Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Removal of imaged layers
Reexamination Certificate
2002-08-20
2004-07-27
Schilling, Richard L. (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Removal of imaged layers
C134S001100, C134S001200, C134S001300, C216S067000
Reexamination Certificate
active
06767698
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method and system for removing ion implanted photoresist, and more particularly to a method and system for effectively stripping a photoresist layer damaged by high dose ion implantation.
2. Discussion of the Background
Photoresist has been used in the fabrication of very large-scale integrated (VLSI) circuits for years. It works as a masking material for shielding ion implantation in selected areas, and transferring patterns into various layers (e.g., oxide, nitride, polysilicon and metals). After ion implantation or pattern transfer is carried out, the photoresist layer needs to be removed completely. Generally, the removal of photoresist, also called stripping or ashing, is performed in a plasma containing (high amounts of) oxygen. The oxygen atoms react with the C (carbon) and H (hydrogen) in the photoresist, forming volatile products that are then exhausted from the system. The requirements of a photoresist stripping process are summarized as: 1) completeness, 2) or minimal or no charge damage, 3) minimal or no contamination, and 4) high throughput. As the device feature size shrinks, those requirements are becoming increasingly stringent.
While non-ion-implanted photoresist can be stripped fairly easily in an oxygenated plasma, it is difficult to completely strip resist that has been damaged by high dose ion implantation by plasma dry etching. During high dose ion implantation, the surface layer of the photoresist is severely damaged or carbonized such that a crust is formed which is difficult to remove. In the case of some plasma dry etching applications, polymers or other materials generated in the process can deposit onto the sidewalls of the photoresist pattern or etched features and form “veils” along the pattern edges that generally require both dry stripping and wet processing to remove. When a conventional oxygen-only process is used, those crust layers or “veils” will be left on the wafer surface as residues after ashing.
As an additional concern, there are many charged particles (ions and electrons) in the plasma discharge. A wafer in the plasma will draw electrons or ions, depending on the RF cycle applied to the plasma and the bias applied to the wafer itself, onto its surface. As electrons have much lower mass and travel at much higher speed, a negative potential will normally be developed on the wafer as a result of surface electron accumulation. If this negative potential is not balanced across the wafer, the resulting potential difference may cause potential differences across the wafer surface and across the thin oxide layers that may exist between the surface conducting layers and the semiconductor substrate. When the potential difference is high enough, it may cause damage, such as charge traps or even dielectric breakdown, in the thin oxide layer. Furthermore, UV radiation generated in the plasma may also create charge traps in oxide layers. As photoresist stripping is used in many steps of the VLSI fabrication process, minimal or no charge or UV damage is allowed, in order to preserve the yield of manufactured devices.
Metallic contents, although minimal, are found in most of the commercial photoresists and developers. Since those metallic contents cannot be removed by oxygen plasma, they will stay on the substrate surface as “residues” at the endpoint of the resist stripping. Overetching, which is commonly required to completely remove the photoresist over the entire wafer, can drive those metallic contents into the films that are present on the wafer surface and result in metallic contamination. Similar phenomena can happen to those ion implanted impurities (e.g., phosphorous and arsenic) as those impurities are oxidized in the oxygen plasma and driven into the surface layers during overetching.
High throughput, one of the critical specifications for any commercial equipment, requires the stripping process to be as short as possible. Besides a reliable, fast and efficient wafer transfer system and pumping system, the resist stripping rate needs to be very high, e.g., >4 mm/min, to achieve high throughput.
Many apparatuses and processes have been developed for removing photoresist from semiconductor wafer surfaces using oxygen plasma discharge. In the conventional method, wafers with resist coatings are simply placed directly in the oxygen plasma generated in a barrel asher. While the throughput of the batch processing in barrel asher is high, charge damage and contamination can result since the ions are in direct contact with the wafers and long overetching times are used.
U.S. Pat. No. 5,478,403 (Shinagawa et al., 1995) introduces an apparatus for resist ashing applications. The apparatus uses a microwave source to generate the oxygen-containing plasma. As shown in
FIG. 1
, the microwave-generated plasma is introduced to a downstream process chamber, through a plasma-transmitting plate, to where the resist coated wafer is to be treated. While the microwave is efficient in generating oxygen radicals, the ions in the plasma may have high ion energy and cause charge damage and contamination if in direct contact with the wafer surface. Those ions must be eliminated from the flux on their way from the plasma source to the wafer substrate. The transmitting plate captures charged particles in the plasma while allowing the transmission of neutral active species to thereby ash the photoresist coating without accumulating charges on the wafer surface. The wafer is placed on a chuck that is capable of adjusting its position to vary the distance between the wafer and the plasma transmitting plate.
Similar concepts of using microwave-generated plasma in resist stripping can be found in U.S. Pat. No. 5,562,775 (Mihara et al., 1996), U.S. Pat. No. 5,780,395 (Sydansk et al., 1998). U.S. Pat. No. 5,773,201 (Fujimura et al., 1998), and U.S. Pat. No. 5,545,289 (Chen et al., 1996). As described therein, the wafers to be processed are placed downstream from the plasma source chamber. The ions generated by the microwave source recombine on the way to the wafer so that only neutral radicals reach the wafer and affect the ashing process.
In the case of not using a downstream approach the wafer is placed close to the source plasma, and a charge trapping plate or grid is generally used in order to minimize charge damage. The use of a transmitting plate to eliminate the charged particles from reaching the wafer surface is discussed in U.S. Pat. No. 4,859,303 (Kainitsky et al., 1989) and “Advanced photoresist strip with a high pressure ICP source” (Savas et al., Solid State Technology, October 1996, pp. 123-128) (hereinafter “Savas”).
The problem of stripping high-dose ion-implanted photoresist, when the above mentioned microwave source and charge-trapping plate are used, is that the oxygen radicals arriving at the wafer surface are not very effective in removing the carbonized crusted skin of the resist coating. During the stripping process, the skin layer tends to crack due to stress and softening of the underlying uncarbonized resist, and the oxygen radicals penetrating through those cracks can react with the underlying “soft” resist at a very high rate. The volatile products generated from ashing the underlying “soft” resist layer will cause the hardened skin layer to crack even further and eventually break into many small pieces. This is generally referred to as “resist popping.” Those small pieces of hardened resist skin will stick on the substrate surface and become very difficult to be removed completely by oxygen radicals, even with long overetching time.
U.S. Pat. No. 4,861,424 (Fujimura et al., 1989) (hereinafter “the '424 patent”) describes a two-step process designed specifically for stripping ion-implanted photoresist. It uses a plasma containing a mixture of hydrogen and nitrogen in the first processing step and oxygen plasma (or a wet chemical treatment) in the second processing step. In the first step, the hydrogen radicals generated in the plasma react wit
Schilling Richard L.
Tokyo Electron Limited
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