Photolithography definitions

A photoresist is a mixture of several main elements :

  • The polymer that provides the physical characteristics of the film (density, refractive index, thermal and physical resistance). The most common polymer for UV lithography is novolak, a polymer based on phenol and formaldehyde groups. For DUV lithography, the polymer molecules are poly-hydroxystyrenes (PHS).
  • The solvent to dilute the polymer to liquid form to use with spin-coating deposition methods. Propylene glycol methyl ether acetate (PGMEA) is generally used for UV resists, and ethyl lactate is added for DUV resists.
  • UV photoresists uses a photo-active compound (PAC), sometimes referred as dissolution inhibitor (DI), that absorbs light and changes the solubility of the polymer. The most common inhibitor is diazonaphthoquinone (DNQ). Before exposure, the DNQ molecule inhibits the dissolution. The reaction of DNQ with blue-UV light and water, releases nitrogen (N2) to produce a carboxylic acid that breaks the polymer chains, strongly increasing the solubility of the photoresist in alkaline solutions.
  • Optional: Chemical amplification (CA) is the process where an additional chemical compound, the chemical amplifier, binds with the polymer (forming a copolymer) and creates a chain of chemical reactions when reacting with the photo-acid molecule. This way one photo-acid molecule can break multiple polymer chains increasing the sensitivity and contrast of the resist. The chemical amplification mechanism requires a post-exposure bake (PEB) in order to homogeneously diffuse the photo-acid molecules within the polymer film.
  • DUV photoresists do not work with DNQ which is too absorbing at 248nm. Instead they use two compounds: 1) a blocking group that binds with the polymer, inhibits dissolution and provides chemical amplification. Three different blocking groups have been used: t-butoxy-carbonyl (t-Boc) derivatives, acetal derivatives and, t-butyl acrylate (ESCAP) derivatives. 2) a photo-acid generator (PAG), that reacts with DUV light to produce the photo-acid molecule that will initiate the depolymerisation. Several families of molecules are used as PAG: halogenated compounds, sulfonate esters, onium salts,…
  • Optional: The cross-linking agent is an additional molecule, typically composed of amine groups, that is used to change the polarity of the photoresist from positive to negative during the PEB process (which means exposed part will not be dissolved). The cross-linking agent will bind to the base polymer right after the depolymerization reaction between the polymer and the photo-acid molecule. This process is referred as “cross-linking” (the amine group replaces the dissolution inhibitor). Cross-linking has been used to develop negative photoresists with the standard novolak polymer, but also with other type of polymers, such as epoxies (SU-8).

“Broadband”, “i-line”, “h-line”, “DUV” illumination:
The most typical exposure tool is the mask-aligner which uses a high pressure mercury (Hg) vapor lamp as light source to expose the photoresist through a chrome/glass mask. The mercury lamp emits narrow spectral lines as shown in the following graph.

Mercury (Hg) lamp spectral lines

In particular:

  • 250 nm, referred as DUV
  • 365 nm, referred as i-line
  • 405 nm, referred as h-line
  • 435 nm, referred as g-line

A “broadband” mask-aligner does not filter the light source and, consequently all UV spectral lines are illuminating the substrates and are absorbed by the resist. An “i-line” mask-aligner has an additional filter that cuts all spectral lines except the 365nm line. The DUV spectral line can also be used with the mask-aligner if all the optical elements, as well as the mask, are made of fused-silica.

Other exposure equipment are using laser sources to produce narrow emission in the UV and DUV wavelength range. In CMi, InGaN/GaN semiconductor lasers emitting at 375nm and 405nm and frequency-tripled solid-state Nd:YAG lasers emitting at 355nm are used on the direct laser writing equipments. A krypton-fluoride (KrF) gas laser emitting at 248 nm is used on the DUV stepper lithography equipment.

Critical Dimension (CD):
The critical dimension is the width of the smallest layout feature which has to be defined after photolithography. This dimension is not geometrically defined but also processing related. This parameter will influence the choice of photoresist type and thickness, as well as the tool and configuration used to expose the wafers.

Aspect Ratio (AR):
The aspect ratio (AR) is a process parameter used to characterize the resolution power of a photoresist. It is defined as the ratio of the height of the photoresist to the minimum width the resist can print. Thick photoresists typically achieve higher AR compared to thin photoresists. For instance, AZ ECI 3007 is a thin resist with maximum AR of ~1. This means 600 nm features will succesfully be printed in a 600 nm thick film. AZ 40 XT is a thick photoresist with maximum AR of ~8. This means 5 um features will succesfully be printed in a 40 um thick film.

Substractive method (etching):
Conventional photolithography. Material growth or deposition is followed by resist coating and patterning. Requires a controlled method for wet or dry etching the target material with as large as possible selectivity relative to the resist mask.

Additive method (Lift-off):
The resist is used as sacrificial material. It is coated and patterned on top of the substrate. Next, the structural material is deposited on top of the resist and inside openings. A dissolution step in a photoresist stripper (such as Remover 1165) will lift-off the material deposited only on top of the resist.

Edge Bead Removal (EBR):
Spin coating homogenity is based on infinite size substrate. Edge effects creates a large perturbation in the equilibrium between inertial and surface forces resulting in an excess of material close to the external wafer edge (edge bead). Edge effects also contaminates the backside of the wafer. Some coaters are equiped with additional dispense nozzles (top and backside) to remove a small portion of the coating at the outer radius of the wafer. This procedure is called the edge bead removal (EBR).

Wedge error correction (WEC):
The wedge error correction (WEC) is a standard procedure of a mask-aligner to define the “contact position” between the mask and the wafer. When loading a wafer, a pressure sensitive platform will push the wafer against the mask. Piezo-motors will accomodate for any thickness non-homogeneity and adjust z-axis fine tuning until the pressure reads the same on all pressure sensors. At this point, the mask and wafer are parallel and the procedure can continue with the alignment step.

Non-contact exposure equipment such as direct laser writers or projection steppers use a lens that focuses the light (laser spot or mask image) on the photoresist-coated wafer. To obtain a sufficient image quality, the distance between the lens and the wafer needs to be carefully monitored and adjusted so that the photoresist is exposed within the depth of focus of the lens. Several autofocus methods (pneumatic or optical) are used to maintain the focus distance in real time during the printing process. Defocus is a process parameter that lets the user adjust the focus position slightly compared to the autofocus distance to optimize the illumination inside the photoresist, which will have a direct impact on the photoresist profile and resolution after development.