WO2009112655A1

WO2009112655A1 - Method of fabricating photomasks and device for implementing it

Info

Publication number: WO2009112655A1
Application number: PCT/FR2008/052425
Authority: WO
Inventors: Arnaud Favre; Magali Davenet; Jean-Marie Foray
Original assignee: Alcatel Lucent
Priority date: 2008-03-05
Filing date: 2008-12-29
Publication date: 2009-09-17
Also published as: KR101253825B1; JP2011513783A; CN101925860A; CN101925860B; JP5372966B2; KR20100101003A; KR101253948B1; KR20130016404A

Abstract

The method of fabricating photomasks comprises at least one step of cleaning the photomask and at least one step of laying a protective film on the photomask after it has been fabricated. The method according to the invention furthermore includes at least one step of removing the ammoniacal and sulphate residues between the cleaning step and the film application step. This step comprises the following operations: the photomask is placed in a sealed enclosure; a low pressure is established in the sealed enclosure by pumping out the gases that it contains; the photomask is subjected to infrared irradiation; the infrared irradiation is stopped; a check is made that the temperature of the photomask is at most equal to 50°C; atmospheric pressure is re-established in the enclosure; and the photomask is extracted from the enclosure. The device for implementing the method according to the invention comprises a sealed enclosure containing at least one photomask, a pumping unit for creating and maintaining the vacuum inside the enclosure, a system for holding at least one photomask in place, positioned inside the sealed enclosure, infrared irradiation means, and a gas injection system.

Description

Method of manufacturing photomasks and device for its implementation

The present invention relates to a method for manufacturing photomasks, in particular used in the manufacture of microelectronic components with submicron dimensions. It also extends to the device for the implementation of this process.

The micromachining of semiconductor substrates, in particular of silicon, is currently carried out by plasma etching techniques from a pattern transferred onto the substrate from a mask. A photomask is equivalent to a negative in photography: it contains information to be printed on a medium 11 which is generally used in transmission for insolations and printing on semiconductor substrates. Different parameters whose focusing wavelength defines the depth of the active area that is printed directly on the substrate Outside this area, the details are not printed but may have an impact on the transmission of the photomask. Pollution in the active zone has a direct effect on the image printed on the substrate with the impression of a defect But they have only an indirect effect on this image. Suspended outside this zone, as for example the decrease contrast or reduction of photomask transmission.

Otherwise. (Semiconductor industry seeks to reduce the size of the inscribed image in order to obtain ever smaller, integral and less costly electronic components.) Photomask dimensions are shrinking, pollution requirements are becoming more and more important. The photomask is therefore a key element, expensive and complex, that one seeks to keep clean and operational.The active area of the photomasks must absolutely be free of any particles, especially in the focal plane, because these contaminating particles create a defect. which is printed and repeated on the semiconductor substrate.At the end of its manufacture, the mask is cleaned and a film is applied to the photomask to protect its active side from any particles. photomask during its life at the utifisatcur.Laminating consists of a deposition of an optical membrane (parallel multi-layer surfaces) a a good transmission and a reduced impact on the optical rays that pass through this film is deposited on the side of the active face of the photomasque, and separated therefrom by a space. The pollutants likely to be deposited on the active face of the photomask will thus be deposited on the film outside the focusing zone (physical distance from the active surface). Thus, these pollutants will not be printed in the transfer by lithography: the film does not directly protect particulate pollutants but reduces their impact on the image.

Document US-2001/005944 has focused on the removal of gaseous contaminants from the surrounding atmosphere, such as O ₂ , CO ₂ and H ₂ O, which may be present in the space between the photomask and the film . These contaminants are particularly troublesome because they prevent the smooth operation of the photolithography operation by inhibiting the transmission of the 157nm radiation that is usually used. Defaetarnination takes place in a sealed enclosure, under vacuum or under optically inert gas, by exposure to UV radiation, plasma, ozone, and / or heat. This treatment has the effect of accelerating the desorption of gases.

However, the increase in the energy required for insolation for the creation of smaller and smaller patterns has created a new problem. The gases present under the pedicle, such as ammonia, fluorine and organic volatile compounds, βe recombine under the effect of this high energy to generate crystals that will grow with time. These crystals appearing under the film, therefore in the focal zone, produce defects in the area printed on the substrate. These crystals represent a major problem because they generate unpredictable and numerous defects on the substrates and can affect more than 20% of the most advanced current photomasks. One of the chemical reactions occurring can be schematized by the following formula:

Ammonia NH ₃ comes from multiple sources, but essentially from human activity in photomask manufacturing areas and use of these photomasks. In order to reduce the phenomenon of crystalline growth, semiconductor and photomask manufacturers have invested heavily to limit the ammonia present in the clean room and have defined photomask storage and transport strategies under protected environment to reduce their contact with ammonia. Surfactic acid H ₂ SO ₄ is widely used by photomask manufacturers in the manufacturing stages, and in particular during etching and stripping operations. In the most common case, the last cleaning step, before filming, requires sulphates and generates sulphated residues. The last etching step, of removing the resin previously deposited and which precedes the cleaning, also generates sulphate residues. These sulphated residues trapped under the film will therefore desorb, which is the main reason for the appearance of these crystals in the manufacturers of electronic chips. Photomask manufacturers endeavor to reduce the amount of sulfates used in the cleaning steps by modifying the processes or adding steps to reduce the sulfate residue content. However, these new processes or palliative techniques put in place are more expensive and less effective, and it is not possible to completely eliminate the use of sulfates in the manufacturing steps.

The film is put in place after a stripping step followed by a cleaning step. The last cleaning step is done in a clean area. This film will make it possible to guarantee that the particles generated in the clean room or in the production equipment will not rest on the active face of the photomask, One of the palliative solutions consists in periodically inspecting the active surface of the photomask. When the first crystals appear, the photomask is sent back to the manufacturer The film is removed, cleaned and a new film is deposited on the photomask This must be done by the manufacturers of photomasks and not by the users, which causes a loss time and significant additional costs of inventory management related to the shorter use of photomasquβs.

The most important problem today for manufacturers of semiconductor components is therefore the availability and the duration of use of these photomasks for the production of semiconductors. Indeed these photomasks are a major link in the manufacture of electronic chips, and their increased technicality implies a cost more and more important

The present invention therefore aims to allow a longer use of photomasks by decreasing the frequency of cleaning operations. Linventioπ also aims to reduce the risk of degradation of the photomask associated with the poflution by formation of crystals in the volume placed under the film.

It is another object of the invention to provide a process for the reclamation of residual ammonia and sulfated compounds resulting from the manufacture of photomasks.

The object of the present invention is a photomask manufacturing process comprising at least one cleaning step of the photomask and at least one step of placing a protective film on the photomask. The method further comprises at least one step of removing ammonia and sulfated residues between the cleaning step and the step of placing the particle. The step of eliminating ammonia and sulphated residues comprises: placing the photomask in a sealed enclosure; establishing a low pressure in the sealed enclosure by pumping the gases contained therein;

the photomask is subjected to infrared radiation,

- the infrared radiation is stopped,

- it is verified that the temperature of the photomask is at most equal to 50 ° C,

- the atmospheric pressure is restored in the chamber. and the photomask is extracted from the enclosure.

The gases are preferably pumped for a period of between 20 minutes and 5 hours.

Infrared radiation (IR) accelerates the selective desorption of the targeted species, and improves their efficiency Pumping the gases contained in the enclosure to establish a vacuum, used simultaneously with infrared radiation, substantially improves the desorption of residues, and In particular, it is possible to desorb almost all the ammonia and sulphated compounds resulting from the cleaning and stripping steps. The removal of ammonia and sulphated residues allows the laying of the protective film on a perfectly clean substrate. The wavelength of the infrared radiation is the main parameter influencing the description, which will be done more or less in prorounder according to the chosen wavelength. Infrared radiation waves of said wavelength "Short" will go deeper into the material than wavelengths of "medium" or "long" wavelengths that will be more effective on the surface.

Infrared radiation must be carefully controlled because it causes a θcnauffement the photomask bridge temperature must not exceed 300 ^° C imperative Beyond 3CX) ⁹ C. photomask is irreversibly damaged. The temperature may be between 50 ° C and 300 ° C, preferably between 50 ° C and 150 ° C, and more preferably at a temperature of 80 ° C. The increase in temperature resulting from the application of the infrared radiation contributes to acceleration of the desorption phenomenon by diffusion. Advantageously, the atmospheric pressure is re-established in the chamber when the temperature inside the chamber is less than or equal to 50 ° C., which may require a waiting time after stopping the infrared radiation.

According to a particular variant, a clean gas is introduced with a constant flow simultaneously with the pumping of the gases in the chamber. The presence of such a gas is likely to accelerate the desorption of some other organic compounds.

The rise in pressure inside the chamber is preferably performed by injection of a clean non-reactive gas, such as air or a neutral gas such as nitrogen or argon.

To further increase the performance, the ammonia and sulphate residue removal process can be used not only after the last cleaning step, but also after other manufacturing steps prior to cleaning and using sulphated residues, such as exempts the stripping step.

The invention also relates to a device for implementing the previously described method comprising: a sealed enclosure containing at least one photomask,

- a pumping group to install and maintain the vacuum inside Penceinte,

a system for holding at least one photomask, placed inside the sealed enclosure,

infrared radiation means, a gas injection system.

The holding system may advantageously be designed to allow the simultaneous processing of several photomasks. According to an alternative embodiment of the device, the internal walls of the enclosure reflect the transmitted waves.

According to another variant embodiment, the gas injection system comprises one or more shower-shaped injectors. According to another variant embodiment, the gas injection system comprises one or more particulate filters.

The device may further comprise a pressure gauge for controlling the pressure inside the enclosure.

The device may also include a temperature probe for measuring the temperature of the photomask.

Other features and advantages of the present invention will become apparent from the following description of embodiments, given of course by way of illustration and not limitation, and in the accompanying drawing in which:

FIG. 1 shows schematically the various steps of an embodiment of the process according to the invention,

FIG. 2 schematically shows an example of the positioning of the infrared radiation means with respect to the photomasks,

FIG. 3 represents an installation adapted to carrying out the step of eliminating ammonia and sulphated residues,

FIG. 4 represents a variant of an installation adapted to carrying out the step of eliminating ammonia and sulphated residues,

- Figure 5 is a comparison of the residual sulfates content in the photomasks at the end of manufacture.

One embodiment of the photomask manufacturing method, according to the present invention, is schematically shown in FIG. 1. The manufacture of photomasks usually comprises several steps. A substrate, for example made of quartz 1 coated with chromium 2, is covered with a layer of resin 3 on which the pattern to be etched is reproduced by means of a laser or electronic beam, for example

(step A). Step B is an etching step during which the pattern is etched in the chromium layer 2. During a step C _1, the photomask, once etched, is wet etched in order to eliminate the resin 3 and the byproducts of the attack reaction. The photomask obtained then undergoes several successive cleaning (step E), control (steps D and F) and possible repair (step G) operations. during the steps D to Q. Final cleaning is carried out during step H. The cleaning conditions commonly used involve the implementation of sulphates which must be discarded before the photomask recovery step I by 4. Indeed, for the reasons mentioned above, the presence of sulfates in the active zone 5 of the photomaεquβ under the film 4 must absolutely be avoided.

A removal step of the ammonia and sulfated residues, using the combination of infrared radiation and vacuum pumping, is interposed between the cleaning step H and the pelleting process to remove the contamination. photomask, especially by sulphates. This step J includes several operations that make up three distinct phases.

During a first phase, the photomask being in the chamber, the gases present in the chamber are pumped. During this part, the control parameter is the pumping speed. The pressure drop slope is adjusted to prevent water crystallization. Simultaneously, the infrared radiation means are turned on to allow the wavelength control system to be preconditioned. The photomask is subjected to infrared radiation to accelerate the degassing of contaminants while pumping continues

The second phase is carried out on a temperature and pressure stage. The three temperature, pressure and IR wavelength parameters are interdependent. The wavelength of the infrared radiation is adjusted to allow the desorption of the ammonia and sulphated residues. The pressure controls the desorption threshold and the temperature is controlled to allow adjustment of the wavelength regulation. The infrared radiation being stopped, the third phase begins with a rise in pressure in the chamber as soon as it has reached a temperature equal to or less than 50 ° C. The low pressure in the chamber helps to lower your temperature. The control parameter of this phase is the temperature. A pressure control in the enclosure can also be used to control the cooling. The pressure rise is carried out using a non-reactive clean gas. At the end of the cycle, the imposed pressure of clean gas is slightly higher than the atmospheric pressure for a short period of time so as to favor the adsorption of the clean gas on the surface of the photomask, which makes it possible to protect it from external contaminants when it is removed from the enclosure. The photomask is cooled to a temperature at most equal to 50 ° C, so as to emerge from the chamber at a temperature close to room temperature, in order to avoid the re-adsorption of gas present in the atmosphere that could occur during the decline of temperature.

In another embodiment, this step of eliminating ammonia and sulphated residues may also be placed before cleaning step H, in particular after certain steps involving the persistence of sulphated residues. An elimination step J 'could for example be inserted further between the pickling step C and the control step D.

FIG. 2 shows an example of how the infrared radiation generated by the radiation means 21 is reflected on the photomasks 22 on the one hand and on the internal reflective walls 23 of the sealed enclosure 24 on the other hand Heaters can be placed above the photomasks and / or below the photomasks, or interposed between two layers of photomasks for example.

The use of infrared radiation is preferred because the selectivity with respect to the species to be eliminated and the yield under vacuum is high. By judiciously choosing the characteristics of the infrared radiation 20, such as, for example, the wavelength, the desorption will be carried out more or less in depth. Infrared radiation waves of "short" wavelength will go deeper into the material than wavelengths of "medium" or "long" wavelengths that will be more effective on the surface.

A temperature below 300 ° C, for example close to 80 ° C can be applied without damaging the photomask. Advantageously, a regulation of emission of the infrared radiation by hashing, ie by successive application of a voltage V and a voltage nuHe making it possible to obtain power peaks of the infrared radiation, is used. This regulation makes it possible to control the heating of the photomasks without losing the characteristics (wavelength) of the infrared radiation. This regulation also makes it possible to vary the wavelength of the infrared radiation. By combining the emission control of the hash radiation and the modification of the wavelength of the infrared radiation, it is thus possible to cause the desorption of the photomask at several depth levels in the material. Another method to add infrared energy to photomasks is to use a microwave generator coupled to a metal bar that will radiate infrared waves.

In the embodiment of the invention illustrated in FIG. 3, the photomasks 31 (not yet carrying a film) are placed in a sealed enclosure 32 held under vacuum by means of a pumping unit 33. A pressure gauge 34 controls the pressure inside the enclosure 32. The photomasks are placed on superimposed shelves and are supported by non-metallic spacers 36. In this case, the photomasks are subjected to infrared radiation by means of a device 37 placed on the enclosure wall 32, for example a microwave device as mentioned above. The device 37 is controlled by a control loop 38 radiation as a function of the temperature of the photomask 31 measured by the associated temperature probe 39. The geometry and the arrangement of the radiating elements 37 are chosen in order to obtain a homogeneous and optimized action on the entire surface of the photomasks 31.

Advantageously, the surface of the enclosure 32 may be mechanically or electrolytically polished to promote the reflection of the infrared radiation on the photomasks 31. The shape of the enclosure 32 also makes it possible to distribute the infrared radiation homogeneously. One of the important constraints to which the installation is subjected is that the implementation of the method must not generate particles. This is why the gas injection system 40 comprises at least one shower-shaped injector 41 for reducing the injection speed in the chamber 32 under vacuum. The injection system 40 is furthermore provided with particulate filters 42. Advantageously, the injection system 40 comprises one or more gas injectors in the form of a shower 41, which makes it possible to avoid gas turbulence during the delivery of the gas. the chamber 32 at atmospheric pressure The atmospheric pressure resetting step is carried out according to a mathematical equation of the type: y = ax ² + b in which y is the flow and x is the pressure. This way of proceeding makes it possible to have a low injection speed and a low pressure, the porticular contamination being greater at low pressure.

When the removal of ammonia and sulphated residues is carried out, means of measuring outgassing 43 make it possible to ensure that the operations run smoothly, by monitoring at least one of the following parameters:

- the partial pressure of the gases, the pressure limit of the pumping unit 33,

the weight of the photomask 31,

the power reflected by the walls of the enclosure 32. The pumping unit33, the injection system 40 and the degassing measurement means 43 are connected to a control and control device 44 called PLC (for "Programmable Logic Controller ").

FIG. 4, which illustrates another embodiment of a plant 31 suitable for carrying out the step of eliminating ammonia and sulphated residues, in which the radiation device 47 is placed outside the enclosure, will now be considered. 48 under vacuum. An interface 49, for example a porthole, formed in the wall 48 allows the waves to pass in the direction of the photomask 31. The choice of the material constituting the interface 49 between the radiation device 47 and the photomasks 31 is critical because it material must let through the waves for the photomask 31, without posing a problem of dissipation of the radiation they carry. The quartz will advantageously be chosen.

FIG. 5 presents comparative results of the measurement of the residual sulfates content in the photomask, carried out by the ion chromometric method. The sulphate levels 50a, 51a, 52a result from cleaning the photomasks by three different variants I, II, Ht of the cleaning process. The sulphate levels 50b, 51b, 52b are obtained at the end of the elimination step according to one embodiment of the invention which follows a cleaning step for each of the three variants. The comparison of these results shows the removal efficiency of the sulphate content of the photomasks. The current goal of photomasque manufacturers for 1θ3nm technologies is to have a sulphate level of less than 1 ppbv (parts per billion volumetric), so that they do not have a crystal growth problem in their customers. FIG. 5 shows that the values reached 50b, 51b, 52b thanks to the invention are largely below this objective.

Claims

A photomask manufacturing method comprising at least one step of cleaning the photomask and at least one step of placing a protective film on the photomask, and further comprising at least one step of removing the ammonia residues and sulphated between the cleaning step and the step of placing the film comprising

the photomask is placed in a sealed enclosure,

a low pressure is established in the sealed enclosure by pumping the gases contained therein,

the photomask is subjected to infrared radiation,

- the infrared radiation is stopped,

- restoring the atmospheric pressure in the enclosure, and

the photomask is extracted from the enclosure.

2. The method of claim 1, wherein the gases are pumped for a period of between 20 minutes and 5 hours.

3. The method of claim 1, wherein a clean gas is introduced with a constant flow simultaneously pumping.

The method of claim 1, wherein the photomask is heated to a temperature of from 50 ° C to 300 ° C.

5. The process according to claim 1, wherein the atmospheric pressure is re-established by injection of clean nonreactive gas.

The method of claim 1, further comprising a step of removing ammonia and sulfated residues after another manufacturing step and prior to said cleaning step.

7. Device for implementing the method according to claim 1, comprising:

a sealed enclosure containing at least one photomask,

- a pumping unit to install and maintain the vacuum inside the enclosure. a system for holding at least one photomask, placed inside the sealed enclosure,

infrared radiation means,

- a gas injection system.

Device according to claim 7, wherein the internal walls of the enclosure reflect the transmitted waves.

Device according to claim 7, wherein the gas injection system comprises at least one shower-shaped injector and at least one particulate filter.