JPH11174692A - Device and method for removing photo-resist on semiconductor substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a device and method for removing photo-resist on a semiconductor substrate, sustaining efficacy of chemicals used and shortening manufacturing cycle time. SOLUTION: A photo-resist exfoliating tank 21 holds a piranha solution 18 for removing photo-resist on a semiconductor substrate. The photo-resist exfoliating tank has a bathing tank 26 separated from a storing part 27 by a wall 25. The bathing tank is filled with the piranha solution up to the height of the wall. The storing part is filled with sulfuric acid up to a height lower than the wall height. A pump 23 is connected to the storing part and pumps up the piranha solution into the bathing tank via an osmotic-membrane gasification device 28. An ozone generator 29 supplies ozone to the osmotic-membrane gasification device. By pumping up the piranha solution into the bathing tank, the piranha solution flows down over the wall and returns to the storing part. The osmotic-membrane gasification device increases the concentration of molecular ozone in the piranha solution.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

FIELD OF THE INVENTION The present invention relates generally to photoresist removal and, more particularly, to the removal of positive photoresist in a semiconductor wafer process.

[0002]

2. Description of the Related Art Positive photoresists are commonly used to shield regions on a semiconductor chip. Typically,
The use of a photoresist is to protect the underlayer during the doping process. It is also used to protect the underlayer from etchants. In a process of forming a semiconductor element, two types of photoresists (positive and negative) are used. Positive photoresist has some advantages over negative photoresist because it creates a substrate structure with a small feature size.
The use of positive photoresist is expected to increase in the future.

[0003] Photoresist is deposited on a semiconductor wafer, and the wafer is rotated to uniformly coat the photoresist on the wafer. The photoresist contains a solvent that keeps it fluid and can evenly coat the photoresist on a rotating wafer. In a positive photoresist, the photoresist is polymerized by evaporating the solvent. In the polymerized state, the photoresist forms a hard coating, which is very difficult to remove. Patterning of the polymerized photoresist involves exposing regions of the semiconductor wafer that do not require a masking material to light. The light decomposes the positive photoresist into short length molecules, and then tetramethylammonium hydroxide (TM
AH is dissolved or removed using a basic substance such as tetramethylammonium hydroxide. Removal of the exposed photoresist leaves a pattern of polymerized positive photoresist.

Next, the semiconductor wafer is subjected to another process, such as doping or etching. The polymerized positive photoresist is removed after the processing steps are completed. Removing the positive photoresist is not a simple task. In many cases, several steps are required to completely remove the positive photoresist. For example, ashing
Is often used to remove bulk positive photoresist. Although most of the positive photoresist is removed by ashing, there are still residues that must be removed before further wafer processing steps can be performed. To remove the remaining positive photoresist, the semiconductor wafer is immersed in a solution of sulfuric acid and hydrogen peroxide or a solution of sulfuric acid and ozone. Often, two or more cleaning steps are used to ensure that all photoresist residues have been removed.

[0005]

Accordingly, it would be advantageous to have a photoresist removal process that improves cleaning and reduces manufacturing cycle time. In addition, the chemical stripping process (chemical stripping pr
It would be further advantageous if the process could extend the shelf life of the chemical stripper and reduce human contact with hazardous chemicals.

[0006]

DETAILED DESCRIPTION OF THE INVENTION Piranha is the name of the photoresist stripping process used throughout the semiconductor industry. The name piranha comes from the fact that it is overly aggressive for the mixture to strip the photoresist. The semiconductor industry will not hesitate to abolish this process if it can develop comparable secure systems. At present, piranhas are still widely used by almost every semiconductor manufacturer in the world, as no better system exists.

[0007] Sulfuric acid (H ₂ SO ₄ ) is a major component in photoresist stripping solutions commonly referred to as piranha solutions. Sulfuric acid is a very dangerous acid and has potential health hazards to humans. An oxidizing agent such as hydrogen peroxide or ozone is added to the sulfuric acid to speed up photoresist removal. This mixture is typically 9 degrees Celsius.
Heat within a temperature range of 0 to 140 degrees Celsius to further accelerate the reaction with the polymerized photoresist.

[0008] Sulfuric acid hydrolyzes the polymerized photoresist to break down the long chain molecules of the polymerized photoresist into short components that readily react with hydrogen peroxide or ozone. The reaction of this shortened organic compound with oxygen forms carbon dioxide and water. As described above, the acidic solution is heated to further enhance the reaction.

A problem with piranha solutions is that they are not effective for long periods of time, ie, they dilute as they are used and their ability to remove photoresist is reduced. The time period required to remove the photoresist
The longer the solution is diluted, the longer it will be, resulting in increased manufacturing cycle time. Also, the production cycle time is further increased because the mixture is constantly changed. Also, removal / replacement of the acidic mixture will expose humans to dangerous chemicals. Other problems with piranha solutions include heating them to very high temperatures and the exothermic nature of the reaction between hydrogen peroxide and sulfuric acid.
Pouring hydrogen peroxide into sulfuric acid is very dangerous, especially if the water content in the piranha solution is low.

Typically, a piranha solution consists of about 95 percent sulfuric acid and 5 percent hydrogen peroxide solution. About 20 percent of the hydrogen peroxide solution is water. Therefore, the dilution of the piranha solution starts immediately with the introduction of hydrogen peroxide.

An acidic solution is placed in a photoresist stripping tank,
The wafer lot is immersed in a strip bath to remove the polymerized photoresist. 5 for photoresist stripping
Or 10 minutes. After removing the photoresist from each wafer lot, more hydrogen peroxide is added to the acidic solution. Therefore, after processing each wafer lot, water is added to the piranha solution. For example, after each wafer lot, 200 milliliters (ml) of hydrogen peroxide was added to 40 liters (about 10 gallons) of piranha solution.
As each addition is made, the solution is further diluted. As mentioned above, by-products of the oxidation reaction between oxygen and the photoresist are carbon dioxide and water. The high temperature of the piranha solution helps to evaporate some of the water, but the net result is that the solution is more diluted with use. Typically, the hydrogen peroxide based piranha solution is replaced after about 12 hours of use.

Ozone foaming (pizone) in piranha solution
The use of bubbling has the advantage of lasting effectiveness over piranha solutions containing sulfuric acid and hydrogen peroxide. For example, a piranha solution foamed by ozone lasts about 2 days, compared to a hydrogen peroxide piranha solution lasting 12 hours. Disadvantages of ozonated piranha (d
Ownside) is the long time it takes to complete an equivalent photoresist strip. Nevertheless, many semiconductor manufacturers choose an ozonated piranha solution at the expense of increased manufacturing cycle time because it minimizes human contact with hazardous chemicals.

In the ozonated piranha solution, ozone is foamed by sulfuric acid. Some ozone diffuses into the solution and further oxidizes the photoresist. In many cases, elaborate foaming techniques are used to create bubbles in close proximity to the surface of the photoresist, or microbubbles, with the expectation that oxidation will occur through direct contact with ozone bubbles. To further enhance the process. These techniques have not been proven to be effective, nor do they provide uniform stripping of the photoresist.

Ozone does not readily diffuse into sulfuric acid when foamed. The concentration level of ozone dissolved in sulfuric acid by the foaming process is not enough to obtain a photoresist strip rate equivalent to a piranha solution using hydrogen peroxide. Further, the rate at which ozone diffuses into sulfuric acid decreases as the temperature of the solution increases. Thus, there is a trade-off between temperature and the amount of diffused ozone in a bubbled ozone piranha system.

The advantage of the ozone-expanded piranha solution is an extended shelf life due to the fact that no water is added to the system.
As mentioned above, sulfuric acid hydrolyzes, ie degrades, the long chain molecules of the polymerized photoresist. The shorter the molecule, the easier the reaction with ozone occurs. Ozone oxidizes the hydrolyzed photoresist to produce water and carbon dioxide. Due to the high temperature of the piranha solution, much of the water in the solution evaporates, making the ozone foamed piranha solution longer lasting than the hydrogen peroxide piranha solution. Typically, the temperature in the foamed ozone piranha to promote the diffusion of ozone is lower than in hydrogen peroxide piranha. Thus, water generated by reaction with the photoresist burns off as efficiently as in piranha hydrogen peroxide.
f) You cannot.

[0016] Implant dopin
In g), there is a tendency to increase the energy of the implant.
High energy implants make photoresist removal more difficult and affect the photoresist. The photoresist absorbs the energy of the implant and the photoresist hardens further. Implants with even higher energy are used with elements of smaller size.
Extend the time period of the ashing process or perform a more aggressive ashing process to ensure that the photoresist is removed when hardened by the high energy implant. A combination of high energy implants and miniaturization of device geometries, when removing polymerized photoresist by ashing,
Yield loss may increase. For example, a thin gate oxide can be damaged if charged during ashing. Charge accumulates until the gate oxide is destroyed. The thinner the gate oxide, and the smaller the device geometry using it, the less charge is needed to create a breakdown condition.

[0017] Charged particles are created when an oxygen plasma is formed during the ashing process. Charged species (O
^+, O ^-) is removed by placing the charged gate ashing chamber. The charging gate is
Attracts charged oxygen species and prevents them from reacting with the photoresist. Nevertheless, not all charged species are removed by the charged gate. Prolonging the time period of the ashing process and removing the hardened photoresist results in more reactive species being reacted, increasing the potential for damaging the gate oxide. Thus, ashing is used to remove the photoresist, but probably not as much as previously used, due to the sensitivity of the device to charge damage. The photoresist remaining after the ashing process becomes more difficult to remove. In other words, a more aggressive chemical photoresist strip is required.

As noted above, ozone bubbling in sulfuric acid does not strip the photoresist at a rate equal to the hydrogen peroxide piranha process. The concentration of ozone in sulfuric acid is greatly increased by diffusing molecular ozone into the piranha process. For example, by combining ozone gas through a gas permeable membrane, an increase in ozone concentration of about one order or more can be obtained.

FIG. 1 is a diagram showing a process for increasing the concentration of ozone molecules in sulfuric acid according to the present invention. This process uses a membrane 11 having a first side and a second side, which is liquid impermeable but permeable to gases such as, for example, ozone. The membrane 11 is preferably made of a material such as a fluoropolymer that is not attacked by sulfuric acid. Since the membrane 11 is not permeable to liquids, it acts as a barrier to liquids. Thus, the liquid sulfuric acid used in the piranha solution flows along the first side of the membrane 11 but is prevented from flowing from the first side of the membrane 11 to the second side. In other words, liquid sulfuric acid does not flow across the membrane 11. On the other hand, the ozone gas flows across the membrane 11 from the second side to the first side. Arrows 12 parallel to membrane 11 indicate the flow of sulfuric acid and ozone gas along membrane 11. Arrows 13 perpendicular to the membrane 11 indicate the diffusion of ozone gas into sulfuric acid.

The mechanism for introducing ozone gas into a sulfuric acid solution at the molecular level is diffusion. At a given volume, a higher concentration of gas will tend to diffuse to regions having a lower concentration of the gas. Over a long period of time, the gas concentration reaches equilibrium throughout the volume. In FIG. 1, a situation occurs where a high concentration of ozone gas is placed near a liquid having a low concentration of ozone gas. Since the membrane 11 is gas permeable, ozone readily diffuses into the sulfuric acid solution. Typically, membrane 11 is designed to expose a large surface area of sulfuric acid to ozone. Since ozone gas always has a higher ozone concentration than sulfuric acid, the diffusion of ozone into sulfuric acid is continuous. As a result, a very high concentration of molecular ozone diffused into the solution.
lfuric acid).

An efficient means for introducing molecular ozone into a sulfuric acid solution is an osmotic membrane gasifier.
sifier). Osmotic gasifiers are commonly used in the drinking water industry to carbonate drinks like soda pop. Carbonation is a process in which carbon dioxide is introduced into an aqueous solution to produce carbonated water. Another use for osmosis gasifiers is to introduce ozone into water.
Water is often ozonated to prevent biologicals such as algae from forming in the water.

A permeable membrane gasifier compatible with the piranha process chemistry is manufactured by WL Gore & Assoc. The membrane in the WL Gore & Assoc. Osmosis gasifier is made of a fluoropolymer that is resistant to sulfuric acid. In general, a single membrane cannot efficiently transfer gas into solution. Hollow membrane fibers carry a small volume of fluid while maximizing the exposure of the surface area of the fluid to the gas. Typically, a permeable membrane gasifier consists of a number of membrane fibers through which a large volume of fluid flows.

FIG. 2 is a diagram of a photoresist stripping tank 21 commonly used throughout the semiconductor industry. Normal,
The photoresist stripping tank 21 includes a heating / cooling device 22, a pump 23, and a filter 24. The photoresist stripping tank 21 is made of a material that is resistant to the piranha solution, such as quartz or fluoropolymer. The photoresist stripping tank 21 has a wall 25, which divides the photoresist stripping tank 21 into a bathtub 26 and a reservoir 27. The height of the wall 25 separating the bathtub 26 from the storage unit 27 is lower than the height of the outer wall of the photoresist stripping bath 21. The bathtub 26 is filled with sulfuric acid up to the height of the wall 25. The reservoir 27 is filled with the piranha solution 18 to a point below the height of the wall 25. The piranha solution 18 that cascades into the reservoir 27 preferably does not splash back into the bath 26. The piranha solution 18 is pumped from the storage unit 27 to the bathtub 26, overflows from the bathtub 26, and flows out of the bathtub 26.
8 flows down the wall 25 and returns to the reservoir 27. The volume of the piranha solution 18 that is filtered and supplied to the bath 26 by the pump 23 is an amount sufficient to transport the fine particles floating on the surface of the piranha solution 18 in the bath 26 to the storage unit 27. Typically, the input port 31 of the tub 26 is at the bottom of the tub 26 and displaces particulates transported over the surface of the sulfuric acid.

A semiconductor wafer (not shown) having a polymerized photoresist formed on its surface is coated with a fluoropolymer water boat (not shown).
Place inside. By diving the wafer boat into the bath 26, the piranha solution 18 is stripped of the polymerized photoresist. The photoresist stripping process is typically a timed process. The prior art includes an ozone foaming device (not shown) in the bathtub 26,
It teaches ozonation of sulfuric acid. Reservoir 27 includes a drainage port coupled to pump 23. Pump 23
Pumps piranha solution 18 from storage 27 into bath 26. A filter 24 is coupled between the pump 23 and the bath 26 and filters any particulates from the etchant withdrawn from the reservoir 27. The piranha solution 18
It is supplied to the input port 31 of the bathtub 26.

According to one embodiment of the photoresist stripping process, the osmotic gasifier 28 is used to ozonize sulfuric acid at the molecular level. Osmotic gasifier 28 has a fluid input 35, a fluid output 36, a gas input 37, and a gas output 38. Fluid input 35 of osmosis gasifier 28
Is coupled to the particle filter 24 and receives the filtered piranha solution from the reservoir 27. Osmosis membrane gasifier 2
Eight fluid outputs 36 are connected to inputs of the heating / cooling device 22. The fluid output of the heating / cooling device 22 is
6 to complete the fluid loop from storage 27 to bath 26. Ozone generator 29 has an output coupled to the gas input of osmosis gasifier 28.
The gas output of the osmosis gasifier 28 discharges ozone gas for environmental treatment.

The operation of the photoresist stripping tank 21 is to continuously supply ozone dissolved in sulfuric acid through the osmosis gasifier 28. It should be noted that the sequence and connection of the components are not limited to the configuration shown in FIG. 2, but can be significantly changed according to the components used and the requirements for stripping the photoresist. Note that The piranha solution 18 from the reservoir 27 is pumped by the particle filter 24 to remove any particulates. Next, the piranha solution is supplied to the osmosis gasifier 28. The ozone generator 29 supplies ozone gas to the permeable membrane gasifier 28. Osmotic membrane gasifier 28 diffuses ozone gas into piranha solution 18 to form an ozonated piranha solution. As described above, the piranha solution 18
The concentration of molecular ozone dissolved therein is significantly higher than that of ozone foaming. Ozonated piranha solution 1
8 is supplied to a heating / cooling device 22. The heating / cooling device 22 heats the ozonated piranha solution 18 to a temperature that accelerates the reaction with the polymerized photoresist, for example, 90 ° C. to 140 ° C. The heating / cooling device 22 is not always external to the tub 26 or the reservoir 29, but often a heating coil is located in the tub 26 or the reservoir 29 to heat the solution. The heated ozonated piranha solution is then supplied to bath 26 to strip the photoresist from a semiconductor substrate (not shown), such as a semiconductor wafer.

[0027] The heated ozonated piranha solution 18 in the bath 26 hydrolyzes the polymerized photoresist on the semiconductor substrate, breaking down long chain polymers into short molecular chains. Ozone dissolved in piranha solution oxidizes short molecular chains to form carbon dioxide and water. The higher the concentration of ozone dissolved in the piranha solution,
Promotes oxidation of hydrolyzed photoresist.

It should be noted that heat is also a factor in the piranha process. The ozone foaming piranha process has lower heat than the hydrogen peroxide piranha process. The decrease in heat can increase the amount of ozone that diffuses into the sulfuric acid. Operating at lower temperatures reduces the amount of water that evaporates and dilutes the piranha solution during the photoresist stripping process. After the water by-products of the reaction have accumulated in the acidic solution above a predetermined point, the piranha solution is replaced. Also, the reaction is not as aggressive as the hydrogen peroxide piranha process and may not be suitable for removing photoresist hardened by high energy implants.

Since the permeable membrane gasifier efficiently dissolves ozone in sulfuric acid, the temperature of the solution can be increased while maintaining the ozone concentration in the solution at a high level. The increase in temperature reduces the amount of water remaining in the sulfuric acid solution after stripping the semiconductor wafer of each lot, thereby extending the shelf life of the solution. This is important because it also reduces human contact with acids. In addition, the higher the ozone concentration in sulfuric acid, the faster the photoresist is removed,
Manufacturing cycle time is reduced.

Using the sensor 30, the concentration of molecular ozone dissolved in the acidic solution in the bathtub 26 is detected. The piranha solution 18 is circulated through the osmosis gasifier 28 between wafer lots to increase the ozone concentration. Sensor 30 is used to indicate the ozone concentration or to indicate when the ozone concentration is sufficient and a new wafer lot is immersed in bath 26 to remove the photoresist. Ozonation of the piranha solution 18 can be accelerated by using one or more osmotic membrane gasifiers in parallel. Alternatively, depending on the application, a fixed time delay (instead of detecting the ozone concentration) may be appropriate, or no time delay may be appropriate. Osmotic gasifiers are very efficient and require a continuous circulating piranha solution through the gasifier to maintain a high ozone concentration without delay between wafer lots. It may be enough.

The permeable membrane gasifier reduces manufacturing costs. The first cost savings is obtained by extending the shelf life of the acidic solution, thereby reducing the amount of sulfuric acid used. A second cost saving is obtained by reducing the number of times the acid bath is filled and minimizing acidity related accidents. A third cost saving is obtained by increasing the throughput of the wafer in the photoresist stripping process. The shorter cycle time is due to the faster removal of photoresist due to the higher concentration of molecular ozone dissolved in sulfuric acid. A fourth cost saving is due to the reliability and cost of the osmosis gasifier. It is relatively inexpensive (compared to other semiconductor devices) and has no components to break, so it can be used for many years in a manufacturing environment.

By now it should be appreciated that a method and apparatus for removing photoresist on a semiconductor substrate has been provided. The photoresist stripping process according to the invention increases the molecular ozone concentration in the piranha solution. By increasing the ozone concentration, the exfoliation reaction is further activated, and the useful life of the acidic solution is extended. This process is simple and can be easily carried out by using a permeable membrane gasifier to ozonize sulfuric acid.

[Brief description of the drawings]

FIG. 1 illustrates a process for increasing the molecular concentration of ozone in sulfuric acid according to the present invention.

FIG. 2 is a diagram of a photoresist strip bath according to the present invention.

[Explanation of symbols]

 Reference Signs List 11 film 18 piranha solution 21 photoresist stripping tank 25 wall 26 bathtub 27 storage 31 input port 35 fluid input 36 fluid output 37 gas input 38 gas output

Claims (5)

[Claims] An apparatus for stripping a photoresist on a semiconductor, comprising: a photoresist stripper having a first port and a second port; coupled to the first port of the photoresist stripper. (23) having a fluid input and a fluid output; and a fluid input (35) coupled to the fluid output of the pump (23).
A permeable membrane gasifier having a fluid output (36) coupled to the second port of the photoresist stripper (21), a gas input (37) for receiving ozone gas, and a gas output (38). Apparatus (28); 2. A method for removing photoresist on a semiconductor substrate, comprising: diffusing ozone into sulfuric acid through a permeable membrane gasifier (28); and placing the semiconductor substrate in the ozonized sulfuric acid. Dipping to hydrolyze and oxidize the photoresist. 3. A method for increasing the concentration of molecular ozone in sulfuric acid for removing photoresist on a semiconductor wafer, the method comprising: diffusing ozone through a hydrophobic gas permeable membrane (11); Ozonizing; a method comprising: 4. A method for removing a photoresist on a semiconductor substrate, comprising: a film having a first side and a second side.
Providing the gas permeable membrane; flowing sulfuric acid across the first side of the membrane (11), making the membrane (11) a barrier to the sulfuric acid; 11) flowing ozone gas across said second side, wherein there is an ozone concentration difference between said first and second sides of said membrane (11); Diffusing ozone gas into said sulfuric acid to form ozonated sulfuric acid; and immersing said semiconductor substrate in said ozonated sulfuric acid to remove said photoresist. how to. 5. A method for extending the useful life of an acid for removing a photoresist on a semiconductor substrate, the method comprising: receiving sulfuric acid used to remove the photoresist;
Filtering the sulfuric acid to remove fine particles; diffusing ozone gas into the sulfuric acid by an osmotic membrane gasifier (28) to form ozonated sulfuric acid; and heating the sulfuric acid to form a photoresist. Increasing the reaction with and removing water by-products of said reaction.