If cost is a concern look to the Raynox DCR-125 and 250 add on Lenses, these are very usefully and work well, and should do a decent job with your Canon 20-70mm. We used these often and they also work well as a "tube" lens working with infinity type microscope lenses for getting down to detailed chip images. BTW be sure to get the necessary "ring" adapters for the lenses you would like to use with the Raynox.
Dedicated macro lens are usually very good, especially the ones from the camera body OEM, but can be somewhat expensive. Recall the Venus Optics did receive some good reviews.
A couple important things to consider when working at macro levels, the subject to lens distance is important because the further the lens front surface the better because of getting "quality" lighting around the subject. Others are vignetting and image edge distortion and focus.
Also consider a sturdy tripod, rather than trying handheld, our experience with image stabilization at macro and below levels has't been good.
Caution: Quality images fo electronics can become addictive, if you get "bit" prepare to open your wallet, and a deep study into optics
Don't ask how we know this
We can elaborate on our progress and developments from ~2000 if interested, but don't want to clutter up the thread.
Anyway, you have a fun journey ahead enjoy
Best,