Photolithography is the transference of a pattern from a mask to a substrate. Oriel Instruments has long addressed the need for high resolution exposure sources for photolithography. This began with an advanced design mercury source with high intensity, collimation and uniformity unmatched in the industry. Building on that success, a line of mask alignment fixtures soon followed along with all of the building blocks of today's mask alignment systems. Today, Oriel offers an extensive family of tools to meet the everchanging needs of the broad photolithography market. We've continued to update our advanced designs and have added new products. Everything from flood exposure sources, to full mask alignment systems, to photoresist testing systems can be obtained from Oriel.
||Features and Benefits
||Flood Exposure Sources
|| High intensity outputs
Highly uniform, large collimated beams
Efficient out of band rejection
||Mask Alignment Tools
|| Mask Alignment Fixture
Substrate or Wafer Holder
Production of Integrated Circuits
To produce ICs, thin films of various materials are used as barriers to the diffusion or implantation of impurity atoms, or as insulation between conductive materials and the silicon substrate. Holes or windows are cut through this barrier material wherever impurity penetration, or contact is desired.
Masks contain the patterns of windows, which are transferred to the surface of the silicon wafer using the process of photolithography. Photolithography makes use of a highly refined version of the photoengraving process. The patterns are first transferred from the mask to a light-sensitive material called photoresist by illuminating or "exposing" the photoresist. Chemical or plasma etching is then used to transfer the pattern from the photoresist to the barrier material on the wafer surface. Each mask step requires the successful completion of numerous processing steps, and the number of photographic masks used during fabrication often measures the complexity of an IC process.
The Steps Involved
The processing steps are as follows:
1. The wafers are first "cleaned" to remove particulates on the wafer surface, as well as traces of organic, ionic and metallic impurities.
2. After cleaning, the silicon wafer is "covered" with the material, which will serve as a barrier layer. This is commonly silicon dioxide (SiO2).
3. After the formation of a SiO2 layer, the surface of the wafer is "coated" with a light-sensitive material called photoresist in a process referred to as spinning.
4. A drying step known as "soft baking" follows, to improve adhesion and remove solvents from the photoresist .
5. After soft baking, the resist is ready for mask alignment and exposure.
A photomask, a square glass plate with a patterned emulsion or metal film on one side, is placed over the wafer. Note: each mask following the first must be precisely aligned to the previous pattern on the wafer. With manual alignment equipment, the wafer is held on a mask alignment fixture in a substrate holder, and is then carefully moved into position below the mask using an adjustable X-Y stage. The mask, which is held in a mask holder, is spaced 25 to 125 µm above the surface of the wafer during proximity printing. If vacuum contact printing is utilized, the mask is brought into direct contact with the wafer.
Alignment or registration marks are introduced on each mask and transferred to the wafer as part of the IC pattern. The marks are used to register each new mask level to the previous level. For certain mask levels, a cross on the mask is placed over a cross on the wafer. For other levels, a box on the mask is placed over a cross on the wafer. The choice depends on the type of resist being used. A Split Field Microscope is used to simultaneously align two well-separated areas of the wafer. A Stereo Zoom Microscope allows alignment of one registration mark at a time. It is recommended that viewing take place in a vibration isolated environment on a Work Station or Optical Table.
Following alignment, the photoresist is exposed through the mask with a high-intensity, well-collimated and uniform beam of ultraviolet radiation. Resist is exposed wherever silicon dioxide is to be removed. The resist is developed with a process very similar to that used for developing ordinary photographic films, using a developer supplied by the photoresist manufacturer. Any resist exposed to the UV light is washed away, leaving bare silicon dioxide in the exposed area. This is referred to as a "positive" resist. The mask retains a copy of the pattern, which will remain on the wafer surface. Windows are opened wherever the exposing light passes through the mask. A "negative" resist remains on the surface wherever exposed - it is not washed away.
Following exposure and development is a baking step to harden resist and improve adhesion to the substrate. Chemical etching in liquid or gaseous form is used to remove any barrier material not protected by the hardened resist. Lastly, after the windows are etched through the SiO2 layer, the photoresist is stripped from the surface leaving a window in the silicon dioxide.
This completes the process for the transfer of a pattern for one mask. For each additional mask utilized, the entire series of steps is repeated. With each additional layer in a complex IC, mask alignment to the previous patterns becomes absolutely critical.1
1 Jaeger, Richard C. Introduction to Microelectronic Fabrication.