Kurt K. Christenson

All Wet Stripping of Implanted Photoresist

Introduction Photoresist stripping in IC manufacturing has become more challenging. The number of photoresist levels has increased while the allowable material loss and allowable surface damage has decreased. Heavily implanted photoresist is especially challenging due to the dehydrogenated, amorphous carbon layer that forms on the surface [1]. The carbonized layer can be removed by plasma etching, or can be broken up by physical processes such as ion bombardment or the swelling of the underlying photoresist material. Physical processes, however, tend to leave residues where the carbonized resist contacts the wafer at the edges of features and particularly at the inner boundary of the wafer edge bead removal area. In addition, new plasmadoped (PLAD) implant processes provide very high implant doses and require complicated, multi-step ashing sequences for complete resist stripping. Plasma ashing processes used to strip implanted photoresist tend to oxidize the wafer surface and cause an unacceptable increase in Si material loss in subsequent processing steps. Interest in ash-free, all-wet stripping processes is driven primarily by the desire to reduce surface damage and material loss, but is also by a desire for a simplified stripping process for PLAD implants, and the elimination of a process step for all implants (wet strip/clean vs dry ash followed by a wet clean). A liquid mixture of sulfuric acid (H2SO4) and hydrogen peroxide (H2O2), also known as “piranha” or “SPM”, can be used to remove photoresist that is unimplanted or only lightly implanted, up to about 1x10 ions/cm. When H2SO4 is mixed with H2O2, monopersulfuric acid (H2SO5 or “Caro’s acid”) is formed. Caro’s acid, and to some degree H2SO4 itself, breaks down the undamaged carbon polymer chain, eventually forming H2O and CO2 reaction products [2]. Caro’s acid, however, does not effectively remove heavily carbonized resist. Fortunately, H2O2 and Caro’s acid break down to form radicals of OH and HSO4. These radicals rapidly react with the carbonized layer, but are very short-lived (lifetime ~10s,) and so are present at a very low concentrations. Current piranha processes are heated as high as 150°C in order to accelerate radical formation and achieve sufficient reactivity and stripping rates on partially carbonized resists. If we assume Arrhenius behavior, and a radical formation activation energy of 200 kJ/mol, then the radical formation rate (and therefore concentration) will increase by over 400 times with a temperature increase from 150°C to 200°C. The rate of attack by Caro’s acid will be similarly increased. While beneficial for stripping, this rapid decay makes 200° C immersion processing impractical. In this work, achievement of freshly mixed chemistries with 200°C on-wafer temperature has enabled the wet stripping of implanted photoresist exposed to doses of over 1x10 ions/cm. Solid State Phenomena Online: 2007-11-20 ISSN: 1662-9779, Vol. 134, pp 109-112 doi:10.4028/www.scientific.net/SSP.134.109

Steam-Injected SPM Process for All-Wet Stripping of Implanted Photoresist

Photoresist stripping in IC manufacturing has become more challenging as the number of photoresist levels has increased while at the same time allowable material loss and surface damage has decreased. Heavily implanted photoresist is especially challenging due to the dehydrogenated, amorphous carbon layer that forms on the surface [1]. To facilitate implanted photoresist removal, this layer can be attacked by physical processes such as ion bombardment as part of the common dry ashing approach. However, these physical approaches can lead to surface damage and increased material loss. Another approach is to increase the reactivity of the sulfuric acid – hydrogen peroxide mixture (SPM), so that it can penetrate and dissolve the amorphous carbon layer and achieve complete photoresist removal.

Strength Distribution of Megasonic Damage Events

Introduction Achieving high particle removal efficiency on patterned wafers is becoming more difficult as features become smaller and allowable surface etching is decreased. In particular, damage to polysilicon lines has pushed IC manufacturers to eliminate the use of megasonics energy from some of their cleaning processes, resulting in decreased cleaning efficiency. Megasonic damage to brittle polysilicon lines typically takes the form of 1-2 μm long breaks (Figure 1 and Figure 2). In some cases, closely spaced breaks in adjacent lines are seen, sometimes with an unbroken line between a pair of damaged lines. Damage to malleable metal lines often takes the form of one or more lines bent over, as if pushed away from a centralized energy source. In all cases, the damage is consistent with the model of a micron-scale liquid jet impinging normally on the wafer [1]. The jet is created during the asymmetric collapse of a cavitation bubble, with the asymmetry created by the proximity of the wafer surface.

Non-damaging chemical photoresist strip process for copper/low-k interconnects

Photoresist removal using traditional plasma ash chemistries can cause severe degradation of low-k dielectric properties including increases in k-value and changes in critical dimensions. Restoration processes using silyating agents, for example, hexamethydisilazane (HMDS) have been used to partially restore the dielectric properties of films which have been exposed to a plasma ash. However, these processes do not fully restore the k-value of the as-deposited low-k film and as a result, the non-damaging photoresist removal has become a key challenge in ultra low-k integration. We present, herein, the photoresist strip results on CVD organosilicate glass (OSG), low-k films using ozone-saturated, deionized water (DIO3) in a batch spray processor. This process yields no changes in the low-k dielectric properties or changes in critical dimensions. In addition, we also demonstrate the use of corrosion inhibitors to eliminate copper corrosion during the ozone process

Identification and sizing of particle defects in semiconductor-wafer processing

An experimental study of the sizing of submicron particles on semiconductor wafers is presented. The objective of the study was to determine the accuracy of a state-of-the-art optical wafer scanner, by comparing its size response with that of two well established techniques, namely, differential mobility analysis (DMA) and secondary electron microscopy (SEM). Test particles used include polystyrene–latex spheres and SiO2, Si3N4, W, and Cu particles of 64, 107, and 202 nm nominal (DMA) sizes. The scanner-indicated sizes using the oblique and normal operational modes were compared to the DMA sizes and to average sizes determined from the SEM images. Results show that the scanner-indicated size is comparable to the SEM-indicated size. However, both the scanner size and SEM size exhibit large distributions for a given nominal (DMA) size. The discrepancies are due to a combination of factors such as the effects of particle shape and material, and the presence of larger, doubly charged particles among those ana...