Wet Atomic Layer Etching of Copper Structures for Highly Scaled Copper Hybrid Bonding and Fully Aligned Vias

Copper to copper hybrid bonding relies on nanometer level control of copper pad recess to create electrical contacts. To compensate for the loss of recess control during chemical mechanical planarization (CMP) due to feature scaling, a wet atomic layer etch (W-ALE) was proposed as a supplement. The process explored was a two-step cyclical wet etch to create recesses in copper patterns with sub-nanometer level control. This etching process achieved true ALE control with removal rates of ~0.28nm/cycle; roughly the atomic diameter of copper. Additionally, there was no significant impact on dielectric roughness, dielectric thickness, copper roughness or the shape of the copper feature (convexity) up to 30 cycles.

I. INTRODUCTION

Copper recess control is a key factor for multiple areas in the back end of the line. Two areas where this is especially important are hybrid bonding and fully self-aligned vias (FSAV/FAV) [1][2]. Hybrid bonding is a 3D integration process where two wafers with mixed material top surfaces (dielectric/copper) are bonded together. The two wafers are aligned and brought in contact with each other to create dielectric to dielectric bonds. A subsequent anneal step fuses these dielectric bonds together while causing expansion of the copper in each wafer. This expansion leads to the merging of the copper in each wafer and creates the copper/copper bonds. Subsequent thinning of the wafer post anneal can facilitate further stacking [3]. FAVs are the natural evolution of via placement in a dual damascene process where misalignment in both the x and y direction are compensated (Fig. 1). During this process a copper recess is created in the metal layer before covering it with an etch stop layer. After further deposition steps the dual damascene etch is carried out with the etch stop being removed over the copper feature and not the surrounding dielectric. This process virtually increases the distance between the via and a neighboring metal line while also increasing the size of via that can be used at that level resulting in much better electrical performance [4].