Selective Silicon Nitride Etch with Hot Diluted HF– an Alternative to Orthophosphoric Acid
Authors: Garnier Philippe , Neyens Marc , Massin Thomas , Thomassin David , Maurice Carole 
Published: February 2020

Orthophosphoric acid is the worldwide standard chemistry to selectively etch silicon nitride towards silicon oxide [1]. Several alternatives have arisen since few years. Among them, a hot very diluted HF process has been developed in the early 2000’s [2]. This paper deals with its applications and adoption in production.
Until the 45nm node, CMOS transistors manufacturing only one or two silicon nitride etch operations were use, namely to define the Shallow Trench Isolation [3]. Latest nodes require more and more SiN etch operations either to define hardmasks or to generate localized stress: SPT (Stress Proximity Technic) [4] [5] or SMT (Stress Memory Technic) [6].
H3PO4 benefits from tremendous etch selectivity towards silicon oxide. Nonetheless it still remains in reclaim immersion batch tool with drawbacks: particles and metals intra or inter batch cross contamination. Various alternatives are under development: chemical formulation [7], single wafer approach [8] or plasma solutions [9]. There’s still a long way to overpass cross contamination concerns, and compete with high productive, low cost of ownership H3PO4 wet benches. Last alternative relies on hot very diluted HF. It has never really been adopted by the industry due to relatively low SiN etch rates. It perfectly suits etching application of small SiN removal amount. Two applications are hereby described.
First, a SPT technic in a 28nm node is considered. A comparison between two chemicals is made to remove the PECVD SiN spacer (figure 1). Its high etch rate of 9nm/min for the 85% H3PO4 wet bench at 165°C leads to short process (110s), poorly uniform. Moreover the SiN / SiO2 selectivity is only 30:1, and phosphoric acid is incompatible with highly As doped silicon films (figures 1 b) & 2). Si Gate and active areas are both corroded. Indeed, the transistor architecture makes the doped gates quite at risk with thin silicon oxide protection. Whereas H3PO4 has very poor process window, the hot very diluted HF in a spray batch tool enables better selectivity towards silicon oxide (60:1) and reduced corrosion of doped silicon. Moreover, excellent defectivity is obtained on silicon oxide surfaces (figure 3) making this process industrial.
Second described application relies on SiN patterning by wet etching. Phosphoric acid isn’t used for such application due to reclaim issues, and swelling resist with high temperature. Applications exist with low temperature H3PO4 formulations [10] but decreases significantly the etch rate. Fresh, hot very diluted HF enables LPCVD SiN patterning without any resist lift off. The process window is large and without any resist optimization (248nm DUV resist 235nm thick). Last optimization would be the within wafer uniformity (15% 3 sigma) (figure 5).
conclusion:
Hot diluted HF is proposed as an alternative to H3PO4 for thin SiN film etching with or without resist presence. Its benefits are: fresh chemical use, low cross contamination risks, good defectivity, high compatibility with resist.

References

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[8] B.Yu, Solid State Phenomena Vol. 195, pp 46-49
[9] C.D.Buttet, ECS 2014, 1674
[10] D. Huo, J. Electrochem. Soc. 1988, pp 2334-2338

Figure 1