Patent Application
20050147916
This invention relates to the field of fabricating semiconductor devices and, in particular, to negative tone photoresists.
Modern integrated circuits generally contain several layers of interconnect structures fabricated above a substrate. The substrate may have active devices and/or conductors that are connected by the interconnect structure. As these devices become smaller the need for fine patterning through photolithography has become increasingly more important.
To obtain the fine patterns needed for the current generation of devices, KrF (248 nm) and ArF (193 bm) lasers are being used. The dimension of device will continue to scale down, and may require even shorter wavelengths, such as EUV (13.4 nm). To obtain smaller and finer patterns through the use of shorter wavelength light sources, a new generation of photoresists will be required. The design of the next generation of photoresist may be governed by limitation with EUV tools, such as flare.
When extreme ultraviolet (EUV) steppers are used to expose photoresist to radiation with a wavelength of 13.4 nm, the mid frequency roughness of the optics will cause flare. Flare is produced when the light source is reflected off the reflective optics and passes through the mask to expose the photoresist material, and will reduce the contrast of the aerial image. Small amounts of flare may be able to be corrected for by calculating the amount of flare that will occur and scaling the mask CDs accordingly; however, for higher amounts of flare, alternative strategies are necessary. One strategy is the use of a negative tone resist, especially for poly layers, because the amount of flare is proportional to the amount of light that passes through the mask.
A positive tone photoresist becomes more solulable to a developer solution upon exposure to light, whereas a negative tone photoresist becomes less solulable to a developer solution upon exposure to light. Consequently, when a negative photoresist is used at the poly layer, a dark field mask, instead of a bright field mask, may be used to create a dark field pattern. By using a dark field mask the impact of flare on an underlying layer may be significantly reduced.
Current negative tone photoresists utilize a cross-linking mechanism that makes the exposed portion of the photoresist less solulable to the base developer solution. As cross-linking occurs, the molecular weight of the polymer will decrease. However swelling may occur, since the change in solubility is governed by a change in the molecular weight, and the interactions between the resist and the developer are still very favorable. Swelling of the resist structures will prevent correct pattern transfer and affect the resolution.
Because of these limitations, there is a need for a negative tone photoresist that does not swell during the developing process, thereby, allowing the use of a dark field mask to reduce the effect of flare in patterning.
The present invention is illustrated by way of example and not intended to be limited by the figures of the accompanying drawings.
In the following description, numerous specific details are set forth such as examples of specific developer solutions in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that these specific details need not be employed to practice the present invention. In other instances, well known methods and materials, such as polymerization techniques for fabricating the polymer, spin-coating techniques, chemical amplification strategies, and stripping techniques have not been described in detail in order to avoid unnecessarily obscuring the present invention.
Cycloolefin 110 may be any cycloolefin, such as a second aromatic structure or a norbonene structure. The chemical structure of cycloolefin 110 may be varied by changing the functional group X. As an example, X may be no atom (i.e. an aromatic structure), a carbon atom (norbornene), an alkyl, an oxygen atom, or a sulfur atom.
Cycloolefin 110 may be functionalized with di-ol 115. Di-ol 115 may be an alkyl group with two hydroxyls groups that are bonded to cycloolefin 110.
In addition, di-ol 115 may have additional functional groups, R2, R3, and R4 bonded to it. The selection of functional groups R2, R3, and R4, will affect the resist properties, such as dissolution, adhesion, and etch resistance. For example, R2, R3, and R4 may be individually selected from any of the following: hydrogen, alkyl, aromatic, or cage groups. As another illustrative example, if a cage is used for either one or all of R2, R3, and R4, the etch resistance of the photoresist material may be increased.
It is readily apparent that the photoresist may include other elements and structures. For example the negative tone photoresist may include a photoacid generator (PAG) to facilitate chemical amplification. Chemical amplification is discussed in more detail in reference to
Turning to
In reference to
As mentioned above, exposure chemistry in step 305 may occur via a chemical amplification strategy. To increase the sensitivity of the photoresist (i.e. reduce the amount of radiation needed to cause the necessary chemical reaction in the exposed portions 315 and 320) the photoresist may be chemically amplified. Chemical amplification occurs when a PAG reacts during exposure to produce an acid catalyst. The acid catalyst then mediates a cascade of reactions as it diffuses through the resist. A post exposure bake (PEB) step may occur after the exposure step to increase the diffusion length of the acid catalyst.
Step 305 may also include the generation of water 415. If the pinacol arrangement deprotection occurs while the resist is still under vacuum in the EUV tool, the water may outgas. Outgassing is in general considered very problematic, because it can contaminate the optics. However early research shows that the presence of water or oxygen in the chamber may actually help to clean EUV optics. However high concentrations of water can cause oxidation of the optics, which will damage the optics. The optics may be protected from oxidation by the use of capping layers that are known in the art.
Another method for protecting the optics from oxidation is to design copolymer 100 so that the pinacol rearrangement deprotection has a high activation energy. For reactions with high activation energies, the pinacol rearrangement deprotection will only occur at high temperatures. In this case the acid will be generated under exposure, but the deprotection will not occur until after the wafer has been removed from the exposure toll and is subjected to a PEB. Water that is generated during the PEB may outgas during the PEB, but this should not be problematic since water will not cause damage to the bake tools. The polymer can be designed to have a high deprotection energy for the pinacol rearrangement by selecting the correct functional groups R2, R3, and R4, such as alkyl groups.
As shown in
In reference to
Therefore, as shown above, a negative tone photoresist that is not based on cross-linking chemistry may be made and used. Negative photoresist may have applications at EUV wavelengths to reduce flare. Negative tone resists will reduce flare, because a dark field mask can be used at the poly layer, which reduces flare compared to a bright field mask. The deprotection chemistry may show improved performance over current negative resists based on cross-linking chemistry because swelling may be lower. When portions of the photoresist are exposed to light, the PAGs may generate acids. These acids may react with the di-ol deprotecting group to form ketone/aldehyde and water. The newly generated ketone or aldehyde is less soluble in a developer solution, such as 2.38% TMAH, than the original diol. Consequently, the photoresist that was not exposed will dissolve in the developer solution, while the exposed portions will not dissolve leaving the exposed portions of the photoresist.
In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.
