Environment-friendly process method for improving silicon wafer surface metal cleaning efficiency

Abstract

The invention relates to an environment-friendly process method for improving the surface metal cleaning efficiency of a silicon wafer, and belongs to the technical field of semiconductor silicon wafer processing, and the process method comprises the following operation steps: S1, pre-cleaning and activation: placing the silicon wafer in a first cleaning tank, introducing ozone into ultrapure water, and processing in cooperation with low-frequency ultrasound; and S2, main cleaning and complexing: placing the silicon wafer subjected to pre-cleaning and activating treatment in a second cleaning tank, and treating the silicon wafer in a dilute citric acid solution in cooperation with high-frequency ultrasound. And S3, overflow rinsing: placing the silicon wafer subjected to main cleaning and complexing treatment in a third cleaning tank. And S4, drying: carrying out drying treatment on the silicon wafer subjected to the overflow rinsing treatment. Through the synergistic effect of physical stripping, chemical oxidation and complexing dissolution, under the condition that highly toxic chemicals such as hydrofluoric acid are not used, the cleanliness of metal on the surface of the silicon wafer reaches the ppt level, and the problems that traditional RCA cleaning is serious in environmental pollution and high in operation risk are effectively solved.

Description

Technical Field

[0001] This invention relates to the field of semiconductor silicon wafer processing technology, and more specifically to a process method that improves the efficiency of metal cleaning on the surface of silicon wafers and is environmentally friendly. Background Technology

[0002] In silicon wafer processing, the permissible threshold for the concentration of metal impurities on the silicon wafer surface typically requires the concentration of a single metal ion to be less than 1E¹⁰ atoms / cm². 2 Its harmful effects extend through subsequent processing steps ("cleaning-photolithography-etching-thin film deposition-device packaging"), ultimately leading to device performance failure, a sharp drop in process yield, and deterioration in product reliability.

[0003] 1. Leakage current increases sharply, and power consumption soars. Metal ions form "deep level traps" on the silicon wafer surface or in the gate oxide layer, capturing or releasing charge carriers (electrons / holes), resulting in a large number of charge carriers still conducting through the traps even when the device is in the "off state".

[0004] 2. Threshold voltage (Vth) drift, causing switching characteristic disturbances. Alkali metals (Na) + K + It has extremely strong mobility and will diffuse to the interface between the gate oxide layer (SiO2) and the silicon substrate during high-temperature processes (200-1000℃) such as photolithography and annealing, forming a "fixed positive charge" and changing the gate control capability of the MOSFET.

[0005] 3. Abnormal photoresist coating leads to pattern transfer failure. Metal impurities alter the hydrophilicity / hydrophobicity of the silicon wafer surface. Photoresist that should be evenly spread may form "shrinkage" (localized thinning of the photoresist layer) or "pinholes" in areas contaminated with metal. After exposure, this results in "jagged defects" at the pattern edges (uneven photoresist exposure in metal areas) and excessive deviations in critical dimensions, directly causing subsequent etching to fail to form a qualified device structure.

[0006] 4. Thin film deposition defects lead to interconnect reliability collapse. During the deposition of metal interconnect layers or insulating layers on silicon wafers, surface metal impurities can cause "voids" and "abnormal growth of columnar crystals" in the thin film, and trigger "interlayer diffusion," ultimately resulting in interconnect short circuits or open circuits.

[0007] 5. Decreased etching selectivity and excessive substrate damage. Metallic impurities react with etching gases to form "non-volatile metal compounds" (such as CrF3 and TiCl4), which adhere to the silicon wafer surface and form "etching residues." If these residues remain at the bottom of the etched trenches, they will lead to incomplete filling of subsequent thin films; if they remain on the surface of the silicon substrate, they will cause "silicon lattice damage" in subsequent high-temperature processes, further deteriorating device performance. Summary of the Invention

[0008] This invention addresses the shortcomings of existing technologies by providing an environmentally friendly process for improving the efficiency of metal cleaning on silicon wafer surfaces. This method offers advantages such as ease of operation, high effectiveness, and strong operational stability. Through the synergistic effect of "physical stripping + chemical oxidation + complexation dissolution," a ppt-level cleanliness of the silicon wafer surface is achieved without the use of highly toxic chemicals such as hydrofluoric acid. This effectively solves the problems of severe environmental pollution and high operational risks associated with traditional RCA cleaning, significantly improving silicon wafer yield and the long-term reliability of semiconductor devices.

[0009] The above-mentioned technical problems of the present invention are mainly solved by the following technical solutions: A process method that improves the efficiency of metal cleaning on silicon wafer surfaces and is environmentally friendly includes the following steps: S1. Pre-cleaning and activation: The silicon wafer is placed in the first cleaning tank, ozone is introduced into ultrapure water, and low-frequency ultrasound is used to treat it, so that the metal contaminants on the surface of the silicon wafer are oxidized into high-valence metal ions and particulate contaminants are removed.

[0010] S2. Main cleaning and complexation: The silicon wafers that have undergone pre-cleaning and activation are placed in the second cleaning tank and treated with high-frequency ultrasound in a dilute citric acid solution, so that the high-valence metal ions and citric acid form a stable water-soluble complex and detach from the silicon wafer surface.

[0011] S3. Overflow Rinse: The silicon wafers after the main cleaning and complexation treatment are placed in the third cleaning tank and rinsed with continuously flowing ultrapure water to remove residual citric acid and complexes.

[0012] S4. Drying: The silicon wafers that have undergone overflow rinsing are dried.

[0013] Preferably, the concentration of ozone in ultrapure water is 5–20 ppm; the frequency of the low-frequency ultrasound is 20–40 kHz; and the treatment time is 1–5 min.

[0014] Preferably, the ozone acts as a strong oxidant, oxidizing the metal on the silicon wafer surface from a zero-valence or low-valence state to a high-valence state ion; the low-frequency ultrasound loosens and peels off particulate metal contaminants attached to the silicon wafer surface through cavitation effect.

[0015] Many metals (such as copper) exhibit extremely strong adsorption to silicon wafer surfaces in their zero or low valence states (they may even embed themselves in the silicon lattice), making them difficult to remove through direct cleaning. Ozone oxidizes these metals into ionic states (such as Cu²⁺), which not only breaks the direct bond with silicon but also increases their positive charge, making them more susceptible to complexation reactions with negatively charged citrate ions. This is a chemical thermodynamic technique known as "playing hard to get."

[0016] Preferably, the concentration of the dilute citric acid solution is 0.1%–1% w / v, and the pH value is controlled between 2.0 and 4.0; the frequency of the high-frequency ultrasound is 80–130 kHz; and the treatment time is 5–10 min.

[0017] Preferably, the citric acid acts as a chelating agent to form a water-soluble complex with the high-valence metal ions generated during pre-washing and activation, wherein the complex includes [Cu(H2O)6]. 2+ + and [Fe(H2O)6] 3+ .

[0018] In addition to being safe and environmentally friendly, citric acid is a natural tricarboxylic acid chelating agent with a strong selective binding ability to transition metals (Fe, Cu, Ni). Moreover, in a weakly acidic environment of pH 2 to 4, it can ensure complexation efficiency without corroding the oxide layer or gate material on the silicon wafer surface like strong acids.

[0019] Preferably, the cavitation effect bubbles generated by the high-frequency ultrasound are smaller than those generated by the low-frequency ultrasound, which is used to enhance the cleaning of tiny gaps and improve the removal efficiency of ionic metals.

[0020] Preferably, low-frequency ultrasound is used to assist in the rinsing process, with the frequency of the low-frequency ultrasound being 40 kHz and the rinsing time being 5 to 10 minutes.

[0021] Low-frequency cavitation bubbles are large and burst violently, making them suitable for "overturning" large particulate pollutants; high-frequency bubbles are small and numerous, and the resulting jets are finer, making them suitable for entering nanoscale gaps to clean ionic residues, and causing less damage to the microstructure of the silicon wafer surface.

[0022] Preferably, isopropanol vapor drying is used to dry the silicon wafers during the drying process.

[0023] Preferably, the device includes a first cleaning device, a second cleaning device, a third cleaning device, and a drying device; the first cleaning device is used to introduce ozone solution and emit low-frequency ultrasonic waves; the second cleaning device is used to contain dilute citric acid solution and emit high-frequency ultrasonic waves; the third cleaning device is used to introduce flowing ultrapure water and perform rinsing; and the drying device is used to dry the silicon wafer with isopropanol vapor.

[0024] The present invention can achieve the following effects: This invention provides an environmentally friendly process method that improves the efficiency of metal cleaning on silicon wafers. Compared with existing technologies, through the synergistic effect of "physical stripping + chemical oxidation + complexation dissolution", the surface cleanliness of silicon wafers reaches the ppt level without the use of highly toxic chemicals such as hydrofluoric acid. This effectively solves the problems of serious environmental pollution and high operational risks associated with traditional RCA cleaning, and significantly improves the yield of silicon wafers and the long-term reliability of semiconductor devices.

[0025] (1) Environmentally friendly: Non-corrosive acid solution: Citric acid (an organic acid) is used instead of traditional hydrofluoric acid (HF) or hydrochloric acid / sulfuric acid mixtures. Citric acid is biodegradable, places less pressure on the wastewater treatment system, and has low corrosiveness to equipment.

[0026] Ozone decomposes into oxygen, leaving no harmful residue.

[0027] (2) High- and low-frequency ultrasound synergy: Segmented optimization: Stage S1 uses low frequency to focus on "powerful physical stripping". Stage S2 uses high frequency to focus on "refined chemical mass transfer". Stage S3 returns to low frequency to ensure "thorough cleaning". This combination is more effective than cleaning with a single frequency.

[0028] (3) High cleaning efficiency: The dual mechanism of oxidation and complexation first oxidizes the sparingly soluble metal into ions, and then chelates and dissolves it, solving the problem that simple cleaning is insufficient to remove certain stubborn metals (such as copper).

[0029] Preventing redeposition: The complex formed by citric acid and metal has high stability, effectively preventing the secondary adsorption (re-deposition) of metal ions during the cleaning process.

[0030] (4) High drying quality: Isopropanol vapor drying avoids the mechanical stress that may be generated by rotary drying, and can completely remove the moisture in the deep pores of the pattern structure, avoiding water spots. Detailed Implementation

[0031] The technical solution of the invention will be further described in detail below through examples.

[0032] Example: The silicon wafer cleaning system includes a first cleaning device, a second cleaning device, a third cleaning device, and a drying device; the first cleaning device is used to introduce ozone solution and emit low-frequency ultrasonic waves; the second cleaning device is used to contain dilute citric acid solution and emit high-frequency ultrasonic waves; the third cleaning device is used to introduce flowing ultrapure water and perform rinsing; the drying device is used to dry the silicon wafer with isopropanol vapor.

[0033] A process method that improves the efficiency of metal cleaning on silicon wafer surfaces and is environmentally friendly includes the following steps: S1. Pre-cleaning and activation: The silicon wafer is placed in the first cleaning tank, ozone is introduced into ultrapure water, and low-frequency ultrasound is used to treat it, so that the metal contaminants on the surface of the silicon wafer are oxidized into high-valence metal ions and particulate contaminants are removed.

[0034] The ozone concentration in ultrapure water is 5–20 ppm; the frequency of low-frequency ultrasound is 20–40 kHz; and the treatment time is 1–5 min. Ozone, as a strong oxidant, oxidizes the metal on the silicon wafer surface from zero-valence or low-valence states to high-valence ions; the low-frequency ultrasound loosens and peels off particulate metal contaminants adhering to the silicon wafer surface through cavitation.

[0035] In the oxidation-stripping process, ozone (O3) in water generates strong oxidizing agents such as hydroxyl radicals, which rapidly attack the metal surface. Example chemical formula: 2Cu + O3 + 2H2O + →2Cu 2+ +H2O+O2.

[0036] S2. Main cleaning and complexation: The silicon wafers that have undergone pre-cleaning and activation are placed in the second cleaning tank and treated with high-frequency ultrasound in a dilute citric acid solution, so that the high-valence metal ions and citric acid form a stable water-soluble complex and detach from the silicon wafer surface.

[0037] The concentration of the dilute citric acid solution is 0.1%–1% w / v, and the pH value is controlled between 2.0 and 4.0; the frequency of high-frequency ultrasound is 80–130 kHz; and the treatment time is 5–10 min. Citric acid acts as a chelating agent, forming water-soluble complexes with the high-valence metal ions generated during pre-cleaning and activation. These complexes include [Cu(H₂O)₆]. 2+ + and [Fe(H2O)6] 3+ .

[0038] The complexation-dissolution process utilizes citric acid, an environmentally friendly chelating agent. Complexation reaction: Fe... 3+ +H3Cit→[FeCit]+3H + .

[0039] High-frequency ultrasound produces smaller cavitation bubbles than low-frequency ultrasound, which can be used to enhance the cleaning of tiny gaps and improve the removal efficiency of ionic metals.

[0040] High-frequency ultrasound generates smaller, more numerous, and denser cavitation bubbles, which can penetrate deep into the micro-nano structures on the silicon wafer surface, accelerating ion exchange and mass transfer processes and improving the utilization rate of chemical agents.

[0041] S3. Overflow Rinse: The silicon wafers after the main cleaning and complexation treatment are placed in the third cleaning tank and rinsed with continuously flowing ultrapure water to remove residual citric acid and complexes.

[0042] During the rinsing process, low-frequency ultrasound is used for assistance. The frequency of the low-frequency ultrasound is 40kHz, and the rinsing time is 5-10 minutes. Dilution and replacement are performed, and a large flow of ultrapure water is used to ensure that the residual ion concentration is reduced to an extremely low level.

[0043] S4. Drying: The silicon wafers, after overflow rinsing, are dried. During the drying process, isopropanol vapor drying is used. When isopropanol vapor condenses on the silicon wafer surface, it reduces the surface tension gradient, "pulling" water molecules away. Finally, the isopropanol itself evaporates, leaving a dry, residue-free surface.

[0044] (1) Environmentally friendly and non-toxic alternative: using dilute citric acid (mild organic acid) to replace highly corrosive hydrofluoric acid (HF) and highly toxic ammonia (NH4OH) significantly reduces waste liquid treatment costs and safety risks.

[0045] (2) Physical-chemical synergy: Abandoning the traditional approach of relying solely on chemical corrosion, a synergistic mechanism of "strong ozone oxidation + multi-frequency ultrasonic physical stripping + citric acid complexation and dissolution" is adopted.

[0046] (3) Precise application of multi-frequency ultrasound: Low frequency (20-40 kHz): Utilizing strong cavitation effect, it mainly targets particulate pollutants for "brutal" stripping.

[0047] High frequency (80–130 kHz): Utilizes dense microbubbles to perform “fine” cleaning, primarily targeting crevices and ionic contaminants, reducing surface damage.

[0048] (4) Highly targeted: Specifically designed to remove the most troublesome metal contamination (Cu, Fe, etc.) in semiconductor manufacturing step by step (oxidation valence state increase -> complexation and dissolution to remove).

[0049] In summary, this environmentally friendly process method, which improves the efficiency of silicon wafer surface metal cleaning, achieves a silicon wafer surface metal cleanliness level of ppt through the synergistic effect of "physical stripping + chemical oxidation + complexation dissolution" without using highly toxic chemicals such as hydrofluoric acid. It effectively solves the problems of serious environmental pollution and high operational risks of traditional RCA cleaning, and significantly improves silicon wafer yield and the long-term reliability of semiconductor devices.

[0050] The above description is only a specific embodiment of the present invention, but the structural features of the present invention are not limited thereto. Any changes or modifications made by those skilled in the art within the scope of the present invention are covered by the patent scope of the present invention.

Claims

1. A process method that improves the efficiency of metal cleaning on silicon wafer surfaces and is environmentally friendly, characterized in that... The following steps are included: S1. Pre-cleaning and activation: The silicon wafer is placed in the first cleaning tank, ozone is introduced into ultrapure water, and low-frequency ultrasound is used to treat it, so that the metal contaminants on the surface of the silicon wafer are oxidized into high-valence metal ions and particulate contaminants are removed. S2. Main cleaning and complexation: The silicon wafers that have been pre-cleaned and activated are placed in the second cleaning tank and treated with high-frequency ultrasound in a dilute citric acid solution, so that the high-valence metal ions and citric acid form a stable water-soluble complex and detach from the surface of the silicon wafer. S3. Overflow Rinse: The silicon wafers after the main cleaning and complexation treatment are placed in the third cleaning tank and rinsed with continuously flowing ultrapure water to remove residual citric acid and complexes. S4. Drying: The silicon wafers that have undergone overflow rinsing are dried.

2. The environmentally friendly process method for improving the efficiency of metal cleaning on silicon wafer surfaces according to claim 1, characterized in that: The concentration of ozone in ultrapure water is 5–20 ppm; the frequency of the low-frequency ultrasound is 20–40 kHz; and the treatment time is 1–5 min.

3. The environmentally friendly process method for improving the efficiency of metal cleaning on silicon wafer surfaces according to claim 2, characterized in that: The ozone, as a strong oxidant, oxidizes the metal on the silicon wafer surface from zero-valence or low-valence states to high-valence ions; the low-frequency ultrasound loosens and peels off particulate metal contaminants attached to the silicon wafer surface through cavitation effect.

4. The environmentally friendly process method for improving the efficiency of metal cleaning on silicon wafer surfaces according to claim 1, characterized in that: The concentration of the dilute citric acid solution is 0.1%–1% w / v, and the pH value is controlled between 2.0 and 4.0; the frequency of the high-frequency ultrasound is 80–130 kHz; and the treatment time is 5–10 min.

5. The environmentally friendly process method for improving the efficiency of metal cleaning on silicon wafer surfaces according to claim 4, characterized in that: The citric acid, acting as a chelating agent, forms water-soluble complexes with the high-valence metal ions generated during pre-washing and activation. These complexes include [Cu(H₂O)₆]. 2+ + and [Fe(H2O)6] 3+ .

6. The environmentally friendly process method for improving the efficiency of metal cleaning on silicon wafer surfaces according to claim 1, characterized in that: The cavitation effect bubbles generated by the high-frequency ultrasound are smaller than those generated by the low-frequency ultrasound, which is used to enhance the cleaning of tiny gaps and improve the removal efficiency of ionic metals.

7. The environmentally friendly process method for improving the efficiency of metal cleaning on silicon wafer surfaces according to claim 1, characterized in that: During the rinsing process, low-frequency ultrasound is activated for assistance. The frequency of the low-frequency ultrasound is 40kHz, and the rinsing time is 5 to 10 minutes.

8. The environmentally friendly process method for improving the efficiency of metal cleaning on silicon wafer surfaces according to claim 1, characterized in that: During the drying process, isopropanol vapor drying is used to dry the silicon wafers.

9. A silicon wafer cleaning system for implementing the method according to any one of claims 1 to 8, characterized in that: It includes a first cleaning device, a second cleaning device, a third cleaning device, and a drying device; the first cleaning device is used to introduce ozone solution and emit low-frequency ultrasonic waves; The second cleaning device is used to contain a dilute citric acid solution and emit high-frequency ultrasonic waves. The third cleaning unit is used to introduce flowing ultrapure water for rinsing; the drying unit is used to dry the silicon wafers with isopropanol vapor.