The choice of the mask material for silicon gate etching depends on the process requirements. These materials can be grouped into carbon based materials (photoresists, bottom antireflective coatings (BARC) and carbon hardmasks) and silicon based dielectric masks (oxides, nitride, dielectric antireflective coatings (DARC)).
Slide 1 shows results of ellipsometry studies of the influence of the mask materials on the gate oxide etch rate. Under identical process conditions (HBr/Cl2/O2 standard chemistries), the gate oxide consumption is increased by a factor of 4 when going from a hardmask to a resist mask. This corroborates the common notion that resist masks tend to impact gate oxide selectivity negatively.
One possible explanation for the lower gate oxide selectivity is that carbon is liberated from the resist mask during the etch process and deposited on the gate oxide. Oxide tends to etch faster in the presence of carbon due to the formation of volatile carbon oxides. In-situ XPS studies of the gate oxide surface show that while Carbon is present on the gate oxide with the resist mask it is absent on the gate oxide with the SiO2 hardmask (slides 2 and 3).
The loss of gate oxide loss and the carbon concentration on the gate oxide surface both increase with the local resist coverage. When etching resist masked poly silicon gates, the poly-Si/SiO2 selectivity across the wafer is strongly affected by the local resist coverage (slide 4).
Besides concerns about the gate oxide selectivity, other reasons to use dielectric hardmasks in advanced gate etching include the dramatically reduced resist thickness / budget for advanced gate etching as well as mask charging (slide 5).
In-situ reflectometry measurements with a commercial predictive endpoint system provide additional evidence that the gate oxide erodes faster in the presence of photoresists on the wafer. In addition, the experiment reveals that the presence of silicon also lowers the gate oxide selectivity. This effect is smaller than for resist but measurable. A very uniform etch rate across the wafer is therefore mandatory to avoid local gate oxide pitting or punch through (slide 6).
Advanced poly-Si gate stack for high performance devices are frequently doped. Fluorine addition is frequently used to reduce the doping effect in advanced gate etching. CF4 addition is much more efficient than non-carbon containing gases like NF3. With respect to dielectric hardmasks, this has a double negative impact on mask selectivity: Both, fluorine and carbon increase the oxide or nitride etch rate and lower therefore the mask selectivity (slide 7).
The need for fluorocarbon addition drives the resurgence of resist schemes and the emergence of carbon and other alternative hardmasks (slide 8).
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Showing posts with label reactive layer. Show all posts
Showing posts with label reactive layer. Show all posts
Saturday, February 3, 2007
Friday, December 8, 2006
Resist Trimming
The need for resist trimming arises from the gap between the line widths advanced lithography can print today and the desired gate length of the transistor. For the 90 nm technology node, the printed line width is typically somewhere between 90 and 100 nm and the final gate length 50 to 60 nm. Hence, a line width reduction has to be achieved at some point in the process flow, typically during the first step of the gate patterning etch (slide 1). The requirements for this so called resist trim step are: no deterioration of the critical dimension (CD) uniformity across wafer, adjustable dense / iso CD bias, line integrity (no resist bending or line clipping), adjustable aspect ratio of post trim resist shape, linear dependence of the CD reduction on trim time (i.e. linear trim curve).
Oxygen is the main etch gas in many resist trim processes because oxygen based plasmas etch polymers isotropically. Halogens such are HBr, HCl, Cl2 or fluorocarbon gaeses are added to provide control over the ratio of lateral vs. vertical etch rate. These gas additives inluence the trim rates for the dense and isolated lines. It iwas found that the trim rate is faster in HBr/O2 chemistry than in Cl2/O2 chemistry. Larger difference in trim rate between dense and isolated lines are found for the HBr/O2 chemistry (slide 2).
With HBr and HCl, the trim rate is faster in isolated than in dense resist patterns. Cl2/O2 chemistries result in a trim process which is faster in dense resist lines than in isolated resist lines (slide 3).
The effect of bromine and chlorine addition to an oxygen based trim process wass studies with in situ XPS (E. Pargon, O. Joubert, T. Chevolleau, G. Cunge, Songlin Xu, Thorsten Lill; JVST B, 23 (2005) 103). In HBr/O2 plasmas, very little Br concentrations are found on all resist surfaces exposed to the plasma. O only is involved in the resist transformation. Br is not reactive with respect to carbon. Thin reactive layers are typical of a very chemical type of etch. In Cl2/O2 plasmas, large chlorine concentrations are found on top and sidewall of the resist patterns indicating a competitive absorption between O and Cl. The increase in reactive layer thickness on the resist sidewalls with Cl2/O2 indicates that it is formed from resist etch products redeposition (slide 4).
For O2/Cl2 trim processes, the composition of the reactive layers changes when the O2 concentration in the gas phase is increased. The Cl concentration and thickness of the the top reactive layer decrease. The Cl concentration and thickness of the reactive layer on the resist sidewall decrease and the O/Cl ratio increases. A direct correlation between trim rate, O/Cl ratio and thickness of the reactive layer on the resist sidewalls exists (slide 5).
For O2/HBr trim processes, very little difference in reactive layer composition and thickness as a function of O2 concentrations was found. A slow down in trim rate at low O2 concentration (10% in HBr/O2) may be attributed to the presence of Br in the sidewall layer. At higher O2 concentration (60%), no bromine is present on the resist sidewalls, the trim rate increases by more than 50% (slide 6).
The pressure dependence of the Cl2/O2 trim process is shown in slides 7 and 8. An increase in pressure in Cl2/O2 generates a slow down in trim rate in dense as well as isolated resist patterns. The composition of top resist patterns is not affected by the change in pressure. For the sidewalls of the resist patterns, an increase in chlorine concentration, decrease in oxygen concentration and increase in reactive layer thickness is observed. This is consistent with the decrease in trim rate as a function of pressure.
Similarly to the Cl2/O2 chemistry, an increase in pressure of the HBr/O2 gas mixture generates a decrease in trim rate in dense as well as isolated resist patterns. XPS shows no difference in reactive layer formation in dense resist patterns as a function of pressure of the HBr/O2 gas mixture (slide 9).
In Cl2/O2 plasmas, increasing bias power has the same impact than increasing pressure: the trim rate decreases in dense as well as isolated lines (slide 10).
XPS studies of the active layer showed the following trends for increasing bias power in Cl2/O2 plasmas: 1. The top resist patterns not affected by the increase in bias power, their composition is only driven by O2/Cl2 ratio, 2. The idewalls of the resist patterns exhibit an increase in chlorine concentration, a decrease in O concentration and an increase in reactive layer thickness consistent with the decrease in trim rate as a function of bias power, and 3. the deposition of carbon etch products increases with bias power (slide 11).
Similarly to Cl2/O2, increasing bias power in HBr/O2 leads to a decrease in trim rate for both isolated and dense lines. An increasing bias power generates a decrease in trim rate difference between isolated and dense lines whereas there is almost no effect in Cl2/O2 (slide 12).
In summary, resist trimming is a very powerful and widely used method to achieve gate lengths which are beyond the resolution limits of photolithography. The trim process is chemical in nature and very dependend on the choice of gases. Slide 13 shows some of the limits of the resist trim process. Firstly, resist consumption and internal stress leads to profile deformation (resist bending). Secondly, initial resist roughness (line edge roughness) becomes a problem after ultimate trim.
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Oxygen is the main etch gas in many resist trim processes because oxygen based plasmas etch polymers isotropically. Halogens such are HBr, HCl, Cl2 or fluorocarbon gaeses are added to provide control over the ratio of lateral vs. vertical etch rate. These gas additives inluence the trim rates for the dense and isolated lines. It iwas found that the trim rate is faster in HBr/O2 chemistry than in Cl2/O2 chemistry. Larger difference in trim rate between dense and isolated lines are found for the HBr/O2 chemistry (slide 2).
With HBr and HCl, the trim rate is faster in isolated than in dense resist patterns. Cl2/O2 chemistries result in a trim process which is faster in dense resist lines than in isolated resist lines (slide 3).
The effect of bromine and chlorine addition to an oxygen based trim process wass studies with in situ XPS (E. Pargon, O. Joubert, T. Chevolleau, G. Cunge, Songlin Xu, Thorsten Lill; JVST B, 23 (2005) 103). In HBr/O2 plasmas, very little Br concentrations are found on all resist surfaces exposed to the plasma. O only is involved in the resist transformation. Br is not reactive with respect to carbon. Thin reactive layers are typical of a very chemical type of etch. In Cl2/O2 plasmas, large chlorine concentrations are found on top and sidewall of the resist patterns indicating a competitive absorption between O and Cl. The increase in reactive layer thickness on the resist sidewalls with Cl2/O2 indicates that it is formed from resist etch products redeposition (slide 4).
For O2/Cl2 trim processes, the composition of the reactive layers changes when the O2 concentration in the gas phase is increased. The Cl concentration and thickness of the the top reactive layer decrease. The Cl concentration and thickness of the reactive layer on the resist sidewall decrease and the O/Cl ratio increases. A direct correlation between trim rate, O/Cl ratio and thickness of the reactive layer on the resist sidewalls exists (slide 5).
For O2/HBr trim processes, very little difference in reactive layer composition and thickness as a function of O2 concentrations was found. A slow down in trim rate at low O2 concentration (10% in HBr/O2) may be attributed to the presence of Br in the sidewall layer. At higher O2 concentration (60%), no bromine is present on the resist sidewalls, the trim rate increases by more than 50% (slide 6).
The pressure dependence of the Cl2/O2 trim process is shown in slides 7 and 8. An increase in pressure in Cl2/O2 generates a slow down in trim rate in dense as well as isolated resist patterns. The composition of top resist patterns is not affected by the change in pressure. For the sidewalls of the resist patterns, an increase in chlorine concentration, decrease in oxygen concentration and increase in reactive layer thickness is observed. This is consistent with the decrease in trim rate as a function of pressure.
Similarly to the Cl2/O2 chemistry, an increase in pressure of the HBr/O2 gas mixture generates a decrease in trim rate in dense as well as isolated resist patterns. XPS shows no difference in reactive layer formation in dense resist patterns as a function of pressure of the HBr/O2 gas mixture (slide 9).
In Cl2/O2 plasmas, increasing bias power has the same impact than increasing pressure: the trim rate decreases in dense as well as isolated lines (slide 10).
XPS studies of the active layer showed the following trends for increasing bias power in Cl2/O2 plasmas: 1. The top resist patterns not affected by the increase in bias power, their composition is only driven by O2/Cl2 ratio, 2. The idewalls of the resist patterns exhibit an increase in chlorine concentration, a decrease in O concentration and an increase in reactive layer thickness consistent with the decrease in trim rate as a function of bias power, and 3. the deposition of carbon etch products increases with bias power (slide 11).
Similarly to Cl2/O2, increasing bias power in HBr/O2 leads to a decrease in trim rate for both isolated and dense lines. An increasing bias power generates a decrease in trim rate difference between isolated and dense lines whereas there is almost no effect in Cl2/O2 (slide 12).
In summary, resist trimming is a very powerful and widely used method to achieve gate lengths which are beyond the resolution limits of photolithography. The trim process is chemical in nature and very dependend on the choice of gases. Slide 13 shows some of the limits of the resist trim process. Firstly, resist consumption and internal stress leads to profile deformation (resist bending). Secondly, initial resist roughness (line edge roughness) becomes a problem after ultimate trim.
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