WO1999059191A2 - Method and device for drying photoresist coatings - Google Patents

Abstract

The present invention relates to a method and device for drying photoresist coatings. A substrate (12) provided with the photoresist coating is impinged upon by infrared radiation from an infrared radiation source (4) with an adjustable output. The temperature of the area around the photresist layer is measured during drying. The output of the infrared radiation source is temperature-controlled so as to achieve a specific chronological temperature cycle. For this purpose, the inventive device is provided with a control unit (8) and a temperature measuring device (6, 7). The inventive method and device pertaining thereto enable especially thick photoresist coatings (≥20 νm) to be dried in an optimum manner over a short period of time. The photoresist mask that is subsequently produced has a high resolution.

Description

 Method and device for drying photoresist layers

The present invention relates to a method and a device for drying photoresist layers, in particular for microsystem and precision engineering.

Within the manufacturing technology in microsystem and precision engineering, the production of a mask using photoresist materials represents a crucial process step.

Photoresists are customized multicomponent systems that are used to manufacture microelectronic components,

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Multilayer systems and micromechanical ^" parts are used. The variety of photographic, chemical and mechanical requirements in the manufacturing process can only be met by suitably adapted photoresists. The photoresists are multicomponent systems which consist of a polymeric binder, a photoactive component and a solvent mixture. Determined the polymeric binder the physical

Properties, the photoactive component acts on the photochemical process, and the solvent mixture influences the behavior of the resist system during the drying process. The solvent mixture is composed in such a way that it contains a solvent which has a high vapor pressure in order to accelerate or promote the expulsion of the solvent mixture from the photoresist during the drying process.

The drying of the photoresist as an immediate precursor before the photolithographic step in the Manufacturing process is considered a very sensitive process step. The physical drying of the photoresists must be carried out in such a way that complete removal of the solvent mixture is achieved.

In modern production lines in the field of microelectronics, the resist coating of the wafers (wafers) is usually carried out on a centrifugal centrifuge at about 5000 min ^"1. The resist thickness generally ranges between 0.5 μm with flat or leveled surfaces and 2 The surface is dried on a hot plate at approximately 100 ° C, completely removing the solvent, and then the photoresist is adjusted and exposed in a special disk exposure device.

However, the drying time for thicker layers (> 40 μm), as they are required in particular in micromechanics, is usually 16 to 20 hours per batch with this conventional technology, so that this represents a bottleneck in the production line.

Furthermore, resist bubbles can form during the drying process for thicker layers, since photoresist has a high binder content and a low viscosity for these applications. The bubbles appear more often when drying in the oven and on a hot plate. These are solvent gas bubbles that stick to the dried photoresist. These bubbles can reach several 100 μm high and extremely deteriorate the structure resolution during the subsequent exposure in the production line (proximity effect). EP 0 509 962 A1 discloses a process for drying photopolymers on metallized substrates, in which the layers are dried by means of infrared radiation (IR radiation). This publication deals specifically with the

Curtain coating in printed circuit board technology, whereby thin layers in the range of 15 μm can be dried quickly and efficiently. However, this method cannot be integrated into a production line for microsystem technology. In addition, the mere exposure of thick layers (> 20 μm) to IR radiation, as required in microsystem technology, does not lead to satisfactory results with regard to the surface quality of the dried layers. In contrast to printed circuit board technology, the surface quality of the photoresists is of great importance in microsystem technology for the generation of high-resolution structures.

It is an object of the present invention to provide a

To provide methods and an apparatus with which process-integrated drying of photoresist layers with thicknesses of more than 20 μm is possible within a reasonable time. Furthermore, the drying process should be suitable for different resists of different thicknesses and for different combinations of resist / substrate and should enable the production of masks with high image accuracy.

The object is achieved with the method and the device according to the applicable patent claims 1 and 8. Advantageous refinements are the subject of the dependent claims.

With the method and the associated device, a drastic reduction in the drying time times by around 80% compared to known drying processes as well as significant energy savings. The drying process leads to homogeneous, evenly dried photoresists and thus allows a reduction in exposure times for highly built-up layers.

So far, no drying technology is known for microsystem technology that dries high-build liquid photoresists with a layer thickness> 20 μm in such a short time. The drying does not cause any chemical change in the resist.

The method and the device according to the invention provide an essential technological prerequisite for application in microsystem technology which meets the demand for ever shorter technological processing times for components. At the same time, the method according to the invention opens up the possibility of electroplating micromechanical components true to size and building up multilayer systems on one another.

With the method and the device according to the invention, it was possible to manufacture products which had previously not been able to be produced with the precision achieved with any other drying method.

In the method according to the invention, the applied photoresist layer is exposed to IR radiation in a vented process chamber, while at the same time the temperature or a temperature-dependent variable in the vicinity of the photoresist layer is measured. The output of the IR radiation source is dependent on the measured temperature or temperature-dependent variable (eg electrical resistance) in real time in order to achieve a predetermined temperature profile in the vicinity of the photoresist layer. This regulation makes it possible for the course of drying of the layer to be optimal for the respective combination of

Resist material and the substrate can be selected.

The space enclosed by the process chamber can be regarded as the surroundings of the photoresist layer. However, a temperature measurement as close as possible to the photoresist layer is preferable.

The temperature profile T (t) (T: temperature; t: time) can be chosen to be constant (T (t) = T ₀ = const) so that the temperature does not change during drying. The level of the temperature is adjusted according to the selected resist and substrate materials. The optimal parameters, ie the level of the temperature and the duration of the irradiation, and a possible change in the temperature over the drying time can be optimally determined by experimental tests. At the level of the temperature, of course, an upper limit must be observed above which the respective photoresist is destroyed.

The device according to the invention preferably consists of a ventable chamber with an air inlet and an air outlet for removing the solvents emerging from the photoresist. A preferably height-adjustable IR emitter is arranged in the chamber above a substrate holder. The substrate holder is preferably rotatable and can accommodate several substrates at the same time. A temperature measuring sensor detects the temperature during drying. Furthermore, a control unit is provided which determines the power of the IR radiator as a function of the measured one Temperature controls so that a predeterminable temperature profile over time can be realized at the measuring point of the temperature sensor.

The invention is described in more detail below using exemplary embodiments in conjunction with the drawings. Show here

1 shows an embodiment of the device according to the invention for drying photoresist layers;

2 shows an example of a predetermined temperature profile over time, which includes a ramp;

3 shows an example of an application of the drying method according to the invention for the production of pressure springs of read / write heads for hard disks;

4 shows a microscopic picture of a structure which can be realized using the drying process according to the invention;

5a shows an example of a particularly advantageous one

Receiving device for substrates or wafers in the device according to the invention in plan view; and

Fig. 5b is a sectional view of a circular

Single holder of the receiving device of Fig. 5a.

Fig. 1 shows a schematic diagram of an embodiment of an IR drying system according to the invention. It essentially consists of three functional parts, the actual furnace (ventable chamber) 1 with a holding option 5 for a defined number of wafers 12 of dimensions 4 "and 6", an IR radiation source 4 with associated power supply unit 9 and the control module 8. The control module combines the control hardware and software and the necessary computer technology, which take over the regulation of the power of the IR radiation source.

The footprint of the entire system is approximately 0.9 m ² in this example. The power consumption of the IR radiation source is 4 kW. The power to be absorbed is adjustable from 0 to 100%.

The detection of temperature is of fundamental importance for controlled process control. For this purpose, two different temperature measurement sensors 6, 7 are provided in the present case. The two temperature sensors, a pyrometer 7 and a temperature-dependent resistor 6 (PT100) can be used to complement the process. It goes without saying that other temperature measuring sensors, such as thermocouples, can also be used. It is also not necessary, as in the present case, to provide two separate temperature sensors. Rather, a temperature sensor, preferably a PT100, is sufficient to supply the control module 8 with the temperature data or a measurement variable that is in a fixed relation to the temperature.

On the basis of the drying processes previously carried out with the system shown on different combinations of photoresist / substrate and with different layer thicknesses of the photoresist, it was possible to recognize that an IR radiation source with a Power of 2.5 kW is sufficient for most applications.

Since a precise temperature measurement in the IR beam path is very complex, a relative measurement of the temperature of the surrounding air or the surrounding gas is carried out in the present device by arranging the temperature sensors below the wafer holder 5. Overall, a temperature measurement is outside the radiation, i.e. out of the range between the IR radiator and the wafer holder.

In the device shown in FIG. 1, an air inlet 2 and an air outlet or exhaust air outlet 3 are provided in the chamber 1. A controllable blower 13 is additionally arranged at the air inlet 2. The infrared radiation source 4 is height-adjustable via an adjusting device 10 above the rotary holder 5 for the wafers 12 with the applied photoresist layer. The IR radiation source 4 can in this case be formed, for example, from a holder for four IR tubes lying parallel next to one another at a distance of approximately 10 cm. The radiation source is supplied via an adjustable power supply 9. The power of the power supply 9 is regulated by the control unit 8.

The receiving possibility 12 for wafers is formed by a rotating sample plate, which receives several wafers in a star-shaped arrangement. In the present case, this turntable has a diameter of approximately 40 cm and can rotate at a speed of approximately 1 to 5 min. ^"1. The speed of rotation is likewise specified by the control unit 8. A speed of less than 5 min. ^{" 1} selected to cause the photoresist to run due to centrifugal forces prevent. The rotation is realized by the motor 11. The distance between the IR radiation source and the turntable is about 20 cm in the present case. The rotation of the wafers under the radiation source advantageously brings about a uniform drying of the layers on the wafers, it being possible for several wafers to be dried at the same time.

The air supply (cold air) and suction (warm air) realized in the process lead to the formation of a dynamic equilibrium of the temperature.

An example of a particularly advantageous receiving device for substrates or wafers in the device according to the invention is shown in FIG. 5a in

Shown top view. The substrate holder (5) consists of stainless steel and in the present example has six individual holders (14) arranged in a star shape for receiving six wafers (12). Of course, an arrangement with a larger or smaller number of individual holders can also be selected. For round wafers, circular rings with a recess (15) are used as a single holder in order to enable the wafers to be deposited into the rings using tweezers. The wafers (12) are advantageously only at the edge over a width of approximately 0.5 mm, so that no significant heat transfer to the wafer holder can occur.

This configuration of the receiving device therefore has the advantage on the one hand that the wafer (with

Photoresist) can be heated more quickly in the IR beam due to a lack of heat transfer to the holder. On the other hand, it is advantageously achieved that the same conditions prevail with regard to the heat transfer with each drying, since the heat coupling to the substrate is eliminated. A full-surface edition on one In contrast, the base plate would not allow constant heat transfer conditions due to possibly uneven contact.

Fig. 5b shows a sectional view of a circular single holder (14) of the receiving device of Fig. 5a. The individual holder has a height of approximately 10 mm on the outer circumference. The support surface (16) with a support width of the wafers (12) of approximately 0.5 mm can be clearly seen in the sectional view.

The realization of the control function in relation to the temperature in the vicinity of the layers is necessary in order to achieve good drying results. Experiments have shown that from the point of view of good process management, a temperature deviation from the specified temperature profile of less than 0.5 ° C. should be maintained. The exact description and measurement of the temperature behavior of the system depending on the IR emitter power is

Precondition for an exact regulation. These values must be incorporated in the control algorithm of the control unit. Software is advantageously used here. This takes advantage of flexible software control. There is the

Possibility to specify or develop and use specific control algorithms for the different photoresist and substrate combinations.

With the control it is possible to control the IR radiation source in a power range from 0 to 100%. Staircase-like and ramp-shaped temperature curves are possible by entering support points. Table 1 shows a list of different carrier substrates on which a 50 μm thick photoresist layer could be subjected to drying in accordance with the invention.

When drying photoresists based on novolac, it is important to ensure that the resist is not changed or decomposed. The thermal stability of the photosensitive component limits the maximum temperature that may occur during the drying process. Novolac based photoresists are stable up to around 100 to 110 ° C. The exact decomposition temperature of some photoresists can be determined with UV-VIS spectroscopy. Here, the absorption spectrum of the photoresists (light-sensitive component) at different drying temperatures is compared and the decomposition is concluded from the change. The photoresists for microelectronics and microsystem technology are mostly UV-sensitive and absorb between 340 nm and 405 nm wavelength.

The exact determination of the reaction rate of the decomposition and evaporation reaction should first be determined for each resist. Is for the development of time-optimized drying parameters

Determination of these reaction variables is very important, since this allows the upper limit of the drying temperature and the drying time to be optimally determined.

Table 2 shows different combinations of

Substrate and resist (commercially available under the name AZ 4562 from Hoechst or ma-PlOO from micro resist technology GmbH) as well as different layer thicknesses of the resist, which with the drying parameters specified there, i.e. Lamp power and

Drying time (time) could be optimally dried.

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I ftio O 2. The radiator power here refers to the maximum power of the radiation source used here of 4 kW. The structure resolution of the masks that can subsequently be produced from the photoresist layers is also specified.

Silicon wafers with nickel surfaces can be dried with the same parameters. The resist thickness in the table is to be regarded as the upper limit. Thinner layers can be dried in a shorter time.

An example of a generated structure, which could be produced using the mask produced from a photoresist layer dried according to the invention, is shown in FIG. 4. For the production, a 60 μm thick photoresist layer was IR-dried using the method according to the invention, a mask was produced therefrom by means of photolithography and electroplated with nickel. The thickness of the webs shown in the micrograph is approximately 20 μm.

The main areas of application for IR drying are high-viscosity and high-resolution photoresists. These are mainly exposed with contact imagesetters. Contact platesetters work according to the shadow casting principle. The mask structure is transferred 1: 1 into the resist. This means that the resolution of the lithography correlates with the distance from the mask to the photoresist.

After IR drying, the resist surface must therefore be as flat as possible so that there is a small distance from the lithography mask. However, this is offset by the formation of an edge bead when the resist is spun on and a bubble formation when drying.

The cause of the bead is the surface tension from the resist to the substrate and the high viscosity. In the "spin-on" process (coating process), resist is applied to the center of the wafer and the wafer is set in rotation. Depending on the time and speed of rotation, a resist layer of different thickness forms. An excess of resist remains on the edge of the wafer, which contracts to form a bead. Before exposure, the bead can be removed by a solvent spinning process.

The situation is different for resist bubbles that can form during the drying process. These bubbles can reach several 100 μm high and therefore extremely deteriorate the structure resolution during exposure. Although bubble formation is already significantly reduced by IR drying, it can also be almost completely suppressed by choosing a suitable temperature profile during drying.

For this purpose, the photoresist is kept sufficiently liquid, for example by a continuous increase in temperature during drying, so that the gas formed can still leave the resist surface. It is important that the temperature increases at the end of drying. The resist remains sufficiently viscous, although solvent evaporates continuously.

A temperature curve for suppressing the formation of bubbles is shown in FIG. 3 as a function of time. The temperatures for the constant temperature range and the maximum temperature at the end of the temperature curve are, for example, 90 and 105 ° C. However, these depend on the resist materials to be dried. This driving of a temperature ramp can be easily implemented with the device according to the invention on the basis of the control unit 8 in connection with the temperature sensors and the regulation of the IR radiation source. This is particularly advantageous because the tendency to form bubbles increases with increasing thickness of the photoresist layer to be dried.

With the method according to the invention or the device according to the invention, for example

Pressure springs of read / write heads for hard drives can be manufactured with high accuracy. Such a manufacturing process, during which IR drying according to the invention takes place, is shown in FIGS. 3A and 3B. Here, a silicon wafer (4 "or 10 cm in diameter) serves as the carrier substrate. A metallic layer, which functions as a galvanic starting layer, is applied to this substrate. Photoresist is then spun on (step no. 3), dried according to the invention (step No. 4), exposed and developed, the micro spring is now created by galvanically filling the lacquer structure, and finally the micro spring is detached from the silicon substrate by two etching processes.

In this example, a maximum radiation power of 4 kW was also used. Such pressure springs for read / write heads have so far not been able to be produced with the required accuracy due to the lack of suitable drying processes.

In the previous exemplary embodiments, an IR radiation source with a power of 4 kW was used in each case. With suitable drying conditions, the maximum of the IR radiation is about 2.6 μm. However, this is only an example. It goes without saying that radiation sources with different power and at different maximum wavelengths can be used depending on the application.

Claims

claims 1. A method for drying photoresist layers, in which a substrate (12) with an applied photoresist layer in a vented chamber is subjected to IR radiation from an IR radiation source (4) which can be regulated in terms of power, the temperature or a temperature-dependent variable in the The environment of the photoresist layer is measured and the power of the IR radiation source is regulated on the basis of the measured temperature or temperature-dependent variable in such a way that a predetermined temperature profile over time is maintained during drying. A method according to claim 1, characterized in that the predetermined temporal temperature profile is selected so that the temperature remains constant over the drying time. A method according to claim 1, characterized in that the predetermined temporal temperature profile is selected so that the temperature increases linearly with time. 4. The method according to claim 1, characterized in that the predetermined temporal temperature profile is selected so that the temperature is initially constant over the drying time, and then increases linearly, stepwise or in some other form. 5. The method according to any one of claims 1 to 4, characterized in that the temperature is measured below the substrate. 6. The method according to any one of claims 1 to 4, characterized in that the temperature of the photoresist-covered surface is measured from the top by a pyrometer, taking into account the different emissivity of the substrate on which the photoresist layer to be dried is taken into account. 7. The method according to any one of claims 1 to 6, characterized in that the predetermined temperature profile over time for each new combination of materials for the photoresist layer and substrate is first determined experimentally. 8. The method according to any one of claims 1 to 7, characterized in that the amount or content of solvents, preferably in an outgoing from the chamber line of air circulation, is detected, and in that the drying process is terminated by falling below a predetermined limit Reduction of the power of the IR radiation source is initiated. 9. The method according to any one of claims 1 to 8, characterized in that an IR radiation source is used which has its maximum IR radiation in the range of 1 to 3 microns. 10. The method according to any one of claims 1 to 9, characterized in that with simultaneous drying of the Photoresist layers of a plurality of substrates which perform a rotational movement about an axis in the chamber, the measurement of the temperature is timed in such a way that a measurement takes place each time one of the substrates passes through a measuring field in which the temperature is measured. 11. Device for drying photoresist layers, consisting of a ventable chamber (1) which has an air inlet (2) and an air outlet (3), one in the chamber above a substrate holder (5) attached IR radiation source (4), the the power is adjustable, one provided in the chamber Temperature measuring sensor (6, 7) and a control unit (8) which controls the output of the IR radiation source as a function of the measured temperature so that a predeterminable temperature profile in the chamber is maintained during drying. 12. The apparatus according to claim 11, characterized in that the IR radiation source (4) is arranged adjustable in height above the substrate holder (5). 13. Device according to one of claims 11 or 12, characterized in that the substrate holder (5) is designed so that it can accommodate several substrates (12) in a star-shaped arrangement next to each other. 14. Device according to one of claims 11 to 13, characterized in that the substrate holder (5) several Has individual substrate holders (14) which are designed such that the substrate rests only with a narrow edge. 15. The apparatus according to claim 13 or 14, characterized in that the substrate holder (5) is rotatably mounted and can be set in rotation at a predeterminable rotational speed by means of a motor (11) which can be regulated in speed. 16. The device according to one of claims 11 to 15, characterized in that a controllable fan (13) is provided at the air inlet (2). 17. The device according to one of claims 11 to 16, characterized in that the temperature measuring sensor (6) is formed by a temperature-dependent resistor, a pyrometer or a thermocouple. 18. Device according to one of claims 11 to 17, characterized in that the IR radiation source (4) has a maximum power consumption between 2.5 and 4 kW. 19. Device according to one of claims 11 to 18, characterized in that, preferably in the air outlet or in an outgoing line from the air outlet of the air circulation, a sensor for measuring the amount or content of solvents is provided, the output signal for determining the End of the drying process and can be used to regulate the power of the IR radiation source.