File paths when using fs

So I know that the fs module uses process.cwd() and not the actual location of the module but I'm still having a little issue with file paths.

So I've a app structure like this:

- app
   - controllers
   - models
   - views
   - tmp
   - index.js

I'm creating csv files in a controller, and hope to store it in tmp.

So considering that despite being in controllers,

process.cwd() is '/app'

I did this:

fs.open('tmp/FILENAME','w',function (err,fd) {

err is null so that seems to work fine. So that should be in /app/tmp/

On the other hand, in index.js I have

res.attachment(__dirname + '/tmp/FILENAME')

which when console.logged is

/app/tmp/FILENAME

which is correct. So clearly the other half is causing the problem.

So my question is - What is the correct way to use fs in this scenario, taking into consideration process.cwd() ? Kinda stumped, any ideas?