How to access current scene's depth texture?

Hi

I require to access current scene’s depth texture for the (last step) volume primitive’s shader that will use this depth.

I tried to get in this manner, but it does not work.
var depthtex = viewer.scene._view.sceneFramebuffer._depthStencilTexture;

I tried czm_globeDepthTexture, but it only has depth of the globe/terrain. I require the 3D objects added as scene primitives to be also accounted in my depth.

Please help.