solenoid coil array; The third copper film is obtained by electroplating, and the copper pillar (6) is obtained by removing the photoresist; the seed layer is removed by dry etching. Step 4: Coat the insulating resin as the substrate of the lower coil layer (2) and cure it; coat the semi-cured insulating resin and place the magnetic core in the central area of the lower solenoid coil array; then coat the insulating resin and cure it to obtain the magnetic core layer (3). Step 5: The surface of the magnetic core layer (3) is treated to expose the copper pillar (6); a fourth copper film is deposited by magnetron sputtering as the seed layer for the upper solenoid coil array (401); Step 6: Coat the photoresist, expose and develop to reveal the solenoid coil array pattern and the electrode sheet (402) pattern; The fifth copper film was prepared by electroplating, and the photoresist was removed to obtain the upper solenoid coil array (401) and electrode sheet (402). Step 7: Coat with photoresist, expose and develop to reveal the pattern of the external electrode (7), and prepare the external electrode (7) by electroplating; remove the photoresist and remove the seed layer by dry etching; Step 8: Coat with insulating resin to achieve encapsulation, and polish to expose the external electrode (7).
7. The method for fabricating a toroidal solenoid thin-film transformer based on an alloy strip magnetic core according to claim 6, characterized in that, The magnetic core is prepared into a ring structure by femtosecond laser cold processing of amorphous / nanocrystalline alloy strip and then vacuum annealed.
8. The method for fabricating a toroidal solenoid thin-film transformer based on an alloy strip magnetic core according to claim 6, characterized in that, The thickness of the first copper film is 100–300 nm, and the thickness of the second copper film is 8–20 µm.
9. The method for fabricating a toroidal solenoid thin-film transformer based on an alloy strip magnetic core according to claim 6, characterized in that, The thickness of the third copper film is 60-100 µm.
10. The method for fabricating a toroidal solenoid thin-film transformer based on an alloy strip magnetic core according to claim 6, characterized in that, The thickness of the fourth copper film is 100–300 nm, and the thickness of the fifth copper film is 8–20 µm.