My reserach concerns segregation of immigrants in US by nationality. My study is at a county level.
Since you are interested in county level data, I would recommend looking at IPUMS NHGIS, which provides summary data at multiple geographic levels. However, since the tables available through IPUMS NHGIS are summaries of individual level responses, many tables included aggregated response categories, so that respondents that indicated they were born in Lithuania or Latvia would be grouped into the “Place of Birth: Europe”.
If you find you need more detail than is available in the IPUMS NHGIS summary tables, you can look into IPUMS USA. IPUMS USA provides far grater individual level detail as it provides microdate as apposed to aggregate data. The trade off here is that not all counties are identifiable due to confidentiality constraints.
In either case, I would recommend starting with the FAQ sections (IPUMS NHGIS FAQ, IPUMS USA FAQ) as well as the short tutorials (for IPUMS NHGIS, and for IPUMS USA) to get a feel for the website and how to get the data you are looking for.
I hope this helps.
Thank you very much for your help. I already found census data on factfinder.census.gov that provide aggregate data on the number of foreign-born by nationality and detailed location. Best