It's OK to use a popup here
Assuming you need positive user affirmation of the legal disclaimer (ie it's important to you that the user directly clicks to affirm the legal content), a modal pop-up is not only fine but it has several benefits:
The pop-up channels user focus onto the legal content (assuming you are masking or otherwise de-emphasizing the background). So lawyers feel comfortable that the user has been given a clear and unambiguous opportunity to read the content.
It provides clear choices to the user. The blocking pop-up interaction with agree/cancel provides helps frame and force a clear choice for the user in a way that doesn't make the user not feel trapped (she can exit the dialog and cancel).
It takes advantage of behavioral sunk cost. By sequencing the disclaimer after the Submit button, you are reducing the up-front friction for completing the form. By the time the user sees the disclaimer dialog, he has already invested time completing the form and pressing Submit, so he is more likely to accept the legal terms, because he's already invested in the completion process.
- I forget the exact name of this behavioral principle (anyone?) but it's the same principle that causes people to buy an item even after discovering that there is a sales or service tax on top of the expected price...they have already invested cognitively in making the purchase so they're likely to accept the incremental cost to finish the transaction.
- Not all designers would be comfortable with the moral position of this approach, but if you are not keeping the user's details if she doesn't consent perhaps you can feel comfortable, but in any case you should pause to give appropriate thought to the moral pros and cons here.
If you decide to do this...
I would recommend using animations, transitions, color palette, fonts, and layout to make the pop-up interaction as smooth and harmonious as possible.
This will present the interface in a way that makes the added cognitive disruption and decision cost of the disclaimer as low-friction as possible.
A good functional example to think about for inspiration is to look at how pharmaceutical companies read out their legal disclaimers during drug TV ads. They use a calm color palette, soothing voice, and smooth, flowing and happy visuals to help calm and even distract users from the difficult content of the legal disclaimer.
- For example, see this ad for Lunesta. Most of the airtime is actually devoted to the legal disclaimer, but the smooth design allows the legal language to feel less painful.
- Again, it's up to you to consider the moral issues here, I am only pointing out the technical design considerations.
By the way, legal departments are where good UX goes to die....