MEMS structure polyimide removal method based on two-step dry etching and application

Abstract

The invention discloses an MEMS structure polyimide removal method based on two-step dry etching and application, and belongs to the technical field of MEMS device manufacturing. The method comprises the following steps: carrying out first dry etching on polyimide at 120-200 DEG C by utilizing first oxygen plasma formed by ionization of oxygen and H2 / N2 mixed gas, and processing for 15-25 minutes to realize rapid mass removal; and performing second dry etching on the residual polyimide at 80-100 DEG C by using second oxygen plasma formed by ionization of oxygen and fluorine-containing gas, and processing for 10-60 seconds to realize accurate residue removal. According to the method, a synergistic effect is generated through specific sequential combination of two functional complementary gas systems, the technical problem that the removal rate of polyimide and the nondestructive property of a microstructure in a high aspect ratio (greater than or equal to 3: 1) MEMS structure cannot be achieved at the same time is successfully solved, sensitive structures such as a vanadium oxide thermosensitive layer and a metal bridge leg are perfectly protected while polyimide is thoroughly removed, and the service life of the MEMS structure is prolonged. And the method is particularly suitable for manufacturing microbridge structure devices such as high-performance uncooled infrared detectors and the like.

Description

Technical Field

[0001] This invention belongs to the field of MEMS device manufacturing technology, specifically relating to a two-step dry etching method for removing polyimide from MEMS structures. This method is particularly suitable for the manufacturing of microbridge structure devices such as uncooled infrared detectors and thermal imaging chips, which have extremely high requirements for structural integrity. Background Technology

[0002] Polyimide (PI) is widely used as a sacrificial layer material in MEMS devices due to its excellent mechanical and thermal stability. After fulfilling its supporting function, it needs to be completely removed to release suspended microstructures (such as microbridges and cantilever beams). However, as MEMS devices develop towards higher integration and more complex geometries, the aspect ratio of internal structures is constantly increasing (commonly ≥3:1). For such high aspect ratio suspended regions or narrow gaps, traditional resist removal processes face severe challenges:

[0003] Wet cleaning processes (such as CN103092009A) have high liquid surface tension, making it difficult to penetrate and wet the deep parts of the microstructure, resulting in polyimide residue. At the same time, the capillary forces generated during the drying process can easily cause structural adhesion or collapse, leading to device failure.

[0004] Single dry ashing processes (such as pure oxygen plasma) have slow resist removal rates and low selectivity when dealing with thick or highly cross-linked polyimides. Extending the etching time to completely remove deep residues can cause over-etching damage to the underlying or surrounding sensitive functional layers (such as metal bridges, vanadium oxide thermistor layers, and silicon nitride insulating layers).

[0005] While existing improved dry etching processes attempt to introduce auxiliary gases (such as H2 / N2 mixtures or fluorine-containing gases) to enhance resist removal, most are bulk etching processes operating in a single-gas environment. When dealing with high aspect ratio structures, the etching capability of these methods decreases sharply with depth, making it difficult to balance thorough removal with structural non-destructiveness. They may fail to effectively remove cross-linking residues in narrow areas such as bridge legs and the bottom of microcavities; or, due to the introduction of highly chemically reactive etching gases (such as CF4), they may easily damage sensitive materials such as metals. Based on the prevalent technical bias in this field that "introducing fluorine-containing gases inevitably damages metals," to avoid damage, the use of fluorine-containing gases is usually avoided in highly sensitive MEMS structures, or they are only used in areas insensitive to metals.

[0006] Therefore, there is an urgent need in this field for a fully dry, highly selective, and non-destructive polyimide removal process specifically designed for high aspect ratio MEMS structures, which can efficiently remove the sacrificial layer while ensuring the integrity and performance stability of the microstructure. Summary of the Invention

[0007] The purpose of this invention is to provide a fully dry, highly selective, and non-destructive polyimide removal process specifically designed for high aspect ratio MEMS structures.

[0008] In a first aspect, the present invention provides a method for removing polyimide from MEMS structures based on a two-step dry etching process, specifically employing the following technical solution:

[0009] A method for removing polyimide from MEMS structures based on a two-step dry etching process, comprising the following steps:

[0010] Step 1: Perform the first dry etching on the polyimide sacrificial layer of the MEMS structure in the first oxygen plasma environment of the first process chamber. The process conditions are: temperature 120-200℃, processing time 15-25min; the process gas used to ionize and form the first oxygen plasma includes oxygen and H2 / N2 mixed gas, and the source power of the plasma source is 750-900W.

[0011] Step 2: Transfer the MEMS structure processed in Step 1 from the first process cavity to the second process cavity;

[0012] Step 3: Perform a second dry etching on the MEMS structure processed in Step 1 in the second oxygen plasma environment of the second process chamber. The process conditions are: temperature 80-100℃, processing time 10-60s. The process gas used to ionize and form the second oxygen plasma includes oxygen and fluorine-containing gas. The source power of the plasma source is 750-900W.

[0013] The execution order of steps one and three is irreversible; the first oxygen plasma and / or the second oxygen plasma are generated by a radio frequency plasma source; and both steps two and three are performed when the water vapor partial pressure is ≤1.0×10⁻⁶. -3 Performed under the condition of torr.

[0014] This invention achieves an unexpected synergistic effect by combining two traditionally contradictory gas systems—a hydrogen-nitrogen mixed gas removal system and a fluorine-containing gas removal system—in a specific sequence. Not only does it achieve complete removal of polyimide from high aspect ratio structures, but the third step, fluorine-containing gas etching, selectively attacks residues without damaging exposed sensitive microstructures under extremely short time (10-60 seconds) and low temperature (80-100°C), successfully resolving the long-standing technical contradiction in the industry of the incompatibility between "removal rate" and "structural integrity." It should be noted that steps two and three are performed when the chamber water vapor partial pressure is ≤1.0×10⁻⁶. -3 The etching process is carried out under torr conditions to avoid accelerating metal corrosion due to moisture absorption and causing uncontrollable etching.

[0015] Furthermore, the H2 / N2 mixed gas is composed of hydrogen and nitrogen, with hydrogen accounting for 3%-4% by volume. This proportion of hydrogen not only fully utilizes the reducing effect of hydrogen, working synergistically with reactive oxygen species in the plasma to efficiently decompose the high-molecular-weight polyimide into volatile small molecules, thus achieving rapid volume removal, but also effectively suppresses the risks of plasma instability, over-polymerization, or hydrogen damage to sensitive metal layers that may be caused by excessive hydrogen concentration. This ensures the optimal balance between efficiency and safety in the first step of the process, laying a reliable foundation for the precise residue removal in the third step.

[0016] Furthermore, the flow rate ratio of oxygen and H2 / N2 mixed gas used for ionization to form the first oxygen plasma is (6-30):1. Sufficient oxygen ensures efficient oxidative breakage of the polyimide backbone, while the appropriate ratio of hydrogen and nitrogen mixed gas not only effectively inhibits the potential damage to the microstructure caused by excessive oxidation, but its reducing atmosphere also significantly promotes the disintegration and volatilization of the deep cross-linked structure.

[0017] Furthermore, the flow rate ratio of oxygen to fluorine-containing gas used for ionization to form the second oxygen plasma is (25-100):1. The high proportion of oxygen not only ensures the complete oxidative decomposition of the residual polyimide carbon skeleton, but also creates a strongly oxidizing environment, which greatly inhibits the tendency of fluorine-containing gas to corrode sensitive structures such as metal bridge legs. This allows the low proportion of fluorine-containing gas to precisely target and attack highly cross-linked carbides that are difficult to remove with conventional oxygen in this environment, achieving "selective fluorination under oxidizing conditions" and solving the technical dilemma in traditional processes that "introducing fluorine will inevitably damage the metal structure".

[0018] Furthermore, the fluorine-containing gas is CF4. The highly symmetrical molecular structure of carbon tetrafluoride enables it to stably generate a suitable amount of moderately reactive fluorine radicals in plasma. Under the "guidance" of excess oxygen, these fluorine radicals can selectively react with cross-linked carbon residues, while hardly interacting with exposed metal surfaces. At the same time, no long-chain fluorocarbon polymer byproducts are generated during the reaction, thus avoiding the formation of harmful deposits on the microstructure surface from the source.

[0019] Furthermore, in steps one and / or three, the chamber pressure is maintained at 0.7–1.2 Torr. This stable low-pressure environment also promotes the effective removal of reaction byproducts and prevents the formation of a “micromask” effect in microstructures with high aspect ratios.

[0020] Furthermore, the MEMS structure includes at least one overhanging region or gap with an aspect ratio greater than 3:1, which is particularly suitable for solving the problem of polyimide (DRIE-based inertial sensors, optical MEMS, etc.) removal in MEMS microbridges, microcavities, or cantilever beam structures with aspect ratios greater than 5:1 (high-performance micro-gyroscopes, complex three-dimensional MEMS structures, uncooled infrared detectors, etc.), especially greater than 10:1. Unless otherwise specified, in this application, "aspect ratio" refers to the ratio of the depth (or height) of the structure to its minimum width; the calculation formula is: Aspect ratio = Depth / Minimum width, where "depth" refers to the vertical distance from the opening of the structure to its bottom, and "minimum width" refers to the dimension at the narrowest point of the opening or channel of the structure.

[0021] Furthermore, the MEMS structure includes at least one of a vanadium oxide thermistor layer, a silicon nitride capping layer, and a metal leg structure.

[0022] Secondly, this invention provides an application of the aforementioned two-step dry etching-based polyimide removal method for MEMS structures in the fabrication of microbridge structure devices. Traditional processes, when releasing the sacrificial layer beneath the microbridge, are prone to causing bridge surface warping and thinning of the bridge leg metal due to stress or excessive etching, thereby degrading device performance. This invention, while thoroughly removing the polyimide under the bridge and around the bridge legs, maintains the intrinsic stress state of the microbridge's ultrathin film layer and the geometric morphology of the bridge legs. This results in a microbridge structure with structural integrity and performance consistency, providing a technological foundation for the mass production of high-performance uncooled infrared focal plane arrays and other devices.

[0023] The present invention has the following beneficial effects:

[0024] Firstly, this invention constructs a division-of-labor etching path of "first bulk removal, then precise residue removal" by combining two complementary gas combinations (O2 / H2 / N2 and O2 / CF4) in a specific order, thus achieving a balance between "polyimide removal rate" and "microstructure non-destructiveness" in the manufacturing of high aspect ratio MEMS structures.

[0025] Secondly, in step three of this invention, the introduction of trace amounts of fluorine-containing gas in a strong oxidizing environment unexpectedly achieved targeted removal of stubborn residues, while effectively inhibiting the erosion of sensitive functional layers (such as metals and vanadium oxide).

[0026] Thirdly, the present invention adopts a single-chamber, constant-pressure continuous operation, which avoids pollution and damage caused by sudden environmental changes, significantly improves process stability, repeatability and production efficiency, and lays the foundation for industrial application.

[0027] Fourth, the method of the present invention is particularly suitable for solving the problem of polyimide removal in complex structures such as high aspect ratio microbridges, providing a reliable process guarantee for the manufacture of high-performance uncooled infrared detectors and other devices. Attached Figure Description

[0028] Figure 1a Figure 1 shows the effect of removing polyimide adhesive from the MEMS structure in Example 1.

[0029] Figure 1b Figure 2 shows the effect of removing polyimide adhesive from the MEMS structure in Example 1;

[0030] Figure 1c The effect of removing polyimide adhesive from MEMS structure in Example 1 Figure 3 ;

[0031] Figure 1d Figure 4 shows the effect of removing polyimide adhesive from the MEMS structure in Example 1;

[0032] Figure 2a Figure 1 shows the effect of removing polyimide adhesive from the MEMS structure in Example 1.

[0033] Figure 2b Figure 2 shows the effect of removing polyimide adhesive from the MEMS structure in Comparative Example 1;

[0034] Figure 3 This is a comparison image of the effect of removing polyimide adhesive from MEMS structures in Example 2;

[0035] Figure 4a Figure 1 shows the effect of removing polyimide adhesive from MEMS structures, as shown in Example 3.

[0036] Figure 4b Figure 2 shows the effect of removing polyimide adhesive from MEMS structures, as shown in Example 3.

[0037] Figure 5a Figure 1 shows the effect of removing polyimide adhesive from MEMS structures, as shown in Example 6.

[0038] Figure 5b Figure 2 shows the effect of removing polyimide adhesive from MEMS structures, as shown in Example 6.

[0039] Figure 5c As a comparison of the polyimide adhesive removal effect on MEMS structures in Example 6 Figure 3 ;

[0040] Figure 5d Figure 4 shows the effect of removing polyimide adhesive from MEMS structures, as compared to Example 6. Detailed Implementation

[0041] To enable those skilled in the art to better understand the technical solutions in this invention, the present application will be further described in detail below.

[0042] [Example 1]

[0043] A two-step dry etching method for removing polyimide from MEMS structures, the specific steps of which are as follows:

[0044] (I) MEMS Chip Preparation

[0045] After completing the patterning of the polyimide and the construction of the microbridge structure, the MEMS chip structure undergoes a resist removal process. The MEMS chip structure comprises a readout circuit substrate, a polyimide layer to be released on the readout circuit substrate, and MEMS microstructures on the polyimide layer. The MEMS microstructures include a metal reflective layer, piers, arms, and a bridge deck. The arms electrically connect the piers and the bridge deck, and the bridge deck includes a vanadium oxide thermistor layer. The polyimide layer is formed by coating, drying, and curing a polyimide resin solution (e.g., P12610), serving as a sacrificial layer to support the microbridge structure above. This structure includes a suspended region with an aspect ratio of 5:1.

[0046] (II) Two-step dry etching

[0047] A radio frequency (RF) plasma system is used to perform two-step etching in different reaction chambers to release the polyimide sacrificial layer. The plasma source (RF power generator, frequency 13.56MHz) is located outside the main process chamber and the excited plasma is guided to the wafer surface through a waveguide, avoiding direct exposure of the processed object to the high-energy discharge region, thereby minimizing the physical damage of the plasma to sensitive microstructures.

[0048] The specific steps of dry etching are as follows:

[0049] Step 1: Introduce process gases oxygen (O2) and a 4% H2 / N2 mixture into the RF plasma system. The O2 flow rate is 2700 sccm, and the 4% H2 / N2 mixture flow rate is 300 sccm. Set the plasma source power to 800W, the chamber temperature to 180℃, the pressure to 1.1 Torr, and the processing time to 20 minutes. During this process, the RF plasma provides a high concentration of active oxygen atoms, and with the assistance of the H2 / N2 mixture, ensures the efficient decomposition of polyimide.

[0050] Step 2: Transfer the MEMS structure processed in Step 1 from the first processing cavity to the second processing cavity. The transfer of the MEMS structure can be achieved using conventional techniques, such as: setting up a transfer cavity between the first and second processing cavities, and installing a vacuum robotic arm within the transfer cavity; before transferring the MEMS structure, adjusting the pressure in the transfer cavity; then, connecting the first processing cavity and the transfer cavity, and using the vacuum robotic arm to transfer the MEMS structure from the first processing cavity to the transfer cavity; subsequently, disconnecting the connection between the first processing cavity and the transfer cavity, adjusting the pressure, and connecting the second processing cavity and the transfer cavity; finally, using the vacuum robotic arm to transfer the MEMS structure from the transfer cavity to the second processing cavity. The vacuum robotic arm is not the focus of this invention; any vacuum robotic arm suitable for transferring MEMS structures under vacuum conditions is acceptable, and its specific construction will not be described here.

[0051] Step 3: Next, process gases oxygen (O2) and carbon tetrafluoride (CF4) are introduced into the second process chamber, with an O2 flow rate of 1500 sccm and a CF4 flow rate of 40 sccm. The plasma source power is set to 750W, the chamber temperature to 90°C, the pressure to 1.0 Torr, and the processing time to 30 seconds. In this precise residue removal step, remote radio frequency plasma ensures that CF4 is efficiently dissociated into an appropriate amount of fluorine radicals. Simultaneously, under oxygen-rich conditions and low ion bombardment, these fluorine radicals selectively react with residues without significantly corroding exposed metal bridge legs and other structures. To avoid the problem of accelerated metal corrosion due to moisture adsorption, both steps 2 and 3 are performed under conditions where the water vapor partial pressure within the chamber is consistently below 1.0 x 10⁻³ Torr.

[0052] (III) Results and Testing

[0053] To observe the residual state of polyimide within the hollowed-out structure after release and the integrity of the microstructure, the MEMS chip structure processed in this embodiment was tested.

[0054] Reference Figures 1a to 1d Scanning electron microscopy (SEM) observation of the bottom of the chip structure revealed no flocculent or granular polyimide residue on the bottom where the metal reflective layer is located, indicating that the polyimide layer has been completely removed. The SEM images show that all microstructures, including the vanadium oxide bridge deck, as well as the bridge arms and piers, are intact, without tilting or collapse. In particular, the morphology of the metal bridge legs remains intact, without thinning, bulging, or over-etching. This demonstrates that the two-step dry etching process employed in this invention completely removes the polyimide without causing etching damage to the surrounding sensitive MEMS microstructures.

[0055] [Examples 2-3]

[0056] Examples 2-3 are all based on Example 1, differing only in the setting of process parameters, as detailed in Table 1:

[0057] Table 1 - Two-step dry etching process parameters

[0058]

[0059] The experimental results of Examples 2-3 show that the polyimide was completely removed, the MEMS structure was extremely intact, and the microbridge structure was undamaged, indicating that the method remains stable and effective even when the parameters are adjusted within the scope of this invention.

[0060] [Examples 4-6]

[0061] Examples 4-6 are all based on Example 1, with the only difference being the aspect ratio of the MEMS structure: the aspect ratios of the MEMS structures in Examples 4-6 are 3:1, 8:1, and 12:1, respectively.

[0062] Experimental results show that: in Example 4, no polyimide residue was found and the MEMS structure remained intact, proving that the method of the present invention is effective for conventional high aspect ratio structures; in Example 5, the polyimide was completely removed without residue, the structure did not collapse, and the bottom sensitive functional layer remained intact; in Example 6, under ultra-high aspect ratio conditions, the present invention can still achieve complete removal of polyimide with zero structural damage, demonstrating its ability to handle extreme structures.

[0063] [Compare with Example 1]

[0064] In Comparative Example 1, only the same step one as in Example 1 was performed, but the processing time was extended to 20.5 min, and steps two and three were not performed.

[0065] Experimental results show that: (refer to) Figure 2a and Figure 2b Obvious polyimide residue ("skirt" effect) can be observed; at the same time, the device resistance test is abnormal and the performance is unqualified.

[0066] [Compare with Example 2]

[0067] In Comparative Example 2, only step three of the same process as in Example 1 was performed, but the processing time was extended to 20.5 min, and steps one and two were omitted.

[0068] Experimental results showed that the polyimide substrate was not effectively etched, leaving significant residual adhesive. (Refer to...) Figure 3 The surface of the metal bridge legs showed obvious etching and bulging, indicating that the fluorine-containing gas directly attacked the metal structure without the first step of pretreatment.

[0069] [Compare with Example 3]

[0070] Example 2 is based on Example 1, except that the execution order of steps one and three is reversed: that is, step three (O2 / CF4), which is the same as in Example 1, is executed first, and step one (O2 / H2 / N2) is executed last.

[0071] Experimental results show that: (refer to) Figure 4a and Figure 4b The metal bridge legs exhibited severe over-etching and bulging, resulting in reduced width and bridge surface warping. The sensor's electrical performance deteriorated significantly, possibly due to the CF4 step first damaging the passivation layer on the metal surface.

[0072] [Compare with Example 4]

[0073] Comparative Example 4 is based on Example 1, with the difference that: step one is the same as in Example 1; in step three, the temperature is increased to 150°C, while other parameters remain unchanged.

[0074] Experimental results show that higher temperatures significantly enhance the chemical reactivity of fluorine-containing gas (CF4), leading to a sharp increase in its lateral etching rate on the metal bridge legs (typically Ti / Al composite layers). The width of the metal bridge legs is significantly reduced compared to their original dimensions. This reduction in the cross-sectional area of ​​the bridge legs results in a significant increase in their resistance, causing an impedance imbalance in the entire microbridge structure. Electrical testing of the fabricated sensor revealed that the resistance variation of the bridge legs exceeded the tolerance range, leading to device malfunction.

[0075] [Compare with Example 5]

[0076] Comparative Example 5 is based on Example 1, with the following differences: Step 1 is the same as in Example 1; in Step 3, the CF4 flow rate is increased to 120 sccm (the O2 / CF4 ratio is reduced to 12.5:1), and the processing time is extended to 90 seconds.

[0077] Experimental results show that excessive fluorine-based plasma, under strong bombardment, damages the natural oxide layer on the metal surface and reacts with the underlying metal to generate volatile fluorides or cause metal lattice distortion. This results in bulging defects on the metal leg surface. These bulges not only alter the metal's conductive path, leading to excessively high local current density and increased thermal noise, but may also detach during subsequent processes, causing open circuits. Electrical tests of the sensor revealed unstable IV characteristics and extremely high noise levels, rendering it completely unusable.

[0078] [Compare with Example 6]

[0079] Comparative Example 6 is based on Example 1, with the difference that: step one is the same as in Example 1; in step two, the source power is increased to 1200W while the chamber pressure is reduced to 0.3 Torr.

[0080] Reference Figures 5a-5dExperimental results show that the extremely high power and low pressure environment generated plasma with a strong physical bombardment effect. This plasma, due to its strong directionality, struggled to effectively penetrate the bottom of high aspect ratio structures for residue removal, resulting in polyimide residue. Furthermore, it subjected the exposed microstructure tops (such as bridge legs and bridge surface edges) to intense physical sputtering. The end result was incomplete removal of bottom adhesive residue, while sensitive top structures (such as vanadium oxide films and metal leads) suffered over-etching. The device simultaneously suffers from performance degradation (due to damage to functional layers) and reliability risks (due to residual adhesive potentially causing short circuits or stress concentrations).

[0081] The above are all preferred embodiments of this application and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A method for removing polyimide from MEMS structures based on a two-step dry etching process, characterized in that, The specific steps are as follows: Step 1: Perform the first dry etching on the polyimide sacrificial layer of the MEMS structure in the first oxygen plasma environment of the first process chamber. The process conditions are: temperature 120-200℃, processing time 15-25min; the process gas used to ionize and form the first oxygen plasma includes oxygen and H2 / N2 mixed gas, and the source power of the plasma source is 750-900W. Step 2: Transfer the MEMS structure processed in Step 1 from the first process cavity to the second process cavity; Step 3: Perform a second dry etching on the MEMS structure processed in Step 1 in the second oxygen plasma environment of the second process chamber. The process conditions are: temperature 80-100℃, processing time 10-60s. The process gas used to ionize and form the second oxygen plasma includes oxygen and fluorine-containing gas. The source power of the plasma source is 750-900W. The execution order of steps one and three is irreversible; the first oxygen plasma and / or the second oxygen plasma are generated by a radio frequency plasma source; and both steps two and three are performed when the water vapor partial pressure is ≤1.0×10⁻⁶. -3 Performed under the condition of torr.

2. The method for removing polyimide from MEMS structures based on two-step dry etching according to claim 1, characterized in that: The H2 / N2 mixture is composed of hydrogen and nitrogen, with hydrogen accounting for 3%-4% by volume.

3. The method for removing polyimide from MEMS structures based on two-step dry etching according to claim 2, characterized in that: The flow rate ratio of oxygen and H2 / N2 mixture used for ionization to form the first oxygen plasma is (6-30):

1.

4. The method for removing polyimide from MEMS structures based on two-step dry etching according to claim 1, characterized in that: The flow rate ratio of oxygen and fluorine-containing gas used for ionization to form the second oxygen plasma is (25-100):

1.

5. The method for removing polyimide from MEMS structures based on two-step dry etching according to claim 4, characterized in that: The fluorine-containing gas is CF4.

6. The method for removing polyimide from MEMS structures based on two-step dry etching according to any one of claims 1-5, characterized in that: In step one and / or step three, the chamber pressure is maintained at 0.7-1.2 Torr.

7. The method for removing polyimide from MEMS structures based on two-step dry etching according to any one of claims 1-5, characterized in that: The MEMS structure includes at least one overhanging region or gap with an aspect ratio greater than 3:

1.

8. The method for removing polyimide from MEMS structures based on two-step dry etching according to claim 7, characterized in that: The MEMS structure includes at least one of the following: a vanadium oxide thermistor layer, a silicon nitride capping layer, and a metal leg structure.

9. The application of the two-step dry etching method for removing polyimide from MEMS structures as described in any one of claims 1-8 in the fabrication of microbridge structure devices.

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