CN117348345A - A glue coating method to optimize the thickness of photoresist in the center of the wafer - Google Patents

Abstract

The invention discloses a glue coating method for optimizing the thickness of photoresist in the center of a wafer. The present invention adds an air jet step in the glue coating process, which is beneficial to optimizing the center thickness of the wafer, controlling the uniformity of the photoresist thickness, and reducing the probability of photoresist accumulation in the middle of the wafer. The distance between the gas nozzle and the wafer surface and the pressure of the gas ejected from the gas nozzle determine the change in film thickness in the middle area of the wafer during the ejection step. Reasonable air jet rate and air pressure prevent the formation of ripple effect on the glue and the loss of the attached photoresist. The invention proposes two glue leveling steps, which significantly improves the uniformity of photoresist thickness in the middle area and surrounding area of the wafer, and improves the reliability of subsequent photolithography and etching.

Description

A glue coating method to optimize the thickness of photoresist in the center of the wafer

Technical field

The invention belongs to the technical field of semiconductor manufacturing and relates to a glue coating method for optimizing the thickness of photoresist in the center of a wafer.

Background technique

In the photolithography process, spin coating is one of the important steps before exposure. The process can be simplified into three stages:

1) Dropping glue: When the wafer is stationary or rotating very slowly, drop photoresist on the center of the wafer surface. 2) High-speed rotation: The wafer is quickly rotated to a higher speed, and the photoresist is stretched to the entire wafer surface.

3) Shake off excess glue: Shake off excess photoresist to obtain a more uniform photoresist covering layer on the wafer.

Because the linear speed of the wafer is slower than the outer ring when it is closer to the center point when rotating, the thickness of the photoresist center is likely to be greater than the thickness of the outer ring. Also, because of the surface tension at the edge of the wafer, the photoresist will accumulate on the edge. When the viscosity of the photoresist is high, the overall film thickness may eventually appear uneven, that is, the central area is thick, gradually becomes thinner outward, and the edges are also thick, as shown in Figure 2.

Because there are wafer edge processing steps later in the process, the larger glue thickness at the edge can be effectively processed. However, as the viscosity of the photoresist increases, the photoresist in the middle of the wafer is slightly thicker than the surrounding areas, which will cause the overall resist thickness to be uneven and mismatch the target film thickness, resulting in deviations from the ideal conditions during subsequent exposure, causing the light to The CD (critical dimension), LWR (line width roughness), and other poor morphology of the etched glue affect the quality of photolithography.

In response to the above problems, the current methods for adjusting the thickness and shape of the photoresist mainly include: controlling the dripping dose of the nozzle during glue coating, switching frequency, wafer rotation speed and acceleration, temperature and time of baking after glue coating, and cavity humidity wait. However, these methods can only solve the overall thickness of the photoresist, and cannot effectively solve the middle thickening phenomenon caused by the increase in photoresist viscosity.

Contents of the invention

In view of the shortcomings of the existing technology, the present invention provides a glue coating method that optimizes the thickness of the photoresist in the center of the wafer. By adding a simple thinning step of central air pressure during the glue coating process, it optimizes the thickness of the wafer as the viscosity of the photoresist increases. The photoresist accumulation in the middle area and the thicker film thickness make the photoresist covering the wafer surface more uniform. For critical layers of advanced sizes or when the photoresist has high viscosity, the glue coating method of the present invention can be used to improve the reliability of subsequent photolithography and etching.

A glue coating method to optimize the thickness of photoresist in the center of the wafer, including:

Step S1: Wafer preprocessing:

Place the wafer that needs to be glued on the rotating table and pre-rotate it at any speed; then clean the surface of the wafer;

Step S2: Add photoresist dropwise;

After adding photoresist dropwise, the invention determines the rotation speed according to the viscosity of the photoresist to achieve rapid expansion of the photoresist; after the photoresist is rapidly expanded, the speed is reduced to make the photoresist in the center area flat.

Step S3: Rotate and homogenize the glue:

3-1 Initial rotation to evenly distribute the glue:

Increase the rotation speed of the wafer after dropping the photoresist in step S2 to n1, and rotate it at a constant speed so that the thickness of the photoresist attached to the current wafer reaches T+δ; where T represents the target thickness of the photoresist, δ Represents redundant thickness;

3-2 Air pressure thinning:

Move the gas nozzle to the center above the wafer. The distance between the vertical position of the nozzle and the wafer surface is d, d>0, and use the gas nozzle to spray gas that does not react with the photoresist onto the photoresist surface at the center of the wafer;

The present invention uses air jet operation. Based on the mechanical effect of air pressure on the surface of the liquid film, the air pressure is used to thin the thickness of the photoresist on the wafer surface (especially the thickness of the central area). The air jet simultaneously reduces the wafer rotation speed, so that the next The film thickness changes in the pressed area are relatively uniform, and the formed depressions spread to the surroundings, making the overall film thickness uniform.

3-3 Second rotation to homogenize the glue:

Restore the wafer rotation speed to the target speed n2 of the target resist thickness for secondary dispersion, n2>n1>1000rpm, so that the thickness of the photoresist attached to the current wafer is reduced to T; the second dispersion time is shorter than the initial rotation dispersion time time;

Step S4: Wafer edge and backside rinse;

Step S5: post-processing.

Preferably, the wafer surface is an advanced size AA layer or POLY layer.

Preferably, the pre-rotation duration in step S1 is less than or equal to 1 s.

Preferably, in step S1, the surface is cleaned at a rotation speed of less than 50 rpm, and the cleaning solvent is dripped and rinsed for 2s-3s; after the rinse is completed, the rotational speed is increased to at least 1000rpm and rotated for a duration of less than or equal to 0.5s, and the excess cleaning solvent is thrown away.

Preferably, step S2 is specifically: drop photoresist in the center of the upper surface of the wafer after pretreatment in step S1. The photoresist adding process keeps the rotation speed at 1000rpm-2000rpm, and the dripping time is controlled within 2s-3s; then reduce the rotation speed to After turning below 200rpm, keep rotating for less than or equal to 1s.

Preferably, in step S3, the first rotation time is 10s-20s, and the second time time is 5s-10s.

Preferably, the distance d between the vertical position of the nozzle and the wafer surface in step S3-2 satisfies 0<d≤50mm.

Preferably, in step S3-2, the flow rate of the nozzle gas is controlled to ≤500ml/s, the rotation speed of the jetting process is reduced to at least 500rpm, and the jetting time is ≤1s.

As a preference, in step 3-1

Preferably, the gas that does not react with the photoresist in step 3-2 is nitrogen.

As a preferred method, step S4 is specifically: use ultrapure water to rinse the edge and lower surface of the wafer after homogenization in step S3, and process the edge photoresist accumulation. During the rinse process, keep the rotation speed at 800rpm-1200rpm, and control the wafer rotation and rinse time to 5s-10s.

Preferably, in step 3-2, the film thickness h of air pressure thinning satisfies the following relationship:

Where p is the pressure difference between the gas and the outside world when the gas is ejected, ρ is the photoresist density, v is the photoresist viscosity, and d is the distance from the nozzle to the photoresist surface.

Preferably, step S5 is specifically:

Increase the rotation speed of the rotating table to at least 2000 rpm, and keep rotating at a constant speed for 5s-10s to remove excess ultrapure water to dry the wafer.

The beneficial effects of the present invention are:

The present invention adds an air jet step in the glue coating process, which is beneficial to optimizing the center thickness of the wafer, controlling the uniformity of the photoresist thickness, and reducing the probability of photoresist accumulation in the middle of the wafer. The distance between the gas nozzle and the wafer surface and the pressure of the gas ejected from the gas nozzle determine the change in film thickness in the middle area of the wafer during the ejection step. Reasonable air jet rate and air pressure prevent the formation of ripple effect on the glue and the loss of the attached photoresist.

The present invention proposes two glue leveling steps, which significantly improves the probability that the thickness of the photoresist middle area and the surrounding area will be consistent, and improves the reliability of subsequent photolithography and etching.

Description of drawings

Figure 1 is a flow chart of the process of the present invention;

Figure 2 shows the results after the conventional process of rotating and uniforming. The film thickness will eventually show an uneven shape, that is, the central area is thick, gradually thinning outward, and the edges are also thick.

Figure 3 is a schematic diagram of nine positions of film thickness measurement on the wafer after photoresist coating treatment.

Figure 4 shows the relationship between wafer rotation speed and time in Example 1 and Comparative Example 1.

Detailed ways

The present invention will be further analyzed below in conjunction with specific embodiments.

Example 1: Glue coating method to optimize photoresist thickness in the center of the wafer (performed at 23°C), see Figure 1

Step S1: Wafer Preprocessing

The 12-inch wafer in the AA layer that needs to be glued (the front layer includes Silicon nitride and/> Hard mask layer), place it on the rotating table, use 2000rpm for pre-rotation, lasting 1s; then perform surface cleaning on the wafer, maintain the rotational speed of 30rpm during the cleaning process, drop cleaning solvent for flushing, lasting 3s; After rinsing is completed, increase the speed to at least 1000rpm and rotate for 0.1s to shake off excess cleaning solvent;

Step S2: Add photoresist dropwise

Add 32ml of photoresist AM20732 ml to the center of the upper surface of the wafer after pretreatment in step S1. Keep the rotation speed at 1850rpm during the addition process and control the dripping time within 2s; then reduce the rotational speed to 100rpm and keep rotating for 1s;

Step S3: Rotate the glue evenly

3-1 Initial rotation and uniform glue

Increase the rotation speed of the wafer after dropping the photoresist in step S2 to 1200 rpm, and rotate it at a constant speed for 15 seconds, so that the thickness of the photoresist attached to the current wafer reaches

(Angstroms) is the unit of photoresist thickness,/>

3-2 air pressure thinning

Move the gas nozzle to the center above the wafer. The vertical position of the nozzle is 50mm away from the wafer surface. Use the gas nozzle to spray nitrogen on the photoresist surface at the center of the wafer. Control the flow rate of the nozzle gas to ≤500ml/s. The process speed is reduced to 500rpm, and the injection time is 1s;

During the air pressure thinning process, the wafer speed is reduced so that the wafer rotates at a low speed of 500 rpm to prevent bubbles and ripples caused by excessive diffusion in the thinning area, and a very small air pressure is controlled to eject air for a short time to prevent excessive thinning. big. After the air jet step, the speed is increased to make the photoresist thickness reach the target thickness.

Based on the mechanical effect of air pressure on the surface of the liquid film, the film thickness h thinned by air pressure has the following relationship with environmental parameters:

Where p is the pressure difference between the gas and the outside world when the gas is ejected (the nozzle area can be increased to reduce pa), ρ is the photoresist density, v is the photoresist viscosity, and d is the distance from the nozzle to the photoresist surface. Control the pressed film thickness h to be less than 8% of the initial glue thickness to prevent the glue thickness from being less than the final target thickness during the thinning process.

3-3 Second rotation to homogenize the glue:

Restore the wafer speed to the target speed of 1479 rpm for the target resist thickness and perform secondary glue dispersion for 10 seconds to reduce the thickness of the photoresist attached to the current wafer to

The rotation speed of the wafer during the first leveling is adjusted from 1479rpm to 1200rpm (the rotational speed is reduced by 20% relative to the target speed), so that the overall glue thickness of the first leveling wafer is >3% greater than the target thickness, exceeding the range card control range. After the central air pressure is thinned, the target speed is used to spread the glue, adjust the photoresist thickness to the target thickness, and make the overall thickness more uniform.

Step S4: Wafer edge and backside rinse

Use ultrapure water to rinse the edge and lower surface of the wafer after homogenization in step S3 to deal with the accumulation of photoresist on the edge. During the rinse process, keep the rotation speed at 1000 rpm and control the wafer rotation rinse time to 7 seconds;

Step S5: Post-processing

Increase the rotation speed of the rotating table to 2000 rpm and keep rotating at a constant speed for 5 seconds to remove excess ultrapure water to dry the wafer.

Table 1COT New Recipe

Comparative Example 1: Conventional wafer center photoresist thickness coating method (performed at 23°C)

Step S1: Wafer Preprocessing

The 12-inch wafer in the AA layer that needs to be glued (the front layer includes Silicon nitride and/> The hard mask layer) is placed on the rotating table and pre-rotated at 2000 rpm for 1 s; then the surface is cleaned on the wafer. During the cleaning process, the rotation speed is maintained at 30 rpm, and the cleaning solvent is dripped into the wafer for 3 s; rinse After completion, increase the speed to at least 1000rpm and rotate for 0.1s to shake off excess cleaning solvent;

Step S2: Add photoresist dropwise

Add 32ml of photoresist AM20732 ml to the center of the upper surface of the wafer after pretreatment in step S1. Keep the rotation speed at 1850rpm during the addition process and control the dripping time within 2s; then reduce the rotational speed to 100rpm and keep rotating for 1s;

Step S3: Rotate the glue evenly

3-1 Rotate and evenly glue

Increase the rotation speed of the wafer after dropping the photoresist in step S2 to 1479 rpm, and rotate it at a constant speed for 25 seconds, so that the thickness of the photoresist attached to the current wafer reaches

Step S4: Wafer edge and backside rinse

Use ultrapure water to rinse the edge and lower surface of the wafer after homogenization in step S3 to deal with the accumulation of photoresist on the edge. During the rinse process, keep the rotation speed at 1000 rpm and control the wafer rotation rinse time to 7 seconds;

Step S5: Post-processing

Increase the rotation speed of the rotating table to 2000 rpm and keep rotating at a constant speed for 5 seconds to remove excess ultrapure water to dry the wafer.

Table 2COT Old Recipe

RRC: Reduced Resist Consumption, organic solvent rinse, lubrication of photoresist

EBR:: Edge Back rinse, edge and back rinse

Resist: Photoresist

Edge1,2: Different positions on the edge of the wafer, used to rinse the edge and remove accumulated photoresist

Take 9 points on the wafer processed in Example 1 and Comparative Example 1, measure the film thickness data at 9 positions (see Figure 3), and calculate the mean value and range of the 9 positions. The process goals are: the mean stuck value is within ±1% of the target glue thickness, and the range stuck value is within 3%. The maximum range usually occurs at the film thickness gap between the center point and the peripheral point. Embodiment 1: Through the air pressure center thinning process, the gap between the center point thickness and the peripheral thickness will be significantly reduced. Taking the target thickness as Take a certain process as an example. Using the traditional gluing process of Comparative Example 1, under the requirement of 3% range control range, the maximum film thickness difference between the center point and the peripheral point is/> The actual difference is/> When the film thickness at the other eight measuring points except the center point is uniform, the maximum value of uniformity (standard deviation σ) is/> In Embodiment 1, by using the central air pressure thinning process, it is expected that the thickness difference between the center point and the peripheral points will be optimized to be greater than 50%, that is, the range control range can be reduced to 1.5%, and the actual film thickness difference between the center point and the peripheral points will be less than /> The maximum value of its uniformity can be reduced to/> The relationship between the wafer rotation speed and time in Example 1 and Comparative Example 1 is shown in Figure 4.

The basic principles and main features of the present invention and the advantages of the present invention have been shown and described above. It is obvious to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments without departing from the spirit or basics of the present invention. In the case of specific features, the present invention can be implemented in other specific forms. Therefore, the embodiments should be regarded as illustrative and non-restrictive from any point of view, and the scope of the present invention is defined by the appended claims rather than the above description, and it is therefore intended that all claims falling within the claims All changes within the meaning and scope of equivalent elements are included in the present invention. Any reference signs in the claims shall not be construed as limiting the claim in question.

Claims (10)

1. A glue spreading method for optimizing the thickness of photoresist in the center of a wafer is characterized by comprising the following steps:

step S1: wafer pretreatment:

placing a wafer needing to be subjected to a gluing process on a rotary table, and pre-rotating by adopting any rotating speed; then cleaning the surface of the wafer;

step S2: dropwise adding photoresist;

step S3: spin coating:

3-1 first time spin coating:

the wafer after the photoresist is dripped in the step S2 is increased in rotating speed to n1, and uniform photoresist is rotated at a constant speed, so that the thickness of the photoresist attached to the current wafer reaches T+delta; wherein T represents the target thickness of the photoresist, and delta represents the redundant thickness;

3-2 air pressure thinning:

moving a gas nozzle to the center above the wafer, wherein the distance d between the vertical position of the nozzle and the surface of the wafer is d, d is more than 0, and spraying gas which does not react with photoresist on the surface of the photoresist at the center of the wafer by using the gas nozzle;

3-3 times spin coating:

restoring the rotation speed of the wafer to a target rotation speed n2 of the target photoresist thickness, and carrying out secondary photoresist homogenizing, wherein n2 is more than n1 and more than 1000rpm, so that the thickness of the photoresist attached on the current wafer is thinned to T; the secondary spin time is less than the spin time for the first time;

step S4: rinsing the edge and the back of the wafer;

step S5: and (5) post-treatment.

2. The method according to claim 1, wherein step S2 is specifically: dropwise adding photoresist in the center of the upper surface of the wafer pretreated in the step S1, wherein the rotating speed of the photoresist adding process is kept at 1000rpm-2000rpm, and the dropwise adding time is controlled within 2S-3S; then, the rotation speed is reduced to 200rpm or less and kept to be 1s or less.

3. The method according to claim 1, wherein the primary spin time in step S3 is 10S-20S and the secondary spin time is 5S-10S.

4. The method of claim 1, wherein the vertical position of the nozzle in step S3-2 is spaced from the wafer surface by a distance d that satisfies 0 < d.ltoreq.50 mm.

5. The method according to claim 1, wherein the flow rate of the nozzle gas is controlled to be less than or equal to 500ml/S in the step S3-2, the rotating speed of the air injection process is reduced to at least 500rpm, and the air injection time is controlled to be less than or equal to 1S.

6. The method according to claim 1, wherein in step 3-1

7. The method of claim 1, wherein the gas that does not react with the photoresist in step 3-2 is nitrogen.

8. The method according to claim 1, wherein step S4 is specifically: and (3) flushing the edge and the lower surface of the wafer after the photoresist is uniformly coated in the step (S3) by using ultra-pure water, treating photoresist accumulation at the edge, and keeping the rotation speed of 800rpm-1200rpm in the flushing process, wherein the wafer rotation flushing time is controlled to be 5S-10S.

9. The method of claim 1, wherein the wafer surface is an advanced-sized AA layer or POLY layer.

10. The method according to claim 1, wherein in step 3-2, the air pressure thinned film thickness h satisfies the following relationship:

wherein p is the pressure difference between the gas sprayed out and the outside, ρ is the photoresist density, v is the photoresist viscosity, and d is the distance from the nozzle to the photoresist surface.