Acoustic Characterization of Patterning Degradation during Wet Etching

Wet etching in photoresist presence is commonly used in MEMS or integrated circuits manufacturing: metal gates [1], or gate oxides patterning [2]. Nonetheless the resist can’t stand a too long exposure to wet chemicals. Indeed, the liquids diffuse through the resist. Then the polymer enters a plasticizing sequence. It swells under the action of polar water and etchant molecules that break the cohesive hydrogen bonds between the polymer chains. The resist stress increases till fractures and blisters appears (figure 1). Hence these phenomena have been characterized by various acoustic means described hereby.

               First method consists in using a high-frequency echography principle [3]. A ZnO transducer, sputtered on the backside of the silicon wafer emits a 2GHz acoustic wave. The longitudinal wave reflection occurs at the interface between the silicon and the medium on the wafer frontside enabling its characterization by monitoring the transducer electrical impedance (figure 3). In the case of a silicon / air interface, the acoustic wave reflection is total. When air is replaced by water, the reflection is partial and the reflection coefficient value is 0.86. These reflection properties will be modified in the presence of a thin TiN layer coated with a 248nm deep UV resist. As the resist thickness (210 nm) is thin compared with the acoustic wavelengths in the different materials (micrometer range), reflections at each interface cannot be separated (silicon / resist and resist / upper medium) so only one total reflection occurs in the presence of air. Then, the reflection coefficient is measured with water on the resist. Without any resist damage, the reflection coefficient changes from 0.86 to 0.765. This latter value is then modulated by the resist blisters amount due to the modification of the mechanical properties of the silicon / resist interface. Measurements are performed on wafers exposed to SC1 (Standard Clean 1) solutions for different durations (figure 1). The reflection coefficient increases with the blisters apparition (figure 4). By determining the ratio of blisters area on optical microscope images of the resist, the reflection coefficient is theoretically calculated for different blisters mechanical properties (solids, liquids and gas). Experimental and theoretical values perfectly match in the case of gas. This result let us think that gas pockets appeared in the resist during blisters formation. The method sensitivity is excellent and mainly depends on the high contrast between gas and water acoustic impedances.

               A comparison with two other complementary technics will be made. First, picosecond acoustic will characterize the resist adhesion loss. Its principle is similar to a sonar. The acoustic wave into the sample is supplied by a tunable laser [4], and an echo is generated at each interface enabling its characterization. Finally the scanning acoustic microscopy will monitor the resist blister apparition with a thicker resist. This technic is commonly used to monitor wafer bonding voids in 3D integrated circuits assembly [5].

Conclusion

              Photoresist degradation occurs during long exposure to wet etching. Resist delamination from substrate and blisters appear after a certain contact duration. This latter is shorter with thin resist, which occurs more and more along the integrated circuits node evolution to maintain photolithography optical requirements. Three acoustic methods have been compared to monitor this degradation. Results show these methods are far better than optical microscopy to detect the resist degradation starting point.

References

[1] M. Foucaud, solid state phenomena, vol.195, pp58-61

[2] P. Garnier, solid State Phenomena, 2008; 134:71-74

[3] R. Dufour, Langmuir, 2013, 29(43)

[4] A.Devos, Ultrasonics Symposium, 2006, pp 564-567

[5] H .Moriceau, Adv. Nat. Sci.: Nanosci. Nanotechnol. 2010