LDMOS (Laterally Diffused Metal Oxide Semiconductor) device with variable-angle funnel-shaped field oxide structure

Abstract

The invention discloses an LDMOS (Laterally Diffused Metal Oxide Semiconductor) device with a variable-angle funnel-shaped field oxide structure. The STI side wall angle is regulated and controlled in a segmented mode, an inclination angle structure is introduced to the position close to a high-voltage drift region, a gradually-changed field oxygen boundary is formed, transverse expansion and gradient slow release of an electric field at the edge of the drift region can be achieved, the electric field concentration effect at the STI corner is remarkably weakened, and the vertical boundary of the lower portion maintains the structural stability; and stress concentration caused by deep groove etching is avoided. Finally, compared with a traditional STI LDMOS structure with a single side wall angle, breakdown voltage (BV) can be partially improved, on resistance (Ron) of a drift region can be remarkably reduced, collision ionization rate on a drain electrode-field oxygen interface is reduced, a hot carrier injection (HCI) effect is remarkably inhibited, and the service life of a device is prolonged.

Description

Technical Field

[0001] The present invention belongs to the technical field of semiconductor devices, and in particular relates to an LDMOS device with a variable-angle funnel-shaped field oxide structure. Background Art

[0002] As electronic systems rapidly advance toward higher power density and miniaturization, optimizing the performance of power semiconductor devices is crucial for improving energy conversion efficiency. Among various power devices, high-voltage switching elements based on lateral layouts have attracted considerable attention due to their low losses and high integration potential. However, as operating voltages increase and chip area decreases, traditional device architectures face significant challenges in electric field regulation and carrier transport efficiency, necessitating structural innovation to achieve a performance leap.

[0003] Among the current mainstream power device designs, laterally diffused metal-oxide-semiconductor (LDMOS) is widely used in fields such as intelligent power management, RF amplification, and automotive electronics due to its compatibility with standard integrated circuit processes. Traditional LDMOS uses lateral diffusion to form a channel structure, with the source, drain, and gate electrodes all located on the wafer surface, facilitating coplanar integration with other devices. However, with the scaling of process nodes, the core contradiction facing LDMOS lies in how to synergistically optimize the off-state breakdown voltage (BV) and on-state resistance (Ron). Specifically, while a low-doped drift region can improve voltage resistance, it will increase on-resistance due to insufficient carrier concentration. Conversely, while high doping can reduce resistance, it will exacerbate the electric field concentration effect and weaken the breakdown characteristics.

[0004] To improve isolation performance, shallow trench isolation (STI) is commonly used instead of traditional localized oxidation (LOCOS) technology. STI achieves electrical isolation by etching deep trenches and filling them with oxide, offering excellent planarization and dimensional control. However, the interface characteristics between STI and the silicon substrate are complex, particularly at the junction of the drift region and the isolation structure. Differences in dielectric constant and thermal expansion coefficient can easily induce electric field distortion and localized stress concentration, leading to reliability issues. For example, traditional vertical-boundary STI generates sharp electric field peaks under high-voltage conditions, resulting in a drop in breakdown voltage and an exacerbation of hot carrier injection (HCI). Furthermore, the fixed-angle STI structure makes it difficult to achieve lateral electric field diffusion, further limiting the balance of device performance.

[0005] Existing technologies attempt to optimize electric field distribution through methods such as field plate structures, gradient doping, or localized implantation, but these solutions often come at the expense of conduction characteristics. For example, while segmented doping can alleviate electric field concentration, it introduces additional resistive paths; while field plate structures can extend the electric field distribution, they increase parasitic capacitance and process complexity. Consequently, traditional LDMOS struggles to overcome the trade-off between breakdown voltage and on-resistance, necessitating structural innovation to achieve synergistic optimization. Summary of the Invention

[0006] The object of the present invention is to overcome the above-mentioned deficiencies in the prior art and to provide an LDMOS device with a variable-angle funnel-shaped field oxide structure.

[0007] The present invention provides an LDMOS device with a variable-angle funnel-shaped field oxide structure, comprising a substrate (1), a drift region (2), a field oxide structure (3), a first well region (4), a second well region (5), a gate (7), a body region (8), a source (10), a drain (11) and a body electrode (12): the drift region (2) and the first well region (4) are respectively formed on both sides of the substrate (1), and there is a distance between the first well region (4) and the drift region (2); the field oxide structure (3) is formed in the drift region (2); the second well region (5) is formed in the drift region (2); the gate (7) is formed above the substrate (1); the body region (8) is formed in the first well region (4); the source (10) and the body electrode (12) are formed in the body region (8); and the drain (11) is formed in the second well region (5); Wherein, the field oxygen structure (3) adopts a variable angle funnel shape.

[0008] Preferably, a gate oxide layer (6) is formed between the gate (7) and the substrate (1); and sidewalls (9) are respectively formed on both sides of the gate (7).

[0009] Preferably, the doping concentration of the second well region (5) is lower than that of the first well region (4).

[0010] Preferably, the field oxide structure (3) extends to a portion of the top region of the second well region (5) and the non-second well region (5) region.

[0011] Preferably, the drain (11) is close to the field oxide structure (3); the body (12) is located outside the source (10), and the two are short-circuited.

[0012] Preferably, the field oxygen structure (3) is an axisymmetric structure, and the upper vertex angle is smaller than the lower vertex angle; more preferably, the upper vertex angle is less than 90 degrees, and the lower vertex angle is 90 degrees.

[0013] Preferably, the field oxygen structure (3) is an integrally formed structure, comprising an upper part (3-1) and a lower part (3-2) configured from top to bottom; the upper part (3-1) adopts an inverted isosceles trapezoidal structure, and the lower part (3-2) adopts a rectangular structure; the lower side of the upper part (3-1) and the upper side of the lower part (3-2) are of equal length and overlap.

[0014] Preferably, the thickness of the upper portion (3-1) is thicker than the thickness of the lower portion (3-2).

[0015] Preferably, one end of the gate (7) is located above a partial area of ​​the field oxide structure (3), and the other end is located above a partial area of ​​the body region (8).

[0016] The beneficial effects of the present invention include at least: The LDMOS device of the present invention utilizes segmented STI sidewall angle control. A canted-angle structure (i.e., the upper portion) is introduced near the high-voltage drift region, forming a gradual field-oxide boundary. This allows for lateral expansion and gradient release of the electric field at the drift region edge, significantly reducing the electric field concentration effect at the STI corner. The vertical boundary in the lower portion maintains structural stability, preventing stress concentration caused by deep trench etching. This reduces the impact ionization rate at the drain-field-oxide interface, significantly suppresses hot carrier injection (HCI), and extends device life. Compared to traditional STI LDMOS structures with a single STI sidewall angle, this novel structure significantly reduces the on-resistance (Ron) of the drift region while partially improving the breakdown voltage (BV). This balances the integrated design, electrical performance, and reliability of power devices, making it a key structural innovation for high-voltage BCD platforms. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the technical solution of the present invention, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0018] Figure 1 3 is a schematic structural diagram of an LDMOS device with a variable-angle funnel-shaped field oxide structure (denoted as anglevary2sti) provided by an embodiment of the present invention.

[0019] Figure 2 It is a structural diagram of an LDMOS device with a trapezoidal field oxide structure of the same thickness (denoted as 2noanglevary2sti).

[0020] Figure 3 It is a structural diagram of an LDMOS device with a trapezoidal field oxide structure at the same angle (denoted as 1noanglevary2sti).

[0021] Figure 4 It is the preparation process flow of LDMOS device anglevary2sti.

[0022] Figure 5These are the BV breakdown characteristic curves of three different field oxide structure LDMOS devices (anglevary2sti, 2noanglevary2sti, 1noanglevary2sti).

[0023] Figure 6 These are the Id–Vds conduction characteristic curves of three different field oxide structure LDMOS devices (anglevary2sti, 2noanglevary2sti, 1noanglevary2sti).

[0024] Figure 7 This is the impact ionization rate distribution diagram of the LDMOS device (anglevary2sti).

[0025] Figure 8 It is the impact ionization rate distribution diagram of LDMOS device (2noanglevary2sti).

[0026] Figure 9 It is the impact ionization rate distribution diagram of LDMOS device (1noanglevary2sti).

[0027] Figure 10 It is the electric field intensity distribution diagram of LDMOS device (anglevary2sti).

[0028] Figure 11 It is the electric field intensity distribution diagram of LDMOS device (2noanglevary2sti).

[0029] Figure 12 It is the electric field intensity distribution diagram of LDMOS device (1noanglevary2sti).

[0030] Markings in the figure: 1. Substrate; 2. Drift region; 3. Field oxide structure; 3-1. Upper part; 3-2. Lower part; 4. First well region; 5. Second well region; 6. Gate oxide layer; 7. Gate; 8. Body region; 9. Sidewall; 10. Source; 11. Drain; 12. Body. DETAILED DESCRIPTION

[0031] The technical solutions in the embodiments of the present invention will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present invention.

[0032] In the description of the present invention, it should be understood that the terms "length," "thickness," "upper," "lower," "top," "bottom," and the like, indicating positions or locations, are based on the positions or locations shown in the accompanying drawings and are intended solely to facilitate and simplify the description of the present invention. They are not intended to indicate or imply that the devices or components referred to must have, be constructed, or operate in a specific orientation, and therefore should not be construed as limitations on the present invention. A person of ordinary skill in the art will understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0033] like Figure 1 As shown, an embodiment of the present invention provides an LDMOS device with a variable-angle funnel-shaped field oxide structure, denoted as anglevary2sti, to resolve the contradiction that when the STI thickness is increased to increase the off-state breakdown voltage of the STI LDMOS, the on-resistance will be greatly increased. The STI sidewall angle is segmented and controlled, and a tilted angle structure is introduced near the high-voltage drift region 2 to form a gradual field oxide boundary. This can achieve lateral expansion and gradient release of the electric field at the edge of the drift region 2, significantly weakening the electric field concentration effect at the STI corner. Specifically, the anglevary2sti device includes a substrate 1, a drift region 2, a field oxide structure 3, a first well region 4, a second well region 5, a gate 7, a body region 8, a source 10, a drain 11 and a body electrode 12: the drift region 2 and the first well region 4 are respectively formed on both sides of the substrate 1, and there is a distance between the first well region 4 and the drift region 2; the field oxide structure 3 is formed in the drift region 2; the second well region 5 is formed in the drift region 2; the gate 7 is formed above the substrate 1; the body region 8 is formed in the first well region 4; the source 10 and the body electrode 12 are formed in the body region 8; the drain 11 is formed in the second well region 5; wherein, the field oxide structure 3 adopts a variable angle funnel shape.

[0034] A gate oxide layer 6 is formed between the gate 7 and the substrate 1 ; and sidewall spacers 9 are formed on both sides of the gate 7 .

[0035] In one embodiment, the doping concentration of the second well region 5 is lower than that of the first well region 4 .

[0036] In one embodiment, the field oxide structure 3 extends to a portion of the top region of the second well region 5 and the region other than the second well region 5 .

[0037] In one embodiment, the drain 11 is close to the field oxide structure 3; the body electrode 12 is located outside the source 10, and the two are short-circuited.

[0038] Specifically, the field oxygen structure 3 is an axisymmetric structure, and the upper vertex angle α1 is smaller than the lower vertex angle α2. The upper vertex angle α1 of the field oxygen structure 3 is less than 90 degrees, and the lower vertex angle α2 is 90 degrees. For example, in this embodiment, α1=68° and α1=90°. As an example, the field oxygen structure 3 is an integrally formed structure, including an upper part 3-1 and a lower part 3-2 configured from top to bottom; the upper part 3-1 adopts an inverted isosceles trapezoidal structure, and the lower part 3-2 adopts a rectangular structure; the lower side of the upper part 3-1 is equal to and coincides with the upper side of the lower part 3-2. More specifically, the thickness of the upper part 3-1 is thicker than the thickness of the lower part 3-2. In this embodiment, the thickness h1 of the upper part 3-1 is 0.35 um, the upper side length L1 is 3.7 um, the lower side length L1 is 1.9 um, the thickness h2 of the lower part 3-2 is 0.09 um, and the upper and lower side lengths are both 1.9 um.

[0039] In one embodiment, one end of the gate 7 is located above a portion of the field oxide structure 3 , and the other end of the gate 7 is located above a portion of the body region 8 .

[0040] This embodiment also provides a preparation process of the LDMOS device anglevary2sti, see the attached Figure 4 include: Step 1, doping the silicon substrate 1; Step 2: A photoresist mask may be used to define a drift region pattern, and the drift region 2 is doped (N-type doping to form an N-type drift region 2) on the doped substrate 1. Step 3: Etch in two steps to form field oxide structure 3 (i.e. shallow trench isolation): The upper portion 3-1 of the field oxide structure 3 is first etched to form the lower portion 3-2, and then further etched to form the lower portion 3-2 through the added SSTI mask, and finally uniformly filled with oxide; the upper portion 3-1 of the inverted isosceles trapezoidal structure and the lower portion 3-2 of the rectangular structure are assembled into a variable angle funnel shape.

[0041] Step 4: Boron (B) ion implantation can be used to form a first well region 4 (P-type Well region) with a slightly higher doping concentration, and low-dose phosphorus implantation can be used to form a second well region 5 (N-type Well region) with a slightly lower doping concentration. Step 5: growing a gate oxide layer 6 (SiO2); Step 6: deposit a layer of polysilicon and etch to form a gate 7. The raised portion of the gate 7 can be smoothed by CMP (chemical mechanical polishing); Step 7: Locally implant boron to form a body region 8 (e.g., a P-type body region) with a slightly higher doping concentration. Step 8: deposit silicon dioxide (SiO2) and silicon nitride (Si3N4) on both sides of the gate and form sidewalls 9 by anisotropic etching; Step 9: Through a self-aligned process, the source 10, the drain 11 and the body 12 are implanted.

[0042] At the same time, this embodiment also provides a comparative structure: an LDMOS device with a trapezoidal field oxide structure of the same thickness, denoted as 2noanglevary2sti; an LDMOS device with a trapezoidal field oxide structure of the same angle, denoted as 1noanglevary2sti; Compared with the anglevary2sti device of this embodiment, the field oxygen structure of the 2noanglevary2sti device adopts an inverted isosceles trapezoidal structure with the same thickness h1+h2 as the variable angle funnel-shaped field oxygen structure 3, and the lengths of the upper and lower bases are respectively the same as the upper and lower bases of the variable angle funnel-shaped field oxygen structure 3, and the top angle α3 is 77.8°, see the attached Figure 2 The field oxygen structure of the 1noanglevary2sti device adopts an inverted isosceles trapezoidal structure with the same thickness h1 as the upper part 3-1 and an angle α4 equal to α1. The lengths of the upper and lower bases are the same as the upper and lower bases of the variable angle funnel-shaped field oxygen structure 3, and the top angle α4 is 68°. Figure 3 .

[0043] Analysis of the breakdown conduction characteristics advantages of three types of devices: 1. Anglevary2sti device compared to 2noanglevary2sti device: like Figure 5 As shown in the BV curve, under the same drift region thickness and doping conditions, anglevary2sti successfully controls the spatial diffusion of the drift region electric field by virtue of its smaller bevel angle design (α1 is 68°) of the upper STI (i.e., the upper part 3-1), allowing the high electric field region to extend more smoothly to the bottom of the gate, thereby avoiding electric field concentration and reducing local stress concentration ( Figure 7 Electric field distribution diagram). This results in a breakdown voltage slightly higher than that of 2noanglevary2sti, by approximately 2-3V. The STI angle design of anglevary2sti (i.e., upper portion 3-1) redistributes the electric field and reduces local stress concentration, which not only increases the breakdown voltage but also improves device reliability and ESD immunity.

[0044] At the same time, through the Id-Vds curve ( Figure 6) As can be seen, the overall on-current curve for anglevary2sti is slightly higher than that for 2noanglevary2sti, indicating lower on-resistance. This is due to the fact that the angled STI (i.e., upper portion 3-1) reduces the steepness of the potential barrier in the vertical direction of the drift region, optimizing the carrier channel and facilitating smoother electron transport. Therefore, this structure effectively reduces on-resistance while maintaining a high breakdown voltage, demonstrating a significant synergistic optimization effect and significantly improving the LDMOS device power figure of merit. The three devices (anglevary2sti, 2noanglevary2sti, and 1noanglevary2sti) achieve power figures of merit of 167, 158, and 156, respectively.

[0045] Figure 8 、 Figure 11 They are the collision ionization rate and electric field intensity distribution diagrams of the 2noanglevary2sti device respectively.

[0046] Figure 10 It is the electric field intensity distribution diagram of anglevary2sti device.

[0047] 2. Anglevary2sti device compared to 1noanglevary2sti device: 1noanglevary2sti uses the same upper STI tilt angle (i.e., upper portion 3-1), but the overall STI depth is relatively shallow, meaning there is no right-angle portion (i.e., lower portion 3-2). Due to its shallow STI structural design, the current path is shorter, resulting in a smaller specific on-resistance in the on state ( Figure 6 The yellow curve in the middle is significantly higher than the other two devices), but at the same time, its electric field peak is more concentrated ( Figure 12 The electric field peak is close to the source-gate junction), causing the breakdown point to move forward, in the avalanche breakdown region ( Figure 9 ) The impact ionization rate increases significantly, ultimately making its breakdown voltage significantly lower than that of anglevary2sti and 2noanglevary2sti. At the same time, the comparison shows that the impact ionization of anglevary2sti is significantly reduced at the source / STI corner and in the electric field strong area (the color is more yellow-green). Figure 7 As shown, 1noanglevary2sti has high impact ionization in the red area in the drift region and gate edge area, which is the hot spot of hot carrier injection. Therefore, anglevary2sti effectively alleviates the hot electron accumulation area, suppresses the avalanche breakdown effect and HCI failure, and improves reliability.

[0048] While maintaining the angled design (i.e., upper portion 3-1), anglevary2sti achieves longitudinal diffusion of the electric field distribution by increasing the vertical depth and angle transition section of the STI (i.e., lower portion 3-2). This not only mitigates the electric field peak but also expands the electric field buffer zone, significantly improving the breakdown voltage at a slight expense of on-resistance (the blue line is slightly lower than the yellow line).

[0049] Table 1: Comparison of characteristics of three different field oxide structures LDMOS devices

[0050] In summary, the comparison results show that anglevary2sti achieves comprehensive optimization of breakdown voltage and on-resistance, achieving a better balance between reliability and performance.

[0051] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. An LDMOS device having a variable-angle funnel-shaped field oxide structure, characterized in that: The LDMOS device comprises a substrate (1), a drift region (2), a field oxide structure (3), a first well region (4), a second well region (5), a gate (7), a body region (8), a source (10), a drain (11) and a body electrode (12): the drift region (2) and the first well region (4) are respectively formed on both sides of the substrate (1), and there is a distance between the first well region (4) and the drift region (2); the field oxide structure (3) is formed in the drift region (2); the second well region (5) is formed in the drift region (2); the gate (7) is formed above the substrate (1); the body region (8) is formed in the first well region (4); the source (10) and the body electrode (12) are formed in the body region (8); and the drain (11) is formed in the second well region (5); Wherein, the field oxygen structure (3) adopts a variable angle funnel shape.

2. The LDMOS device according to claim 1, wherein: A gate oxide layer (6) is formed between the gate (7) and the substrate (1); and sidewalls (9) are respectively formed on both sides of the gate (7).

3. The LDMOS device according to claim 1, wherein: The doping concentration of the second well region (5) is lower than that of the first well region (4).

4. The LDMOS device according to claim 1, wherein: The field oxide structure (3) extends to a portion of the top region of the second well region (5) and a region other than the second well region (5).

5. The LDMOS device according to claim 1, wherein: The drain (11) is close to the field oxide structure (3); the body (12) is located outside the source (10), and the two are short-circuited.

6. The LDMOS device according to claim 1, wherein: The field oxygen structure (3) is an axisymmetric structure, and the upper vertex angle is smaller than the lower vertex angle.

7. The LDMOS device according to claim 6, characterized in that: The upper vertex angle of the field oxygen structure (3) is less than 90 degrees, and the lower vertex angle is 90 degrees.

8. The LDMOS device according to claim 7, characterized in that: The field oxygen structure (3) is an integrally formed structure, comprising an upper part (3-1) and a lower part (3-2) configured from top to bottom; the upper part (3-1) adopts an inverted isosceles trapezoidal structure, and the lower part (3-2) adopts a rectangular structure; the lower side of the upper part (3-1) and the upper side of the lower part (3-2) are of equal length and overlap.

9. The LDMOS device according to claim 8, characterized in that: The thickness of the upper portion (3-1) is greater than the thickness of the lower portion (3-2).

10. The LDMOS device according to claim 1, wherein: One end of the gate (7) is located above a partial area of ​​the field oxide structure (3), and the other end is located above a partial area of ​​the body region (8).

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