Jim's method is the most general, and will work fine. It is the one to use if your object (chip?) is on a background that you want to disappear in the final picture. As Jim says, making a "cropping circle" that fits your chip exactly is not easy to do in Photoshop.

As a simplification, you can arrange to produce a scan with a neutral background, usually black or white. Then you can use a "cropping rectangle", which is easy to size and move to exactly fit your image. That's the technique I try to use when I can. If the object (chip) in in a holder, for example, and you don't want to remove it for scanning, than the "cropping circle" is necessary. It's just more complicated and takes longer.

By the way, you can easily get a black background by leaving your scanner cover open.

Another comment, you switch between a retctangular cropping device (marquee, in Photoshop terms) and a round one by selecting the marquee tool and doing an ALT-CLICK on it. The marquee can be constrained to have the same dimension in the X and Y directions by switching "mode" from "normal" to "constrained". This mode can be selected in the gray horizontal bar at the top of your Photoshop screen. When "constrained" mode is selected you get to draw precise squares, or circles, rather than rectangles and ovals.