SYSTEMS AND METHODS OF SOLVENT TEMPERATURE CONTROL FOR WAFER COATING PROCESSES

Systems and methods of solvent temperature control for wafer coating processes are provided. In an embodiment, a method for spin coating a wafer includes attaching the wafer to a rotatable chuck. The chuck is then rotated, and solvent is dispensed onto the wafer. The solvent dispensing temperature is controlled while the solvent is dispensed onto the wafer.

TECHNICAL FIELD

The technical field relates to temperature control for solvents used in wafer coating processes, and more particularly relates to temperature control for solvents used in spin coating processes for wafers.

BACKGROUND

The semiconductor industry is continuously moving toward the fabrication of smaller and more complex microelectronic components. Market pressures are expected to force manufactures to continue to reduce the size of microelectronic in the foreseeable future. The reduced size of microelectronic components has resulted in modified production processes handling smaller tolerances, with closer spacing of the various components. One result of the smaller, more closely spaced components is a reduced tolerance for mis-alignment in the manufacturing process.

Microelectronics are often produced on a silicon wafer. Some wafers will undergo hundreds or thousands of different manufacturing steps to produce a product. Spin coating is a common manufacturing technique that is utilized in the production of many microelectronic components. For example, lithographic methods spin coat photoresist compounds onto wafers to position particular features or components on the wafer. Anti-reflective coatings are also spin coated onto wafers in some production processes.

Spin coating is used to provide a layer of material with a relatively uniform thickness on the wafer, but the thickness of the spin coated layer is not completely uniform. As the size and spacing of microelectronic components shrink, variations in thickness of a spin coated layer become more significant. Smaller, more closely packed components require stricter tolerances, so a coating layer with a non-uniform thickness can result in unwanted residues or unwanted over-removal in thicker or thinner regions, respectively. Non-uniform thickness can have other undesired effects as well, such as shading of neighboring regions for angled manufacturing processes. Many manufacturing processes downstream from a spin coating process are calibrated to a specific coating layer thickness, so variations in thickness from one wafer to the next are also problematic.

Therefore, systems and methods of producing more uniform and more consistent coating layer thicknesses are desired. Many coating layers are applied at specified temperatures and spin rates, and the thickness of the coating varies as the manufacturing conditions change. A variety of solvents are used in the spin coating process, and variations in the solvent temperature introduce variations in the manufacturing conditions. These variations can result in inconsistent or non-uniform coating layer thicknesses.

Accordingly, it is desirable to provide a system for controlling the temperature of solvents during the spin coating process to improve the uniformity and consistency of the spin coated layer. In addition, it is desirable to develop a method for controlling the temperature of the solvents during the spin coating process. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.

BRIEF SUMMARY

A spin coating system is provided for coating a wafer. The system includes a rotatable chuck with an attachment point for attaching the wafer to the chuck. A solvent dispensing system is configured to dispense solvent on the wafer, and a solvent temperature sensing element is coupled to the solvent dispensing system. The spin coating system also has a heat transfer system coupled to the solvent temperature sensing element and to the solvent dispensing system, and the heat transfer system is configured to adjust the temperature of solvent in the solvent dispensing system.

In another embodiment, the spin coating system includes a rotatable chuck with an attachment point for attaching the wafer to the chuck. A solvent dispensing system has a nozzle positioned to dispense on the wafer. A heat transfer system includes a heat transfer device coupled to the solvent dispensing system, and a heat transfer temperature sensing element. The heat transfer system is configured to adjust the temperature of a heat transfer fluid in response to readings from the heat transfer temperature sensing element.

A method for spin coating the wafer includes attaching the wafer to the chuck. Solvent is dispensed onto the wafer, and the chuck is rotated. The solvent dispensing temperature is controlled while the solvent is dispensed onto the wafer.

DETAILED DESCRIPTION

Coatings are routinely applied to wafers by spin coating during the microelectronics manufacturing process. The spin coating process often includes the application of various solvents to a wafer. The temperature of the wafer is equilibrated within relatively narrow tolerances before beginning the spin coating process, and relatively small changes in the wafer temperature can change the coating thickness and uniformity. It has been discovered that variations in the temperature of some solvents will change the thickness of the coating. Therefore, a temperature control system is added to the spin coating system to control the solvent temperature during application.

Referring now toFIG. 1, in accordance with an exemplary embodiment, a spin coating system10is used to apply various coatings12to a wafer14. The wafer14is typically made of silicon, and may be essentially pure silicon, doped silicon, or another semiconductor, such as selenium (Se) or other semiconductor materials. The spin coating system10includes a chuck16that rotates about its axis to spin the wafer14. A motor18is typically connected to the chuck16to provide the motive force, but other sources of motive force could also be used as long as the chuck16can be rotated. The chuck16includes an attachment point20for attaching the wafer14to the chuck16. The chuck16and wafer14may be positioned within an enclosure22for the spin coating process. In many embodiments, operation of the spin coating system10is highly automated, so a robot or other mechanical device controls the various steps in the process.

The spin coating process will vary in different embodiments, and many of the steps described below are repeated or omitted in various embodiments. The process begins by connecting the wafer14to the chuck16. In many embodiments, the attachment point20is a vacuum system, similar to a suction cup, that holds the wafer14in place. The attachment point20is generally designed such that there is no damage or permanent change to the wafer14from attachment to the chuck16. The wafer14is thin in many embodiments, such as about 500 to about 1,000 microns, so care is needed to prevent breaking or damaging the wafer14during placement and attachment to the chuck16.

After attachment to the chuck16, the wafer14is brought to a controlled hold temperature24. The hold temperature24varies for different coating materials, solvent loadings, spin rates, etc., but in one embodiment the hold temperature24is about room temperature. For example, a hold temperature24of about 20 degrees centigrade (° C.) is used in some embodiments, but hold temperatures of about 22° C., 25° C., 28° C., or other temperatures can also be used. In other embodiments, the hold temperature24is noticeably above or below room temperature. The wafer14is held on the chuck16for a hold time to allow the wafer14to equilibrate to the hold temperature24, where the space inside the enclosure22is maintained at the hold temperature24. The hold time may be about 1 second to about 2 minutes, but other hold times are also possible.

Once the wafer14has equilibrated to the hold temperature24, various materials are applied to the wafer. The wafer14has a coating side30and a back side32opposite the coating side30. In one embodiment, a solvent for reduced resist consumption solvent, also referred to as a reduced resist consumption solvent (RRC),34is applied near a center point of the wafer14on the coating side30, such as within 1 centimeter (cm) of the center point. However, in other embodiments, the RRC34is applied to the coating side30at locations further removed from the center point. In this description, materials designated as “solvents” are materials that are not intended to remain on the wafer14to aid in further processing or manufacturing steps. Most of the applied solvents do not remain on the wafer14when the wafer14is removed from the spin coating system10, but there may be a residue in some embodiments. The solvents may either spin or drip off the wafer14or evaporate, and no appreciable solvent residue is left behind for manufacturing processes downstream from the spin coating process.

In some embodiments, the RRC solvent34is applied prior to the coating fluid36, where the coating fluid36contains a material that will remain on the wafer14after the wafer14leaves the spin coating system10. In other embodiments, the RRC solvent34is applied while the coating fluid36is applied. The RRC solvent34aids the coating fluid36in spreading over the wafer14, and results in a lower volume of coating fluid36to achieve a target thickness. Many coating fluids36are expensive, so the RRC solvent34saves on coating fluid costs. The RRC solvent34is applied either before the wafer14begins to spin, after the wafer14begins to spin, or the RRC solvent34application begins before the wafer14starts to spin and continues as the wafer14starts spinning. The total amount of RRC solvent34applied can be about 0.2 to about 2 milliliters (ml), but other quantities are also possible. Several different RRC solvents34are available, such as gamma butyrolactone, 1-methoxy-2-propanol aka propylene glycol monomethyl ether (PGME), 1-methoxy-2-propanol acetate (PGMEA), ethyl lactate, n-butyl acetate, and 4-methyl-2-pentanol.

Either after or during the RRC solvent34application, the coating fluid36is applied to the coating side30of the wafer14. The coating fluid36is applied either before or after the wafer14begins spinning in various embodiments. In many embodiments, the RRC solvent34and the coating fluid36are dispensed onto the wafer14at close to the same point, and often within about 5 millimeters (mm) of each other. Centrifugal force from the spinning wafer14spreads the coating fluid36over the surface of the coating side30of the wafer14. The coating fluid34typically includes a residual material that will remain on the wafer14as a coating12, and a solvent that keeps the residual material in a liquid state for the spin coating process. The solvent in the coating fluid34is evaporated to leave a solid coating12on the wafer14. In some photoresist embodiments, the residual material includes a resin, which serves to bind the coating12, and an inhibitor or sensitizer that is the photoactive ingredient. Many other embodiments are also possible for the coating fluid34. The wafer14may be moved to a different location to evaporate the solvent from the coating fluid34, but the evaporation of the solvent is still considered part of the spin coating system10. Moving the wafer14to another location to evaporate the solvent from the coating fluid36frees the chuck16for another wafer14, which lowers the spin coating cycle time. There may be one or more applications of coating fluid36, and the RRC solvent34can be applied or skipped between different coating fluid applications. In many embodiments, the coating12is between about 35 nanometers (nm) to several microns, but other thicknesses are also possible.

The coating fluid36tends to flow to the wafer outer edge26, and sometimes flows onto the wafer back side32. This leaves a residue on the wafer back side32, and the residue can interfere with downstream manufacturing processes. A back side rinse (BSR) solvent38can be applied to the wafer back side32to remove the residue. The BSR solvent38is applied during or after the coating fluid36application, and if applied concurrently with the coating fluid36application the BSR solvent38application continues for a period of time after the coating fluid36application stops. The BSR solvent38dissolves and removes the residue from the wafer back side32.

The coating fluid36also tends to leave a bead, or a slight rise, on the coating side30near the wafer outer edge26, so the coating12tends to be slightly thicker there, as shown inFIG. 2, with continuing reference toFIG. 1. An edge bead removal (EBR) solvent40is applied to the coating side30of the wafer14approximately at the wafer outer edge26to remove the slight rise in the coating12. In some embodiments, the EBR solvent40is applied within about 5 millimeters (mm) to about 0 mm of the wafer outer edge26on the coating side30, but other distances are also possible.

During processing, the use of the BSR solvent38can change the thickness of the coating12on the coating side30of the wafer. Several tests were conducted measuring the coating thickness42, indicated by arrows, at several different locations on the wafer coating side30, so as to determine a coating thickness variation44. Some of the tests were conducted when no BSR solvent38was used, and others were conducted when a BSR solvent38was used. The standard deviation in coating thickness42for coatings12applied without a BSR solvent38was about 0.12 nm, and the standard deviation in coating thickness42for coatings12applied with a BSR solvent38was about 0.25 nm, or about twice that of no BSR solvent38. Coatings12when BSR solvent38was used also had an average thickness of almost 0.3 nm more than coatings12when no BSR solvent38was used. The BSR solvent38and the coating fluid36were dispensed on opposite sides of the wafer14, so there was no direct contact of the BSR solvent38and the coating fluid36over the vast majority of the wafer14. There may have been some minor interaction at the wafer outer edge26, but the larger coating thickness variations44were present over the entire coating side30of the wafer14.

In some tests, the BSR solvent38was added while the coating fluid36was applied, and in other tests the BSR solvent38was added only after the coating fluid36was applied. The test results were essentially the same when the BSR solvent38was added during the coating fluid36application and when the BSR solvent38was added after the coating fluid36application. Bulk BSR solvent38was not kept in a temperature controlled area, and the difference in the hold temperature24(which is the wafer temperature when the coating fluid36is applied) and the bulk BSR solvent38was only about 1° C. to about 2° C. Because the BSR solvent38does not directly interact with the coating fluid36, it was determined the temperature change induced by the BSR solvent38impacted the coating thickness42. The BSR solvent38negatively impacted the coating thickness variation44even when applied after the coating fluid36, so it was theorized that the change in temperature impacted the coating fluid viscosity, thermal expansion, or some other property of the coating fluid36enough to change the coating thickness42.

The coating12has a less consistent coating thickness42when the BSR solvent38is used, and there may be similar results for other solvents, such as the RRC solvent34and the EBR solvent40. Therefore, to reduce the coating thickness variation44, and to improve coating thickness42consistencies between different wafers14, a heat transfer system50is used to control the temperature of the solvents applied during the spin coating process, where an exemplary embodiment is shown inFIG. 3with continuing reference toFIGS. 1 and 2. The heat transfer system50is configured to adjust the temperature of the solvent in response to a reading from a temperature sensing element. Many solvents are relatively volatile, so they will evaporate when dispersed on a wafer14. The evaporation of the solvent will result in evaporative cooling, so the proper solvent dispensing temperature may be different than the hold temperature24for the wafer14. The best solvent dispensing temperature can be determined experimentally, and the best solvent dispensing temperature might vary from one solvent to another and from one coating fluid36to another. Therefore, different experimental testing for each solvent and coating fluid36can be used.

Several tests can be run to experimentally determine the solvent dispensing temperature for use in the spin coating system10. Different experimental testing processes can be used, but one embodiment is described herein. The first step is to establish standard conditions for the coating process, including a hold temperature24and a rotation speed for the wafer14. Next, several wafers14are coated at the standard conditions, except the solvent dispensing temperature of a selected solvent is varied and recorded. The coating12is cured, and the coating thickness42is measured at several points on each wafer14. The coating thicknesses42are measured at the same general locations on each wafer14. The coating thickness variation44and total coating thickness42is determined based on the measurements, and an acceptable range is determined for the solvent dispensing temperature. The desired solvent dispensing temperature can be based on a variety of factors, such as (a) most consistent coating thickness on a single wafer14, (b) most consisting coating thickness for different wafers14, (c) a measured coating thickness closest to a desired coating thickness, or (d) other factors. The testing provides a desired solvent dispensing temperature, or a set point for the temperature, as well as an acceptable range for the solvent dispensing temperature. For example, the most consistent coating thickness42may be a BSR solvent dispensing temperature of about 24° C., but solvent dispensing temperatures ranging from about 23° C. to about 25° C. may give acceptably consistent coating thicknesses42. Once a desired solvent dispensing temperature is determined for one solvent, that solvent dispensing temperature is set as a standard condition and the process is repeated for the next solvent.

Once the solvent dispensing temperature is determined, a heat transfer system50is used to control the solvent dispensing temperature to within an acceptable range. The heat transfer system50has several components. A heat transfer device54is a device that transfers heat from a heat transfer fluid52to the solvent. The heat transfer device54may be a heat exchanger56, a tracing line58, a radiant heat transfer system, or other devices, as shown inFIGS. 4 and 5, with continuing reference toFIGS. 1-3. In some embodiments, the heat transfer device54is coupled to a solvent line70for heat transfer. In many embodiments, insulation71is positioned about the solvent line70to minimize unwanted changes in solvent temperature. In other embodiments, the heat transfer device54is coupled to other components of the solvent dispensing system, as long as the solvent dispensing temperature can be controlled within an acceptable solvent dispensing temperature range. The heat transfer fluid52can be a gas, but in many embodiments it is a liquid such as water or a glycol and water mixture. In many embodiments the heat transfer system50also includes a heat transfer fluid tank60for storing bulk heat transfer fluid52, and a heat transfer pump61moves heat transfer fluid52from the heat transfer fluid tank60through the heat transfer device54and back to the heat transfer fluid tank60. A cooling element62and/or a heating element64are used to increase or decrease the temperature of the heat transfer fluid52, and the cooling and/or heating elements62,64can be positioned in the heat transfer tank60or in other locations. In other embodiments, two streams of different temperature are mixed together to produce a heat transfer fluid52with the desired temperature, and the heat transfer fluid52is discarded after use. Many other embodiments are also possible.

In the heat transfer system embodiment shown inFIG. 3, a heat transfer control unit66is coupled to a heat transfer temperature sensing element68and a solvent temperature sensing element72. The heat transfer temperature sensing element68is positioned to measure the temperature of the heat transfer fluid52, and the solvent temperature sensing element72is positioned to measure the temperature of the solvent. The temperature sensing elements68,72can be thermocouples, thermometers, radiant heat sensing devices, or other devices capable of measuring temperature. In some embodiments, the solvent temperature sensing element72is positioned to measure the solvent temperature close to the point of application onto the wafer14, such as within about 1 to about 4 centimeters (cm) of a dispensing nozzle. The heat transfer control unit66is coupled to the cooling element62and/or the heating element64, and adjusts the amount of cooling/heating based on the temperature of one or both of the heat transfer temperature sensing element68or the solvent temperature sensing element72.

The heat transfer system50can be oversized for the amount of solvent dispensed, so much so that controlling the temperature of the heat transfer fluid52effectively controls the solvent dispensing temperature. For example, in some embodiments less than 3 ml of solvent are dispensed on a wafer14. Therefore, a heat transfer system50that transfers heat from 1,000 ml of heat transfer fluid52to 3 ml of solvent would bring the solvent to the temperature of the heat transfer fluid52. If the heat transfer system50is sized and configured to flow very large quantities of heat transfer fluid52past the solvent line70, the heat transfer system50can control the temperature of the solvent by reading the heat transfer temperature sensing element68. In this embodiment, there is no need for a solvent temperature sensing element72because the solvent is brought to the temperature of the heat transfer fluid52.

In an exemplary embodiment, solvent is dispensed from a solvent dispensing system74, as illustrated inFIG. 6with continuing reference toFIGS. 1-5. In some embodiments, the solvent dispensing system74includes a solvent line70, a solvent storage tank76, a solvent pump78, and a nozzle80. In other embodiments, the solvent tank76can be pressurized so no solvent pump78is needed, and other embodiments are also possible. The solvent storage tank76can be as simple as a drum, bucket, or tote bin, but in other embodiments the solvent storage tank76is a dedicated tank. A wide variety of pumps can be used for the solvent pump78, but the solvent pump78should be able to dispense small quantities of solvent, such as 0.1 to 20 ml at a time. In many embodiments, the total solvent discharged per wafer is less than 30 ml. A wide variety of effective nozzles80are available, and essentially any hole that allows solvent to escape the solvent line70can be used, but some nozzle designs are more effective than others.

The coating fluid36is dispensed from a coating fluid dispensing system82, in an exemplary embodiment, as shown inFIG. 7with continuing reference toFIGS. 1-6. The coating fluid dispensing system82includes a coating fluid tank84, a coating fluid pump86, a coating fluid line88, and a nozzle80. In an exemplary embodiment, the nozzle80is positioned to dispense near the center point of the coating side30of a wafer14positioned on the chuck16, and the coating fluid nozzle80is positioned within 5 mm of the reduced resist consumption solvent line nozzle80. However, other embodiments are also possible. As with the solvent dispensing system74, several variations and different embodiments can be used.

FIG. 8illustrates one embodiment of a spin coating system10with a heat transfer system50coupled to a RRC solvent dispensing system74. The solvent storage tank76is positioned in a temperature controlled storage area90, and the temperature in the temperature controlled storage area90is set at the desired solvent dispensing temperature. The temperature controlled storage area90maintains a desired temperature with a storage area heating and/or cooling element92coupled to a storage area temperature sensing element94. The solvent line70enters a heat exchanger56, which is part of the heat transfer system50, to adjust the temperature. The heat transfer control unit66is coupled to two different heat transfer temperature sensing elements68and a solvent temperature sensing element72. A heat transfer pump61moves heat transfer fluid52through the heat exchanger56, and a coating fluid dispensing system82dispenses coating fluid36on the wafer.

FIG. 9illustrates another embodiment of a spin coating system10with a tracing line58and a BSR solvent dispensing system74. The solvent storage tank76is positioned within the temperature controlled storage area90, and the heat transfer control unit66is coupled to one heat transfer temperature sensing element68.FIG. 10illustrates yet another embodiment of a spin coating system10with a heat exchanger56and a solvent dispensing system74for EBR solvent40. The duty on the heat exchanger56can be reduced or eliminated by using a temperature controlled storage area90. In yet other embodiments, the temperature of the solvent is controlled with just the temperature controlled storage area90and insulation71on the solvent line70.