ARDE in SAC Etching

ARDE (Aspect Ratio Dependent Etching) is an effect where features with a high aspect ration (depth/width) have a higher etch rate then those with a small aspect ratio. Under certain conditions this effect can be reversed and is then called reverse ARDE. RIElag is a synonym for ARDE.

The following experiments were conducted in a magnetically enhanced reactive ion etcher (MERIE). Slide 1 describes a method for measuring ARDE where a series of contact holes with variable CD’s is printed and etched simultaneously. The etch depth for each hole size is measured by cross section SEM. This method allows to determine the average etch rate as a function of CD and etch time, final etch depth and final aspect ration. It requires that features with variable CD's are part of the photomask.

In case these special litho features are not available, an etch time scan can be used. Several wafers with the same pattern are etched for a series of etch time and the etch depth for each hole size is measured. Gives the average etch rate as a function of etch time, final etch depth and final aspect ratio (slide 2).

In order to truely measure the etch rate as a function of aspect ration, the change in etch depth has to be determined for an infinitely small etch time. In this method, several wafers with the same pattern are etched for a series of etch times. The etch depth of each hole size is measured. The same wafers are etched for a short time and the change in depth is measured again (slide 2). Using this method, the etch rate can be expressed as a function of the aspect ratio as shown in slide 3, where BPSG was etched in a C4F8, CO, Ar chemistry. For an aspect ratio of around 1, the etch rate is 350 nm/min and is drops to 100 nm/min for an aspect ratio of 5.

Slide 4 shows the effect of ARDE for several different types of oxides: silane oxide, BSG, BPSG, and PSG. The results show that ARDE varies for different types of oxides. It appears to be more pronounced for slower etching oxides, in particular PSG. This can be explained with the mechanism that leads to ARDE which involves the balance between etching and polymer deposition. Slide 5 shows a more detailed comparison between TEOS and BPSG where the faster etching BPSG exhibits less ARDE than the slower etching TEOS. For a 320 nm hole and the recipe used in this particular experiment, TEOS runs into etch stop after 200 s.

Since ARDE is a result in a change of the balance wetween etch and deposition as the the aspect ration increases, one would expect that any external influence on the polymer precurser density in the plasma, their transport down into the feature and their deposition rate will impact ARDE. Since the mask resist is being etched in the process and the etch by-products of this process contribute to the plasma chemistry, one would expect that ARDE is also a function of the resist type which is being used. Slide 6 shows that for the same type of BPSG oxide and similar feature sizes, i-line resist exhibits severe ARDE much sooner than DUV resist. The surprizing element of this finding is that DUV resist etches faster than i-line resist. The explanation must be in the difference in the chemical composition of the reaction by-products.

If the formation of polymers plays a key role in ARDE during oxide etching, the effect of adding a gas which suppresses polymer formation should have a significant influence. Slide 7 shows that clearly. Addition of oxygen indeed suppresses the formation of polymers which is good for ARDE reduction, however it would also decrease the nitride corner selectivity which is very critical in SAC etching. A balance has to be found in the process. The addition of CO provides a better process window between ARDE reduction and nitride corner selectivity. Larger CO flows do suppress the formation of polymers and hence reduce ARDE and at the same time provide enough carbon to the plasma to maintain the oxide to nitride selectivity. Slide 8 shows that with the addition of 170 sccm CO, ARDE was absent for aspect ratios o up to 6 for contact holes in BPSG.

Slide 9 summarizes the key findings:

The ARDE effect for a C4F8 / CO / Ar chemistry for oxide etching in a MERIE type reactor has been found to depend on:
1. The type of oxide to be etched.
2. The type of resist used as a mask.
3. The process pressure (ion energy and density).
4. The CO flow (influencing polymer deposition).
5. The addition of oxygen (influencing polymer deposition).

ARDE/RIElag models given by Sekine et al. and Joubert et al. which consider the strong deposition component of the given process and possibly charging of the deposits can provide a phenomenological explanation of the above findings.

More Plasma Etch Applications ...

Contact Etching

Silicon oxides, nitrides and advanced low k materials are etched with fluorocarbon based chemistries which provide good resist selectivities. Fluorocarbon deposition plays an important role in etching of dielectrics stopping on silicon based materials like contact etching. Several studies have shown that the silicon etch rate is directly proportional to the inverse of the fluorocarbon film thickness. Figure 1 shows results published by Oehrlein et al. (“Reactive Ion Etching Related Si Surface Residues and Subsurface Damage: Their Relationship to Fundamental Etching Mechanisms”; Gottlieb S. Oehrlein, Young H. Lee; J. Vac. Sci. Technol. A5 (1987) 1595) on the relationship between the silicon etch rate and the thickness of the fluorocarbon film thickness. Etch rate studies and XPS investigations show that the etch rate is constant and at a maximum value for film a film thickness between 0 and 5 Angstrom. As the film growths thicker, the etch rate starts to drop proportionally to 1/thickness and reaches zero for values above 20 Angstrom. These values will depend on the specific etch conditions. In the given example, a successful contact etch process would employ a gas mixture with 50% H2 to ensure infinite selectivity of the oxide etch to silicon. This implies that the oxide etch rate under the same conditions is not zero because the protective film is not formed on silicon oxide due to the presence of oxygen. If the degree of polymerization is further increased, the oxide etch will eventually also shut down.

Joubert et al. (“Fluorocarbon High Density Plasma. IV. Reactive Ion Etching Lag Model for Contact Hole Silicon Dioxide Etching in an Electron Cyclotron Resonance Plasma”; O. Joubert, G.S. Oehrlein, M. Surendra; J. Vac. Sci. Technol. A12 (1994) 665) proposed that there are three different etching regimes during SiO2 etching in high density plasmas: A - Fluorocarbon deposition, B - Fluorocarbon suppression, and C - Chemically enhanced etching (Fig. 2). In the fluorocarbon deposition regime, a relatively weak relationship between the deposition rate and the ion energy can be found. The process is very sensitive to the ion energy in the fluorocarbon suppression regime where the surface process switches rapidly from deposition to etch as the ion energy increases. The etch rate levels off as a function of ion energy in the regime of chemically enhanced etching.

In the follow-on paper of the series about fluorocarbon high density plasmas, Joubert at al. studied the impact of fluorocarbon polymer formation on the etching of high aspect ratio structures in silicon dioxide (“Fluorocarbon High Density Plasma. V. Influence of Aspect Ratio on the Etch Rate of Silicon Dioxide in an Electron Cyclotron Resonance Plasma”; O. Joubert, G.S. Oehrlein, Y. Zhang; J. Vac. Sci. Technol. A12 (1994) 658). Figure 3 shows that the etch depth decreases as a function of the contact hole size (left panel) and the etch rate decreases as a function of time and the polymerizing potential of the chemistry: Faster in C2F4 than in C3F6 than in CHF3. The etch rate stops first in highly polymerizing chemistries (C2F4) (right panel).

Figure 4 shows that there is a linear dependence of the etch rate as a function of the aspect ratio for different hole diameters. This means that the important variable in etching SiO2 contact holes is the aspect ratio (Aspect Ratio Dependent Etch ­ ARDE).

In the case of high aspect contact etching, the etch regime can be different in the dense and open areas (Fig. 5). Even if SiO2 is etched in the chemically enhanced etching regime in open areas, the etch in the high aspect ratio contacts can be in the fluorocarbon suppression or even deposition mode, which would mean etch stop in the dense areas. This behavior can be explained among others by charging effects which in high aspect ratio structures lead to a decrease of the ion current density and energy reaching the SiO2 surface. During the etch, the ion power density decreases (ion energy x ion density). The etching starts in the chemically enhanced etching regime, then moves to the fluorocarbon suppression regime (as the power density decreases), and then ultimately to the fluorocarbon deposition regime (inducing etch stop in high aspect ratio contact holes).

The classic experiment which provided direct evidence for the importance of charging effects was conducted by Sekine, Hayashi, and Kurihara (H. Hayashi, K. Kurihara, M. Sekine; Jpn. J. Appl. Phys. 35 (1996) 2488; K. Kurihara, M. Sekine; Plasma Sources Sci. Technol. 5 (1996) 121). They showed that the ion current density measured by mass spectrometry through a thick quartz plate in which capillary holes have been formed decreases very strongly as a function of the aspect ratio of the capillary holes. When the top of the quartz plate is covered by Cu, the amplitude of the ion current density loss decreases strongly: charging effects are minimized by the presence of a conductor covering the SiO2 surface (Fig. 6).

More Plasma Etch Applications ...

EUV: Who gets the first tool ?

September 5, 2006
Who really got the first EUV tool ? EE Times reported on 8/28 that ASML Holding NV has shipped the world's first extreme ultraviolet (EUV) lithography tool to the College of Nanoscale Science and Engineering (CNSE) of the University at Albany, New York. A few days later, NE Asia online wrote in an article that IMEC has started the installation of the world's first extreme ultraviolet (EUV) alpha demo on August 16 at 7am CET. The tool is also made by ASML.

Full story: http://www.clarycon.com/claryconeuvnews.html

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