A photoresist (also known simply as a resist) is a light-sensitive material used in several processes, such as photolithography and photoengraving, to form a patterned coating on a surface. This process is crucial in the electronics industry.[1]
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The process begins by coating a substrate with a light-sensitive organic material. A patterned mask is then applied to the surface to block light, so that only unmasked regions of the material will be exposed to light. A solvent, called a developer, is then applied to the surface. In the case of a positive photoresist, the photo-sensitive material is degraded by light and the developer will dissolve away the regions that were exposed to light, leaving behind a coating where the mask was placed. In the case of a negative photoresist, the photosensitive material is strengthened (either polymerized or cross-linked) by light, and the developer will dissolve away only the regions that were not exposed to light, leaving behind a coating in areas where the mask was not placed.
A BARC coating (Bottom Anti-Reflectant Coating) may be applied before the photoresist is applied, to avoid reflections from occurring under the photoresist and to improve the photoresist's performance at smaller semiconductor nodes.[2][3][4]
Conventional photoresists typically consist of 3 components: resin (a binder that provides physical properties such as adhesion, chemical resistance, etc), sensitizer (which has a photoactive compound), and solvent (which keeps the resist liquid).
Positive: light will weaken the resist, and create a hole
Negative: light will toughen the resist and create an etch resistant mask.
To explain this in graphical form you may have a graph on Log exposure energy versus fraction of resist thickness remaining. The positive resist will be completely removed at the final exposure energy and the negative resist will be completely hardened and insoluble by the end of exposure energy. The slope of this graph is the contrast ratio. Intensity (I) is related to energy by E = I*t.
A positive photoresist is a type of photoresist in which a portion is exposed to light and becomes soluble to the photoresist developer. The unexposed portion of the photoresist remains insoluble in the photoresist developer.
Some examples of positive photoresists are:
PMMA (polymethylmethacrylate) single-component
Two-component DQN resists:
A negative photoresist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes insoluble in the photoresist developer. The unexposed portion of the photoresist is dissolved by the photoresist developer.
Modulation transfer function
MTF (modulation transfer function is the ratio of image intensity modulation and object intensity modulation and it is a parameter that indicates the capability of an optical system.
The following table[6] is based on generalizations which are generally accepted in the microelectromechanical systems (MEMS) fabrication industry.
Characteristic Positive Negative Adhesion to silicon Fair Excellent Relative cost More expensive Less expensive Developer base Aqueous Organic Solubility in the developer Exposed region is soluble Exposed region is insoluble Minimum feature 0.5 μm 7 nm Step coverage Better Lower Wet chemical resistance Fair ExcellentBased on the chemical structure of photoresists, they can be classified into three types: photopolymeric, photodecomposing, and photocrosslinking photoresist.
In lithography, decreasing the wavelength of light source is the most efficient way to achieve higher resolution.[8] Photoresists are most commonly used at wavelengths in the ultraviolet spectrum or shorter (<400 nm). For example, diazonaphthoquinone (DNQ) absorbs strongly from approximately 300 nm to 450 nm. The absorption bands can be assigned to n-π* (S0–S1) and π-π* (S1–S2) transitions in the DNQ molecule.[citation needed] In the deep ultraviolet (DUV) spectrum, the π-π* electronic transition in benzene[9] or carbon double-bond chromophores appears at around 200 nm.[citation needed] Due to the appearance of more possible absorption transitions involving larger energy differences, the absorption tends to increase with shorter wavelength, or larger photon energy. Photons with energies exceeding the ionization potential of the photoresist (can be as low as 5 eV in condensed solutions)[10] can also release electrons which are capable of additional exposure of the photoresist. From about 5 eV to about 20 eV, photoionization of outer "valence band" electrons is the main absorption mechanism.[11] Above 20 eV, inner electron ionization and Auger transitions become more important. Photon absorption begins to decrease as the X-ray region is approached, as fewer Auger transitions between deep atomic levels are allowed for the higher photon energy. The absorbed energy can drive further reactions and ultimately dissipates as heat. This is associated with the outgassing and contamination from the photoresist.
Photoresists can also be exposed by electron beams, producing the same results as exposure by light. The main difference is that while photons are absorbed, depositing all their energy at once, electrons deposit their energy gradually, and scatter within the photoresist during this process. As with high-energy wavelengths, many transitions are excited by electron beams, and heating and outgassing are still a concern. The dissociation energy for a C-C bond is 3.6 eV. Secondary electrons generated by primary ionizing radiation have energies sufficient to dissociate this bond, causing scission. In addition, the low-energy electrons have a longer photoresist interaction time due to their lower speed; essentially the electron has to be at rest with respect to the molecule in order to react most strongly via dissociative electron attachment, where the electron comes to rest at the molecule, depositing all its kinetic energy.[12] The resulting scission breaks the original polymer into segments of lower molecular weight, which are more readily dissolved in a solvent, or else releases other chemical species (acids) which catalyze further scission reactions (see the discussion on chemically amplified resists below). It is not common to select photoresists for electron-beam exposure. Electron beam lithography usually relies on resists dedicated specifically to electron-beam exposure.
Physical, chemical, and optical properties of photoresists influence their selection for different processes.[13] The primary properties of the photoresist are resolution capability, process dose and focus latitudes required for curing, and resistance to reactive ion etching.[14]: 966 [15] Other key properties are sensitivity, compatibility with tetramethylammonium hydroxide (TMAH), adhesion, environmental stability, and shelf life.[14]: 966 [15]
Photoresists used in production for DUV and shorter wavelengths require the use of chemical amplification to increase the sensitivity to the exposure energy. This is done in order to combat the larger absorption at shorter wavelengths. Chemical amplification is also often used in electron-beam exposures to increase the sensitivity to the exposure dose. In the process, acids released by the exposure radiation diffuse during the post-exposure bake step. These acids render surrounding polymer soluble in developer. A single acid molecule can catalyze many such 'deprotection' reactions; hence, fewer photons or electrons are needed.[16] Acid diffusion is important not only to increase photoresist sensitivity and throughput, but also to limit line edge roughness due to shot noise statistics.[17] However, the acid diffusion length is itself a potential resolution limiter.[18] In addition, too much diffusion reduces chemical contrast, leading again to more roughness.[17]
The following reactions are an example of commercial chemically amplified photoresists in use today:
The e− represents a solvated electron, or a freed electron that may react with other constituents of the solution. It typically travels a distance on the order of many nanometers before being contained;[21][22] such a large travel distance is consistent with the release of electrons through thick oxide in UV EPROM in response to ultraviolet light. This parasitic exposure would degrade the resolution of the photoresist; for 193 nm the optical resolution is the limiting factor anyway, but for electron beam lithography or EUVL it is the electron range that determines the resolution rather than the optics.
One very common positive photoresist used with the I, G and H-lines from a mercury-vapor lamp is based on a mixture of diazonaphthoquinone (DNQ) and novolac resin (a phenol formaldehyde resin). DNQ inhibits the dissolution of the novolac resin, but upon exposure to light, the dissolution rate increases even beyond that of pure novolac. The mechanism by which unexposed DNQ inhibits novolac dissolution is not well understood, but is believed to be related to hydrogen bonding (or more exactly diazocoupling in the unexposed region). DNQ-novolac resists are developed by dissolution in a basic solution (usually 0.26N tetramethylammonium hydroxide (TMAH) in water).
One very common negative photoresist is based on epoxy-based oligomer. The common product name is SU-8 photoresist, and it was originally invented by IBM, but is now sold by Microchem and Gersteltec. One unique property of SU-8 is that it is very difficult to strip. As such, it is often used in applications where a permanent resist pattern (one that is not strippable, and can even be used in harsh temperature and pressure environments) is needed for a device.[23] Mechanism of epoxy-based polymer is shown in 1.2.3 SU-8. SU-8 is prone to swelling at smaller feature sizes, which has led to the development of small-molecule alternatives that are capable of obtaining higher resolutions than SU-8.[24]
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In , OSTE Polymers were shown to possess a unique photolithography mechanism, based on diffusion-induced monomer depletion, which enables high photostructuring accuracy. The OSTE polymer material was originally invented at the KTH Royal Institute of Technology, but is now sold by Mercene Labs. Whereas the material has properties similar to those of SU8, OSTE has the specific advantage that it contains reactive surface molecules, which make this material attractive for microfluidic or biomedical applications.[13]
HSQ is a common negative resist for e-beam, but also useful for photolithography. Originally invented by Dow Corning (),[25] and now produced () by Applied Quantum Materials Inc. (AQM). Unlike other negative resists, HSQ is inorganic and metal-free. Therefore, exposed HSQ provides a low dielectric constant (low-k) Si-rich oxide. A comparative study against other photoresists was reported in (Dow Corning HSQ).[26]
Microcontact printing was described by Whitesides Group in . Generally, in this techniques, an elastomeric stamp is used to generate two-dimensional patterns, through printing the “ink” molecules onto the surface of a solid substrate.[27]
Step 1 for microcontact printing. A scheme for the creation of a polydimethylsiloxane (PDMS) master stamp. Step 2 for microcontact printing A scheme of the inking and contact process of microprinting lithography.
The manufacture of printed circuit boards is one of the most important uses of photoresist. Photolithography allows the complex wiring of an electronic system to be rapidly, economically, and accurately reproduced as if run off a printing press. The general process is applying photoresist, exposing image to ultraviolet rays, and then etching to remove the copper-clad substrate.[28]
This includes specialty photonics materials, MicroElectro-Mechanical Systems (MEMS), glass printed circuit boards, and other micropatterning tasks. Photoresist tends not to be etched by solutions with a pH greater than 3.[29]
This application, mainly applied to silicon wafers and silicon integrated circuits is the most developed of the technologies and the most specialized in the field.[30]
As chipmakers move to advanced technology nodes, they are challenged to resolve ever finer features. One of the major roadblocks involves the material used to transfer chip design to the wafer; that material is rapidly reaching its limit to accurately transfer designs. To keep next-generation device scaling on track, Lam introduced a breakthrough technology: dry resist. To better understand this solution, let’s first take a closer look at the patterning process and current photoresists, then discuss the potential advantages of the new solution.
It can take several hundred individual steps to create an advanced chip, as the microscopic features are built up layer by layer. One of the most critical process steps is called lithography – repeated over and over, along with deposition and etching, these steps are responsible for encoding the information contained in the design into patterns on the wafer.
In lithography, a wafer is coated with a light-sensitive material called photoresist. Light is then streamed through a photomask (a pattern of transparent and opaque areas), exposing the photoresist in some places, but not in others. The exposed regions are then etched away, while covered areas remain protected (in the case of positive photoresist). The end result is a set of features whose size and density are determined by the original photoresist pattern, reproducing the device design onto a film on the wafer.
The size of the smallest possible feature is directly proportional to the wavelength of light being used. With its smaller wavelength, extreme ultraviolet (EUV) lithography systems can create finer chip features than its predecessor – just as smaller pixels increase screen resolution on your smart .
The photoresist, often simply referred to as resist, plays a critical role in the lithography process. Ideally, it would have high resolution, low line edge roughness (LER), and high sensitivity.
Obtaining all three of these at the same time is difficult, as they are interrelated, and improvements in one parameter generally degrades at least one of the others – often referred to as the RLS trade-off relation. To better understand why this trade-off exists, we need to understand how resists work.
(Below, we’ll mention photons, which can be thought of as tiny particles of light.)
Today’s chemically amplified resists are made up of many components in addition to the main polymeric matrix, including absorbers, photoacid generators (PAGs), and additives to control viscosity, adhesion and shelf stability. When a photon strikes the resist, it sets off a chain reaction that changes the structure of the material, making it more soluble so it can be washed away in a subsequent development step. Part of the reaction cascade involves a chemical amplification of the initial photon, where the photon is first converted into several electrons, whereby several photoacid molecules are eventually generated per incident photon. Hence these materials are referred to as chemically amplified resists (CARs).
The advantage of CAR is that resist sensitivity can be increased by increasing the number of photoacid molecules that are generated per photon. However, these additional acids will be located further and further from the site of the original photon, resulting in image blur which reduces resolution and increases line edge roughness.
While CARs have made significant progress over the past decades, they are already facing significant limitations at the 5 nm node, requiring chipmakers to make design accommodations and semiconductor fabs to incorporate multiple passes of lithography to meet resolution. To extend affordable and design friendly EUV patterning to future process nodes, a new resist solution is needed.
Lam, together with ASML and imec, has created a breakthrough resist application technology that’s fundamentally different from spin-on liquid resists. Applied using reactive precursors in a vapor phase, the resulting film is uniform and homogeneous.
This new technology involves the dry deposition of small (<0.5 nm) metalorganic units, which has several advantages. Importantly, this high density framework of density photosensitive particles is inherently more efficient in capturing photons, an important characteristic given that current EUV sources produce many times fewer photons than their their longer wavelength predecessors.
Resolution is higher because the exposure mechanics are fundamentally different from the chain reaction involved with CARs. At the SPIE Advanced Lithography symposium, we showed successful imaging at 26 nm pitch with best in class Z-factor <1 x10-8 mJ nm3.
The dry deposition method gives us the unique ability to change the resist thickness simply by changing the deposition and development time. In contrast, changing spin-on resist coating thickness is much more difficult, as both viscosity and surface adhesion must be addressed which can lead to undesirable tradeoffs in lithography performance. This ability of dry resist thickness to be co-optimized with photon absorption, transfer etch, and underlayer adhesion allows us to break tradeoffs in line edge roughness, sensitivity, and defectivity/device yield.
The dry resist framework has other benefits besides breaking the RLS trade-off. There are no concerns with viscosity, chemistry shelf life, and other limitations that can affect wet spin-on resists. Since additives for adhesion or stability are no longer required, the dry deposited material is a much purer material that is inherently more sensitive and importantly amenable to a dry development process.
The new dry develop process has been co-optimized to minimize line and pillar collapse. Without the capillary forces inevitable during wet processing, dry develop results in a dramatically larger collapse-free process window.
Lam’s dry resist and develop technology will accelerate the industry transition to EUV lithography’s future node applications and enable continued scaling for advanced logic and memory devices.
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